Drent, Jan

English

Proceedings - University of Groningen

   University of GroningenLife history variation of a marine bivalve (Macoma balthica) in a changing worldDrent, JanIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2004Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Drent, J. (2004). Life history variation of a marine bivalve (Macoma balthica) in a changing world. [Thesisfully internal (DIV), University of Groningen]. s.n.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 01-11-2023Summarl,l t  is  beconing inc leasinuly  c lear  that  the u 'or lc l 's  tenrpclature is  i r rcreasing.  ln  t l rcNetlrerlancls. too. tenrperature has beeu on the increase dLrring thc last clecaclcs anclth is  t rencl  is  expectec l  to  cont inue.  These changcs nray substant ia l l l ,  iu tpact  coul lu-n i t ies of  p lants ancl  ani rnals .  Cl inrate has a st ron-e in Íhrcnce on thc c l is t r ibut ion ofspecies.  What  c l ïècts r . r , i l l  a  chaugiug c l imate have or . r  arr  ccological ly  in tpor tantarea l ike thc Wadclen Sea'l The lescarch prcscnted here highlights a part of thisbroader c luest ion.Thc Wadden Sea is  an area of  t ic la l  Í - la ts  in te lsectcc l  by c leepcr  eul l ies.  Thc se-clirrcnt is inhabitccl by lar-ee nunrbcrs of bivalr, 'cs and lvorn-rs. Most l ivc burrowcclin the scclitnent. but sotle s1'rccics. the nrr.rsscl ancl at prescnt l lso thc PaciÍlc oystcr.are attachctl to the substrate ancl can in this way Íbnr largc banks. The benthic Íi iunaconsumes r rost ly  unicel lLr lar  a lgac Í i 'om the water  ancl  Í l -orr  the sedintent  sur Íàce.The bivalvcs and \l,c'rtt.t.ts irr their turn are eaten by lar-uc numbcls o1'bilds ancl l lsh.thus Íbnr ing arr  i rnpor tant  l ink in  the Waclden Sea ccosystcnr .  Wi thout  i ts  bcnth icÍ iu" tna the area would lack i ts  ro le as a nurscry for  Íbr  iustancc p la ice 'o l  l rs  u inte l ingancl stopor"cr arca Íbr r. laders.What  u ' i l l  happcn to the coururuni ty  of  thc t ida l  Í la t  sedinrent  i f  thc c l inratcc l ianges '? To tack le th is  cprest ion.  i t  . " r ,as chosen to lbcus ou one s in-u lc  species.  theBal t ic  c lan Mu<ot t tu ln l t l t icu (L. ) .  The Bal t ic  c lanr  is  a smal l  b i r ,a l r .c  nro l lusc thatri ' i l l  grow to about 25 nrnr. TIrc shells ale 'ul,hitc. pink. yellovn' or oran-ge. The anirrall ives burrou,cd a fèw ccnt inreters in to thc sedinrent .  p l inrar i ly  in  thc t ida l  l la ts .  butit can also bc Íbund ir.r sor.nenhat cleeper vr,atcfs, down to abor"rt 2,5 rrctcrs. TheBal t ic  c larr  sucks up sand and food par t ic les through i ts  long and Í lex ib le s iphon.In the shci l  cav i ty  the animal  sor ts  thc col lectec l  rnater ia l .  in_9csts thc edib le pal t i -c les Íbr  fur ther  c l igcst ion and excrctcs thc rest  as pseucioÍaeces.  Thc Bal t ic  c lanr  isa u,idesprcad and cornuron species that is Íbund Íl 'om the Cii lonclc at thc Freuclrcoast  uear  Bordeaur r " rp to Nclva Zernbla in  the h igh nor th.  For  thc l la l t ic  c lanr  theWaddcn Sea is thcrclt lre a fi i ir lv soLrtlrerly klcation r.r, ithin its _rrco_uraphic clistl ibLr-t ion.  On the basis  of  th is  i t  r ray be expected that  th is  s1 ' rcc ics i le lat ive ly  seusi t iveto u'artrinq of t l ie Wirclclen Sea, lvhich \\ 'as eur irrportant rcasou 1br choosine theBaltic clanr as rcscarch objcct. Morcover, a long rcsearch traclit ion exists Ítrrl, lucortttt hultlt ic'u. as a rcsult ol rvhich prior knou,lecl-9e of its biolo-uy \\ 'as cxtcn-s ivc.  Notably.  a successf i r l  rcar ing techniquc had been dcveloped.  Íac i l i ta t ing bree-ding crperirrclrts ir.r the laboratory.What  happens to the Bal t ic  c lanr  u 'hen the tempcratu le r iscs ' l  Thcrc a le rcal lyonly two possib i l i t ics:  ad-yust  to  the neu,c i rcur .nstanccs or ' lcavc ' lbr  cooler  p lacest10 S' t t  t t t  t t t r t  t ' t 'to thc north, which implies local ertinctior, . Adjr,rstrneuts to thc surroundings opc-ratc at severarl levels. within an inclivicLral or belll 'een individuals. Within ar-r ir-rcli-v idual the capacity to adjust o the euvironment is cal led phcnotypic plast ic i ty.  Thephenotype is the entire indiviclr.ral nd comprises morphological. physiological andbehavioural traits. Phenotypic plasticity can bc diviclecl into phenotypic Ílexibilityancl cleveloprnental p asticity. Phenotypic Ílcribility inrplies reversible changes inbehaviour,  physiology or even morphology: the changes tend to be rapid ar,d cnablcrcsponciing to short tenn alterations of the enviror.rlnent. Devcloprncntal plasticity isthc capacity of an inclividual to ttrhe on a particular phenotype, depending on thcenviror.rr .nent,  dul ing dcvelopment.  The lat ter is l loïe per\asire aud part icular lysuited Íbr respondirrg to environrncntal variation that is relatively srnall during anincliviclual's lifetin-re. The difïèrent Íbru,s of plasticity u'il l cornc into play ciepen-ding on the spat ial  and temporal scales involved.At some point the possibi l i t ies fbr arr  i rTdividLral  to respclnd to changcs r,r ' i l l  bccrhaustecl .  I t  is then possible that at  the populat ion level there are somc individualsthat are stil l capable of coping, while others succumb. If thcsc diÍï-crcnccs arc gcuc-t ical ly deterrnined, the gcnct ic corrst i tut i t rn of the populat ion wi l l  change. Thisprocess is called natural selection ancl takcs place over several -ÍIenerations.Phenotypic plasticity and genetic adaptations compler.nent each other in achanging world.  For the Balt ic c larr  i t  was eran.r ined to what cxtcut phenotypicplasticity and genetic adaptatior-rs arc ilrvolved. ln the most extrelne cases. alladjustments coLrld be either plastic or gcnctic. Molecular r"narkers indicated that atthe European scale genetic diÍï-crcnccs bct'uveen populations are very pronounced.Baltic clarns fi-om the Baltic Seti are even so ertrenrely diÍïèrent that they mi-eht beconsidered to bc a separate species. But other populat ions. too, display clear di Í ïè-rences thal point to a strongly restricted exchauge of individLrals and thus genesbetll,een locations. As a conscqucucc. Baltic cleuls rnay well be c|ftèrentially adap-ted at locations with diÍïcrcnt circurnstances. This result was unexpected because itwas always assnrncd that tsaltic clams eÍÏèctil 'ely tlavel large distances i,vhen theyclriÍi fieely in the water colun-tr as larvae. Unhanrpcrcd larval clispersal lvor"rldguarantcc substantial rnirin-e betw'een populations. inhibiting genetic diÍ1èrenccs{r'orn arising and resr"rlting in the inevitability that adjLrstments to diÍïèrences in envi-ronllelrt carr happen only thror.rgh phcnotypic plasticity.The resnlts of thc moleculerr merrker study indicate that genetic adaptations tocircur.nstanccs spcciÍic to local popr,rlartions are indeed possible. By rearing Balticclams of diÍï-crcnt origins r-urder ider-rtical circurnstances in the laboratory it is pos-sible to detennine to urhat ertent pafticLrlaf traits arc hcritablc (gcnctic). Aclults werecollccted at various localities (Balsfjord. norlhern Norrvay; Wadden Sea and NorthSea, thc Nctherleu-rds: Gironde. France) and indr-rced to spawn in the laboratory. Thclarvae werc rcarcd to the juvenile stage nnder controlled conditions. Any subse-qLrent difÍèrclrccs bctrvcen oflipring of pareuts Íï'om difÍèrent origins are uost like-ly genetically dctcrminecl. Three aspects were str-rdied: Íirst. the Íbraging apparatus,Stt t t t  t t t t  t ' tsccond. scnsit iv i ty of Balt ic c lams to high tcmperatures ancl.  Í inal ly.  srowth auddcvelclprnent clf lalr,ae at diÍfcrent tentperaturcs.Thc u'ater curreltt with which particlcs tl 'onr the seclilnent surfàce are suckecl upt l t rough the siphon is gencrated by ci l ia on thc gi l ls in thc shel l  cavi ty.  Part ic les areÍlltered frou the water curreltt, also by the gills, and tlren transported to the so-cal led palps. On the palps. l ike the gi l ls covered with ci l ia.  the par-t ic lcs arc sortecl .aÍter which thc ediblc Íl 'action is ingcstecl by thc mouth and thc rest is excreted aspsetrdoÍàcces. Strong diÍ1èrences erist betr.veen populations in the ratio betwecn thegi l ls (col lcct ing) ancl  the palps (sort ing).  In the reared Muc'onru hulthi t 'n the di f--fcrences betwcerr populations turned out to be heritable and thercÍbre genetic. Anadditional experitnent den-tonstrated that thc ratio betrveen gills and palps changeswith cl i f tèrent seci iment types. ln course sand the,ei l ls ale larqe ancl the palps srral l .and this is revcrsecl in si l t .  This is probably rclated to thc antount of part ic les thateltters at a particular pr,trnping capacity. A volunre of sancl contains Íar Í-cwcr parti-c les t l iat t  the sanre tolut .uc of s i l t  and therefbre sort in-e out a volume is muclr  lesswork i f  i t  consists of sancl than i f  i t  contains a lot  of  s i l t .  In contrast to the di Í ïe-rel tces betweclt  populat ions. c l i f Ïèrent gi l l /palp rat ios within populat ions arc to alar'-ee ertent flexiblc and lvill respond to the local circur.nstances.tf Baltic clartrs in the nortlt and in the south are both adapted to thcir local cir-cltr.nstances. thc sot-tthern bivalves should be bettcr able to cope u,ith Itigh tcrrpela-tures. This rvas studied by Lrtil izing thc lact that proteins denatule when exposed tohighert l tan opt imal tcr.npet 'atures. I f  this happens inside a cel l  a spccial  type of pro-teins. the hcat shocl< proteins (HSPs). is synthcsized to ci ther repai l  the danrage orre trdcr the danagcd proteius harurle ss and deport thern. The concentration of HSPsis thereforc a lleasure of tentperature stress. A comparison betwcen Íbr.rr groups oflaboratoly reared Baltic clanrs. fi 'our Norlvay, the North Sea, thc Wadclen Sea anclFrattce. shou,cd tltat po;lulations do difÈr in terrlleratllre tolcrance bLrt also thatthese cliÍJèreltces \\'ere not casy to cor"r;llc to tlie original environment. Interestingly.indivic lLrals stntr l lcr  than 5 mur had much higher levels of HSPs than larger ones.This diÍÏèrencc nray be ascribed to telrperatr-u'e diÍÈreuces bctween the placeswhere Balt ic c lans of di Í ïerent sizes tcncl  to be located. Smal ler individuals are bur-rowed closc to 1hc sediment surfacc. while larger ones are burrowed ntore cleeply.Close to the surfàce telxperatllres Í-luctuate nrore and will also reach lti_shcr levels.especially wlrcn a ntuclÍ' lat is exposed around noon. Moreover, small Baltic clarnsare on avera-qc locatecl Irieher up in the tidal zone than larger ones. wltich scrvesrnainly to escape prcdatot's like crabs and shrinp. Thesc places are subnter-{ed lesslong and can thercÍore heat up more dr"rrin-e low ticle than the lowcr ticlal zone. Itwould thcrcfore appear tltat as a Baltic clarl grows, the tcmperature of its sun'olurd-ings f luctuates less and also reaches lor. ler maxima. I t  wi l l  then necd HSPs lessoÍlen to react to proteiu dama-{e caused by hi-uh terrperatures and ther-eÍbrc haveless of thcse proteins in stock.t 7 lt72 Stt t t t t t t t t t ' t 'After  Íèr t i l izat ion of  the eg-9s.  larvac devclop.  The larvae remain in  the vvatercolutnu Íbr a l 'cw lr,eeks. This is a tinre that is charactcrized by high nrurtality ratesand could be a l iniit ing fàctor Íor thc occurrcr.rcc oÍ'Baltic clams. Thc cl}èct of tem-peratl lre on tltc clcvclopmcnt of larvae of Baltic clanrs Íl 'cln.r the Balsljorcl iu Norwayaud fron-t the Ciironde in Francc r,vas studied in a rcalinq experintent in the labora-tory. Larvae of both populations -9rcr.r,and dcvclopccl f irster at high tentperatures butalso clisplayccl a hi-qher nortality latc. Frcnch larvae developccl fhster thanNorwegiitn larvae. Rernarkably. thc Nonvcgian lalvae sti l l  developecl successfullyat  a water  tcr lperature of  20 'C' .  This  is  approximatc ly  l5 'C h igher  than normal  inthe Balsf-1orcl! Lalvac clearly tolcrate a wide range of tentperatures.All three cx;lcl ir.nents lvit lr rcared Baltic clanrs uncovered -eenctically deter-ntined clifÍèrcnces betu,een populations. but thc nrost ltotable observatiorr rvas thatin  a l l  cases thc indi l ' iduals u 'erc capable of  rcs l - roncl ing ef tèct i r . 'e ly  to  var iable c i r -cun-rstances. The Baltic clanr is -gcnctically aclalrtccl to local circumstances. but.fbremost .  reacts pheuotypical ly  p last ic  to  val ia t ions in  i ts  sun'or-urc l ings.The eÍ1'ccts of temperature on the Baltic clam wcrc studied furthcr among indi-v iduals Í l 'ont  thc Wadden Sea usiu-9 l ie ld observat ions and laborato ly  cxpcl i r rents.Temperature has a strolrs inl ' lLrcncc on bioloqical 1-rrocesses. ln colcl-blooded ani-nrals l ike i l ' lut 'orttu hultlt ic'u elrcfgy clcnrand incrcascs steeply as it beconres \\ 'anrer.E,arlier research had already pointccl ont that a hisher energy clemancl in rri ld u'in-ters reduces thc uumber of proclLrcecl eggs courparccl to cold winters. In a starvatiouexperiment. subjccting Mut'ottru hulthicu to diÍïèrcnt terxperatl lres. thc Baltic clanswere Íbuncl to bc capable of allocating their energy effectively tou,arcls synthesis ofeggs even at u,inter tenrpelatrrlcs rbor e a\ erage.Besides har ing an inÍ' lucncc on process rates. tcurperature nray also le_uulatethe breeding scasou. Baltic clanrs spa\v11 after the \\,ater temperature cxcccds a cer-ta in thresholc l .  and i t  is  assunrcc l  that  th is  represents a d i rect  react iorr .  I la l t ic  c larrsthat  l ive in  t ic la l  Í la ts  woulc l  thcrr  possib ly  spawn car l ier  than thei r  consl - rcc i Í ics indeeper water. bccausc at low ticle the sun warms thc Ílats and nruch lcss so thc bot-tot r  of  thc gul l ies.  Whi le t rack ing the reproduct ivc s tatus of  Bal t ic  c lants no d i f fer-ence it.r t irring u,as obsen'ed bctu'een the tidal Í ' lats arrd the gull ics. It r,vas hotvet'eruoted that the timc durirrg u'hich spau'nirr_e takes place is long. at least tu'o nonths.IndividLrals spawn scveral t iurcs cluring this periocl. We suppose that in this u'ay thcyenlarge their chauccs to proclucc oÍl.spring at a l irvolrlable time, a l<incl of bet-hed-ging.To get an idca of thc actual temperature cliÍï 'crences betu,een populations. acot.nparison u,as utade between lleld sites alorrg thc European coasÍ. Baltic claursrvere collcctccl at a lar-ee nunrbcl of sites and tlrc gro\\rth ratc cstiutated from thegrorvth rit 'rgs on thc shells. Thc cxpectation lvas that I lalt ic claurs clf ntorc norther-ly popLrlatiolts grow more slor,vly cll le to the lower tcmperatures. The obscrved pat-tem, however', is not uncquivocal. Thrs points to tlre importance of otltcr Íàctolssuch as Íbocl  avai labi l i ty .  01 ' thc th lee populat ions BalsÍ jord.  Waclc lcn Sea andSummaryGironde the seasonal cycle of body mass and gonad development was estimated.From this, an energy budget could be reconstructed and it appeared that Balticclams from the Balsflord devote more energy to reproduction than the moresoutherly clams from the Wadden Sea and the Gironde. This can mainly be ascribedto the lower temperature, which leaves more energy for body maintenance.Furthermore, it could be deduced that food availability differs between the popula-tions.The results of the separate parts of this study all point to a high level of tole-rance to differences in environmental variables. At the same time genetic adapta-tions to local circumstances exist. It seems that genetic adaptations exist for certainconsistent differences between populations, while superimposed on this, flexibletraits enable the individuals to respond to their variable environment.Do these results imply that the Baltic clam is safeguarded against changes in itsenvironment due to its phenotypic plasticity? The answer is no. Long-term researchof the Royal Netherlands Institute for Sea Research at Íhe Balgzand and other areasin the western Wadden Sea shows that since the end of the nineteen eighties the pop-ulation of Macoma balthica is declining. While average density used to be around100 adults per square meter, now it is merely 10 per square meter. The causes of thisdecline are still uncertain. On the basis of the results in this thesis it is not probablethat direct effects of climate on the physiology of the Baltic clam are responsible.Indirect effects through interactions with other species that respond differently toclimate or other environmental factors are more likely. For example, the spatfall ofBaltic clams may have severely decreased due to predation by shrimp. Shrimp areparticularly numerous on the tidal flats at the time of spatfall after mild winters.However, when conducting research into effects of human-induced changes inecosystems, in this case climate, it must be realised that human influences are var-ied. Pollution, eutrofication and not forgetting fisheries all have undoubtedly influ-enced the current development of Baltic clams and other benthic fauna of theWadden Sea.t73

Life history variation of a marine bivalve (<i>Macoma balthica</i>) in a changing world

ARTS repository - University of Groningen

   University of GroningenLife history variation of a marine bivalve (Macoma balthica) in a changing worldDrent, JanIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2004Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Drent, J. (2004). Life history variation of a marine bivalve (Macoma balthica) in a changing world. [Thesisfully internal (DIV), University of Groningen]. s.n.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 13-02-2023Summarl,l t  is  beconing inc leasinuly  c lear  that  the u 'or lc l 's  tenrpclature is  i r rcreasing.  ln  t l rcNetlrerlancls. too. tenrperature has beeu on the increase dLrring thc last clecaclcs anclth is  t rencl  is  expectec l  to  cont inue.  These changcs nray substant ia l l l ,  iu tpact  coul lu-n i t ies of  p lants ancl  ani rnals .  Cl inrate has a st ron-e in Íhrcnce on thc c l is t r ibut ion ofspecies.  What  c l ïècts r . r , i l l  a  chaugiug c l imate have or . r  arr  ccological ly  in tpor tantarea l ike thc Wadclen Sea'l The lescarch prcscnted here highlights a part of thisbroader c luest ion.Thc Wadden Sea is  an area of  t ic la l  Í - la ts  in te lsectcc l  by c leepcr  eul l ies.  Thc se-clirrcnt is inhabitccl by lar-ee nunrbcrs of bivalr, 'cs and lvorn-rs. Most l ivc burrowcclin the scclitnent. but sotle s1'rccics. the nrr.rsscl ancl at prescnt l lso thc PaciÍlc oystcr.are attachctl to the substrate ancl can in this way Íbnr largc banks. The benthic Íi iunaconsumes r rost ly  unicel lLr lar  a lgac Í i 'om the water  ancl  Í l -orr  the sedintent  sur Íàce.The bivalvcs and \l,c'rtt.t.ts irr their turn are eaten by lar-uc numbcls o1'bilds ancl l lsh.thus Íbnr ing arr  i rnpor tant  l ink in  the Waclden Sea ccosystcnr .  Wi thout  i ts  bcnth icÍ iu" tna the area would lack i ts  ro le as a nurscry for  Íbr  iustancc p la ice 'o l  l rs  u inte l ingancl stopor"cr arca Íbr r. laders.What  u ' i l l  happcn to the coururuni ty  of  thc t ida l  Í la t  sedinrent  i f  thc c l inratcc l ianges '? To tack le th is  cprest ion.  i t  . " r ,as chosen to lbcus ou one s in-u lc  species.  theBal t ic  c lan Mu<ot t tu ln l t l t icu (L. ) .  The Bal t ic  c lanr  is  a smal l  b i r ,a l r .c  nro l lusc thatri ' i l l  grow to about 25 nrnr. TIrc shells ale 'ul,hitc. pink. yellovn' or oran-ge. The anirrall ives burrou,cd a fèw ccnt inreters in to thc sedinrent .  p l inrar i ly  in  thc t ida l  l la ts .  butit can also bc Íbund ir.r sor.nenhat cleeper vr,atcfs, down to abor"rt 2,5 rrctcrs. TheBal t ic  c larr  sucks up sand and food par t ic les through i ts  long and Í lex ib le s iphon.In the shci l  cav i ty  the animal  sor ts  thc col lectec l  rnater ia l .  in_9csts thc edib le pal t i -c les Íbr  fur ther  c l igcst ion and excrctcs thc rest  as pseucioÍaeces.  Thc Bal t ic  c lanr  isa u,idesprcad and cornuron species that is Íbund Íl 'om the Cii lonclc at thc Freuclrcoast  uear  Bordeaur r " rp to Nclva Zernbla in  the h igh nor th.  For  thc l la l t ic  c lanr  theWaddcn Sea is thcrclt lre a fi i ir lv soLrtlrerly klcation r.r, ithin its _rrco_uraphic clistl ibLr-t ion.  On the basis  of  th is  i t  r ray be expected that  th is  s1 ' rcc ics i le lat ive ly  seusi t iveto u'artrinq of t l ie Wirclclen Sea, lvhich \\ 'as eur irrportant rcasou 1br choosine theBaltic clanr as rcscarch objcct. Morcover, a long rcsearch traclit ion exists Ítrrl, lucortttt hultlt ic'u. as a rcsult ol rvhich prior knou,lecl-9e of its biolo-uy \\ 'as cxtcn-s ivc.  Notably.  a successf i r l  rcar ing techniquc had been dcveloped.  Íac i l i ta t ing bree-ding crperirrclrts ir.r the laboratory.What  happens to the Bal t ic  c lanr  u 'hen the tempcratu le r iscs ' l  Thcrc a le rcal lyonly two possib i l i t ics:  ad-yust  to  the neu,c i rcur .nstanccs or ' lcavc ' lbr  cooler  p lacest10 S' t t  t t t  t t t r t  t ' t 'to thc north, which implies local ertinctior, . Adjr,rstrneuts to thc surroundings opc-ratc at severarl levels. within an inclivicLral or belll 'een individuals. Within ar-r ir-rcli-v idual the capacity to adjust o the euvironment is cal led phcnotypic plast ic i ty.  Thephenotype is the entire indiviclr.ral nd comprises morphological. physiological andbehavioural traits. Phenotypic plasticity can bc diviclecl into phenotypic Ílexibilityancl cleveloprnental p asticity. Phenotypic Ílcribility inrplies reversible changes inbehaviour,  physiology or even morphology: the changes tend to be rapid ar,d cnablcrcsponciing to short tenn alterations of the enviror.rlnent. Devcloprncntal plasticity isthc capacity of an inclividual to ttrhe on a particular phenotype, depending on thcenviror.rr .nent,  dul ing dcvelopment.  The lat ter is l loïe per\asire aud part icular lysuited Íbr respondirrg to environrncntal variation that is relatively srnall during anincliviclual's lifetin-re. The difïèrent Íbru,s of plasticity u'il l cornc into play ciepen-ding on the spat ial  and temporal scales involved.At some point the possibi l i t ies fbr arr  i rTdividLral  to respclnd to changcs r,r ' i l l  bccrhaustecl .  I t  is then possible that at  the populat ion level there are somc individualsthat are stil l capable of coping, while others succumb. If thcsc diÍï-crcnccs arc gcuc-t ical ly deterrnined, the gcnct ic corrst i tut i t rn of the populat ion wi l l  change. Thisprocess is called natural selection ancl takcs place over several -ÍIenerations.Phenotypic plasticity and genetic adaptations compler.nent each other in achanging world.  For the Balt ic c larr  i t  was eran.r ined to what cxtcut phenotypicplasticity and genetic adaptatior-rs arc ilrvolved. ln the most extrelne cases. alladjustments coLrld be either plastic or gcnctic. Molecular r"narkers indicated that atthe European scale genetic diÍï-crcnccs bct'uveen populations are very pronounced.Baltic clarns fi-om the Baltic Seti are even so ertrenrely diÍïèrent that they mi-eht beconsidered to bc a separate species. But other populat ions. too, display clear di Í ïè-rences thal point to a strongly restricted exchauge of individLrals and thus genesbetll,een locations. As a conscqucucc. Baltic cleuls rnay well be c|ftèrentially adap-ted at locations with diÍïcrcnt circurnstances. This result was unexpected because itwas always assnrncd that tsaltic clams eÍÏèctil 'ely tlavel large distances i,vhen theyclriÍi fieely in the water colun-tr as larvae. Unhanrpcrcd larval clispersal lvor"rldguarantcc substantial rnirin-e betw'een populations. inhibiting genetic diÍ1èrenccs{r'orn arising and resr"rlting in the inevitability that adjLrstments to diÍïèrences in envi-ronllelrt carr happen only thror.rgh phcnotypic plasticity.The resnlts of thc moleculerr merrker study indicate that genetic adaptations tocircur.nstanccs spcciÍic to local popr,rlartions are indeed possible. By rearing Balticclams of diÍï-crcnt origins r-urder ider-rtical circurnstances in the laboratory it is pos-sible to detennine to urhat ertent pafticLrlaf traits arc hcritablc (gcnctic). Aclults werecollccted at various localities (Balsfjord. norlhern Norrvay; Wadden Sea and NorthSea, thc Nctherleu-rds: Gironde. France) and indr-rced to spawn in the laboratory. Thclarvae werc rcarcd to the juvenile stage nnder controlled conditions. Any subse-qLrent difÍèrclrccs bctrvcen oflipring of pareuts Íï'om difÍèrent origins are uost like-ly genetically dctcrminecl. Three aspects were str-rdied: Íirst. the Íbraging apparatus,Stt t t t  t t t t  t ' tsccond. scnsit iv i ty of Balt ic c lams to high tcmperatures ancl.  Í inal ly.  srowth auddcvelclprnent clf lalr,ae at diÍfcrent tentperaturcs.Thc u'ater curreltt with which particlcs tl 'onr the seclilnent surfàce are suckecl upt l t rough the siphon is gencrated by ci l ia on thc gi l ls in thc shel l  cavi ty.  Part ic les areÍlltered frou the water curreltt, also by the gills, and tlren transported to the so-cal led palps. On the palps. l ike the gi l ls covered with ci l ia.  the par-t ic lcs arc sortecl .aÍter which thc ediblc Íl 'action is ingcstecl by thc mouth and thc rest is excreted aspsetrdoÍàcces. Strong diÍ1èrences erist betr.veen populations in the ratio betwecn thegi l ls (col lcct ing) ancl  the palps (sort ing).  In the reared Muc'onru hulthi t 'n the di f--fcrences betwcerr populations turned out to be heritable and thercÍbre genetic. Anadditional experitnent den-tonstrated that thc ratio betrveen gills and palps changeswith cl i f tèrent seci iment types. ln course sand the,ei l ls ale larqe ancl the palps srral l .and this is revcrsecl in si l t .  This is probably rclated to thc antount of part ic les thateltters at a particular pr,trnping capacity. A volunre of sancl contains Íar Í-cwcr parti-c les t l iat t  the sanre tolut .uc of s i l t  and therefbre sort in-e out a volume is muclr  lesswork i f  i t  consists of sancl than i f  i t  contains a lot  of  s i l t .  In contrast to the di Í ïe-rel tces betweclt  populat ions. c l i f Ïèrent gi l l /palp rat ios within populat ions arc to alar'-ee ertent flexiblc and lvill respond to the local circur.nstances.tf Baltic clartrs in the nortlt and in the south are both adapted to thcir local cir-cltr.nstances. thc sot-tthern bivalves should be bettcr able to cope u,ith Itigh tcrrpela-tures. This rvas studied by Lrtil izing thc lact that proteins denatule when exposed tohighert l tan opt imal tcr.npet 'atures. I f  this happens inside a cel l  a spccial  type of pro-teins. the hcat shocl< proteins (HSPs). is synthcsized to ci ther repai l  the danrage orre trdcr the danagcd proteius harurle ss and deport thern. The concentration of HSPsis thereforc a lleasure of tentperature stress. A comparison betwcen Íbr.rr groups oflaboratoly reared Baltic clanrs. fi 'our Norlvay, the North Sea, thc Wadclen Sea anclFrattce. shou,cd tltat po;lulations do difÈr in terrlleratllre tolcrance bLrt also thatthese cliÍJèreltces \\'ere not casy to cor"r;llc to tlie original environment. Interestingly.indivic lLrals stntr l lcr  than 5 mur had much higher levels of HSPs than larger ones.This diÍÏèrencc nray be ascribed to telrperatr-u'e diÍÈreuces bctween the placeswhere Balt ic c lans of di Í ïerent sizes tcncl  to be located. Smal ler individuals are bur-rowed closc to 1hc sediment surfacc. while larger ones are burrowed ntore cleeply.Close to the surfàce telxperatllres Í-luctuate nrore and will also reach lti_shcr levels.especially wlrcn a ntuclÍ' lat is exposed around noon. Moreover, small Baltic clarnsare on avera-qc locatecl Irieher up in the tidal zone than larger ones. wltich scrvesrnainly to escape prcdatot's like crabs and shrinp. Thesc places are subnter-{ed lesslong and can thercÍore heat up more dr"rrin-e low ticle than the lowcr ticlal zone. Itwould thcrcfore appear tltat as a Baltic clarl grows, the tcmperature of its sun'olurd-ings f luctuates less and also reaches lor. ler maxima. I t  wi l l  then necd HSPs lessoÍlen to react to proteiu dama-{e caused by hi-uh terrperatures and ther-eÍbrc haveless of thcse proteins in stock.t 7 lt72 Stt t t t t t t t t t ' t 'After  Íèr t i l izat ion of  the eg-9s.  larvac devclop.  The larvae remain in  the vvatercolutnu Íbr a l 'cw lr,eeks. This is a tinre that is charactcrized by high nrurtality ratesand could be a l iniit ing fàctor Íor thc occurrcr.rcc oÍ'Baltic clams. Thc cl}èct of tem-peratl lre on tltc clcvclopmcnt of larvae of Baltic clanrs Íl 'cln.r the Balsljorcl iu Norwayaud fron-t the Ciironde in Francc r,vas studied in a rcalinq experintent in the labora-tory. Larvae of both populations -9rcr.r,and dcvclopccl f irster at high tentperatures butalso clisplayccl a hi-qher nortality latc. Frcnch larvae developccl fhster thanNorwegiitn larvae. Rernarkably. thc Nonvcgian lalvae sti l l  developecl successfullyat  a water  tcr lperature of  20 'C' .  This  is  approximatc ly  l5 'C h igher  than normal  inthe Balsf-1orcl! Lalvac clearly tolcrate a wide range of tentperatures.All three cx;lcl ir.nents lvit lr rcared Baltic clanrs uncovered -eenctically deter-ntined clifÍèrcnces betu,een populations. but thc nrost ltotable observatiorr rvas thatin  a l l  cases thc indi l ' iduals u 'erc capable of  rcs l - roncl ing ef tèct i r . 'e ly  to  var iable c i r -cun-rstances. The Baltic clanr is -gcnctically aclalrtccl to local circumstances. but.fbremost .  reacts pheuotypical ly  p last ic  to  val ia t ions in  i ts  sun'or-urc l ings.The eÍ1'ccts of temperature on the Baltic clam wcrc studied furthcr among indi-v iduals Í l 'ont  thc Wadden Sea usiu-9 l ie ld observat ions and laborato ly  cxpcl i r rents.Temperature has a strolrs inl ' lLrcncc on bioloqical 1-rrocesses. ln colcl-blooded ani-nrals l ike i l ' lut 'orttu hultlt ic'u elrcfgy clcnrand incrcascs steeply as it beconres \\ 'anrer.E,arlier research had already pointccl ont that a hisher energy clemancl in rri ld u'in-ters reduces thc uumber of proclLrcecl eggs courparccl to cold winters. In a starvatiouexperiment. subjccting Mut'ottru hulthicu to diÍïèrcnt terxperatl lres. thc Baltic clanswere Íbuncl to bc capable of allocating their energy effectively tou,arcls synthesis ofeggs even at u,inter tenrpelatrrlcs rbor e a\ erage.Besides har ing an inÍ' lucncc on process rates. tcurperature nray also le_uulatethe breeding scasou. Baltic clanrs spa\v11 after the \\,ater temperature cxcccds a cer-ta in thresholc l .  and i t  is  assunrcc l  that  th is  represents a d i rect  react iorr .  I la l t ic  c larrsthat  l ive in  t ic la l  Í la ts  woulc l  thcrr  possib ly  spawn car l ier  than thei r  consl - rcc i Í ics indeeper water. bccausc at low ticle the sun warms thc Ílats and nruch lcss so thc bot-tot r  of  thc gul l ies.  Whi le t rack ing the reproduct ivc s tatus of  Bal t ic  c lants no d i f fer-ence it.r t irring u,as obsen'ed bctu'een the tidal Í ' lats arrd the gull ics. It r,vas hotvet'eruoted that the timc durirrg u'hich spau'nirr_e takes place is long. at least tu'o nonths.IndividLrals spawn scveral t iurcs cluring this periocl. We suppose that in this u'ay thcyenlarge their chauccs to proclucc oÍl.spring at a l irvolrlable time, a l<incl of bet-hed-ging.To get an idca of thc actual temperature cliÍï 'crences betu,een populations. acot.nparison u,as utade between lleld sites alorrg thc European coasÍ. Baltic claursrvere collcctccl at a lar-ee nunrbcl of sites and tlrc gro\\rth ratc cstiutated from thegrorvth rit 'rgs on thc shells. Thc cxpectation lvas that I lalt ic claurs clf ntorc norther-ly popLrlatiolts grow more slor,vly cll le to the lower tcmperatures. The obscrved pat-tem, however', is not uncquivocal. Thrs points to tlre importance of otltcr Íàctolssuch as Íbocl  avai labi l i ty .  01 ' thc th lee populat ions BalsÍ jord.  Waclc lcn Sea andSummaryGironde the seasonal cycle of body mass and gonad development was estimated.From this, an energy budget could be reconstructed and it appeared that Balticclams from the Balsflord devote more energy to reproduction than the moresoutherly clams from the Wadden Sea and the Gironde. This can mainly be ascribedto the lower temperature, which leaves more energy for body maintenance.Furthermore, it could be deduced that food availability differs between the popula-tions.The results of the separate parts of this study all point to a high level of tole-rance to differences in environmental variables. At the same time genetic adapta-tions to local circumstances exist. It seems that genetic adaptations exist for certainconsistent differences between populations, while superimposed on this, flexibletraits enable the individuals to respond to their variable environment.Do these results imply that the Baltic clam is safeguarded against changes in itsenvironment due to its phenotypic plasticity? The answer is no. Long-term researchof the Royal Netherlands Institute for Sea Research at Íhe Balgzand and other areasin the western Wadden Sea shows that since the end of the nineteen eighties the pop-ulation of Macoma balthica is declining. While average density used to be around100 adults per square meter, now it is merely 10 per square meter. The causes of thisdecline are still uncertain. On the basis of the results in this thesis it is not probablethat direct effects of climate on the physiology of the Baltic clam are responsible.Indirect effects through interactions with other species that respond differently toclimate or other environmental factors are more likely. For example, the spatfall ofBaltic clams may have severely decreased due to predation by shrimp. Shrimp areparticularly numerous on the tidal flats at the time of spatfall after mild winters.However, when conducting research into effects of human-induced changes inecosystems, in this case climate, it must be realised that human influences are var-ied. Pollution, eutrofication and not forgetting fisheries all have undoubtedly influ-enced the current development of Baltic clams and other benthic fauna of theWadden Sea.t73

Dissertations of the University of Groningen

   University of GroningenLife history variation of a marine bivalve (Macoma balthica) in a changing worldDrent, JanIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2004Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Drent, J. (2004). Life history variation of a marine bivalve (Macoma balthica) in a changing world. s.n.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 03-06-2022Summarl,l t  is  beconing inc leasinuly  c lear  that  the u 'or lc l 's  tenrpclature is  i r rcreasing.  ln  t l rcNetlrerlancls. too. tenrperature has beeu on the increase dLrring thc last clecaclcs anclth is  t rencl  is  expectec l  to  cont inue.  These changcs nray substant ia l l l ,  iu tpact  coul lu-n i t ies of  p lants ancl  ani rnals .  Cl inrate has a st ron-e in Íhrcnce on thc c l is t r ibut ion ofspecies.  What  c l ïècts r . r , i l l  a  chaugiug c l imate have or . r  arr  ccological ly  in tpor tantarea l ike thc Wadclen Sea'l The lescarch prcscnted here highlights a part of thisbroader c luest ion.Thc Wadden Sea is  an area of  t ic la l  Í - la ts  in te lsectcc l  by c leepcr  eul l ies.  Thc se-clirrcnt is inhabitccl by lar-ee nunrbcrs of bivalr, 'cs and lvorn-rs. Most l ivc burrowcclin the scclitnent. but sotle s1'rccics. the nrr.rsscl ancl at prescnt l lso thc PaciÍlc oystcr.are attachctl to the substrate ancl can in this way Íbnr largc banks. The benthic Íi iunaconsumes r rost ly  unicel lLr lar  a lgac Í i 'om the water  ancl  Í l -orr  the sedintent  sur Íàce.The bivalvcs and \l,c'rtt.t.ts irr their turn are eaten by lar-uc numbcls o1'bilds ancl l lsh.thus Íbnr ing arr  i rnpor tant  l ink in  the Waclden Sea ccosystcnr .  Wi thout  i ts  bcnth icÍ iu" tna the area would lack i ts  ro le as a nurscry for  Íbr  iustancc p la ice 'o l  l rs  u inte l ingancl stopor"cr arca Íbr r. laders.What  u ' i l l  happcn to the coururuni ty  of  thc t ida l  Í la t  sedinrent  i f  thc c l inratcc l ianges '? To tack le th is  cprest ion.  i t  . " r ,as chosen to lbcus ou one s in-u lc  species.  theBal t ic  c lan Mu<ot t tu ln l t l t icu (L. ) .  The Bal t ic  c lanr  is  a smal l  b i r ,a l r .c  nro l lusc thatri ' i l l  grow to about 25 nrnr. TIrc shells ale 'ul,hitc. pink. yellovn' or oran-ge. The anirrall ives burrou,cd a fèw ccnt inreters in to thc sedinrent .  p l inrar i ly  in  thc t ida l  l la ts .  butit can also bc Íbund ir.r sor.nenhat cleeper vr,atcfs, down to abor"rt 2,5 rrctcrs. TheBal t ic  c larr  sucks up sand and food par t ic les through i ts  long and Í lex ib le s iphon.In the shci l  cav i ty  the animal  sor ts  thc col lectec l  rnater ia l .  in_9csts thc edib le pal t i -c les Íbr  fur ther  c l igcst ion and excrctcs thc rest  as pseucioÍaeces.  Thc Bal t ic  c lanr  isa u,idesprcad and cornuron species that is Íbund Íl 'om the Cii lonclc at thc Freuclrcoast  uear  Bordeaur r " rp to Nclva Zernbla in  the h igh nor th.  For  thc l la l t ic  c lanr  theWaddcn Sea is thcrclt lre a fi i ir lv soLrtlrerly klcation r.r, ithin its _rrco_uraphic clistl ibLr-t ion.  On the basis  of  th is  i t  r ray be expected that  th is  s1 ' rcc ics i le lat ive ly  seusi t iveto u'artrinq of t l ie Wirclclen Sea, lvhich \\ 'as eur irrportant rcasou 1br choosine theBaltic clanr as rcscarch objcct. Morcover, a long rcsearch traclit ion exists Ítrrl, lucortttt hultlt ic'u. as a rcsult ol rvhich prior knou,lecl-9e of its biolo-uy \\ 'as cxtcn-s ivc.  Notably.  a successf i r l  rcar ing techniquc had been dcveloped.  Íac i l i ta t ing bree-ding crperirrclrts ir.r the laboratory.What  happens to the Bal t ic  c lanr  u 'hen the tempcratu le r iscs ' l  Thcrc a le rcal lyonly two possib i l i t ics:  ad-yust  to  the neu,c i rcur .nstanccs or ' lcavc ' lbr  cooler  p lacest10 S' t t  t t t  t t t r t  t ' t 'to thc north, which implies local ertinctior, . Adjr,rstrneuts to thc surroundings opc-ratc at severarl levels. within an inclivicLral or belll 'een individuals. Within ar-r ir-rcli-v idual the capacity to adjust o the euvironment is cal led phcnotypic plast ic i ty.  Thephenotype is the entire indiviclr.ral nd comprises morphological. physiological andbehavioural traits. Phenotypic plasticity can bc diviclecl into phenotypic Ílexibilityancl cleveloprnental p asticity. Phenotypic Ílcribility inrplies reversible changes inbehaviour,  physiology or even morphology: the changes tend to be rapid ar,d cnablcrcsponciing to short tenn alterations of the enviror.rlnent. Devcloprncntal plasticity isthc capacity of an inclividual to ttrhe on a particular phenotype, depending on thcenviror.rr .nent,  dul ing dcvelopment.  The lat ter is l loïe per\asire aud part icular lysuited Íbr respondirrg to environrncntal variation that is relatively srnall during anincliviclual's lifetin-re. The difïèrent Íbru,s of plasticity u'il l cornc into play ciepen-ding on the spat ial  and temporal scales involved.At some point the possibi l i t ies fbr arr  i rTdividLral  to respclnd to changcs r,r ' i l l  bccrhaustecl .  I t  is then possible that at  the populat ion level there are somc individualsthat are stil l capable of coping, while others succumb. If thcsc diÍï-crcnccs arc gcuc-t ical ly deterrnined, the gcnct ic corrst i tut i t rn of the populat ion wi l l  change. Thisprocess is called natural selection ancl takcs place over several -ÍIenerations.Phenotypic plasticity and genetic adaptations compler.nent each other in achanging world.  For the Balt ic c larr  i t  was eran.r ined to what cxtcut phenotypicplasticity and genetic adaptatior-rs arc ilrvolved. ln the most extrelne cases. alladjustments coLrld be either plastic or gcnctic. Molecular r"narkers indicated that atthe European scale genetic diÍï-crcnccs bct'uveen populations are very pronounced.Baltic clarns fi-om the Baltic Seti are even so ertrenrely diÍïèrent that they mi-eht beconsidered to bc a separate species. But other populat ions. too, display clear di Í ïè-rences thal point to a strongly restricted exchauge of individLrals and thus genesbetll,een locations. As a conscqucucc. Baltic cleuls rnay well be c|ftèrentially adap-ted at locations with diÍïcrcnt circurnstances. This result was unexpected because itwas always assnrncd that tsaltic clams eÍÏèctil 'ely tlavel large distances i,vhen theyclriÍi fieely in the water colun-tr as larvae. Unhanrpcrcd larval clispersal lvor"rldguarantcc substantial rnirin-e betw'een populations. inhibiting genetic diÍ1èrenccs{r'orn arising and resr"rlting in the inevitability that adjLrstments to diÍïèrences in envi-ronllelrt carr happen only thror.rgh phcnotypic plasticity.The resnlts of thc moleculerr merrker study indicate that genetic adaptations tocircur.nstanccs spcciÍic to local popr,rlartions are indeed possible. By rearing Balticclams of diÍï-crcnt origins r-urder ider-rtical circurnstances in the laboratory it is pos-sible to detennine to urhat ertent pafticLrlaf traits arc hcritablc (gcnctic). Aclults werecollccted at various localities (Balsfjord. norlhern Norrvay; Wadden Sea and NorthSea, thc Nctherleu-rds: Gironde. France) and indr-rced to spawn in the laboratory. Thclarvae werc rcarcd to the juvenile stage nnder controlled conditions. Any subse-qLrent difÍèrclrccs bctrvcen oflipring of pareuts Íï'om difÍèrent origins are uost like-ly genetically dctcrminecl. Three aspects were str-rdied: Íirst. the Íbraging apparatus,Stt t t t  t t t t  t ' tsccond. scnsit iv i ty of Balt ic c lams to high tcmperatures ancl.  Í inal ly.  srowth auddcvelclprnent clf lalr,ae at diÍfcrent tentperaturcs.Thc u'ater curreltt with which particlcs tl 'onr the seclilnent surfàce are suckecl upt l t rough the siphon is gencrated by ci l ia on thc gi l ls in thc shel l  cavi ty.  Part ic les areÍlltered frou the water curreltt, also by the gills, and tlren transported to the so-cal led palps. On the palps. l ike the gi l ls covered with ci l ia.  the par-t ic lcs arc sortecl .aÍter which thc ediblc Íl 'action is ingcstecl by thc mouth and thc rest is excreted aspsetrdoÍàcces. Strong diÍ1èrences erist betr.veen populations in the ratio betwecn thegi l ls (col lcct ing) ancl  the palps (sort ing).  In the reared Muc'onru hulthi t 'n the di f--fcrences betwcerr populations turned out to be heritable and thercÍbre genetic. Anadditional experitnent den-tonstrated that thc ratio betrveen gills and palps changeswith cl i f tèrent seci iment types. ln course sand the,ei l ls ale larqe ancl the palps srral l .and this is revcrsecl in si l t .  This is probably rclated to thc antount of part ic les thateltters at a particular pr,trnping capacity. A volunre of sancl contains Íar Í-cwcr parti-c les t l iat t  the sanre tolut .uc of s i l t  and therefbre sort in-e out a volume is muclr  lesswork i f  i t  consists of sancl than i f  i t  contains a lot  of  s i l t .  In contrast to the di Í ïe-rel tces betweclt  populat ions. c l i f Ïèrent gi l l /palp rat ios within populat ions arc to alar'-ee ertent flexiblc and lvill respond to the local circur.nstances.tf Baltic clartrs in the nortlt and in the south are both adapted to thcir local cir-cltr.nstances. thc sot-tthern bivalves should be bettcr able to cope u,ith Itigh tcrrpela-tures. This rvas studied by Lrtil izing thc lact that proteins denatule when exposed tohighert l tan opt imal tcr.npet 'atures. I f  this happens inside a cel l  a spccial  type of pro-teins. the hcat shocl< proteins (HSPs). is synthcsized to ci ther repai l  the danrage orre trdcr the danagcd proteius harurle ss and deport thern. The concentration of HSPsis thereforc a lleasure of tentperature stress. A comparison betwcen Íbr.rr groups oflaboratoly reared Baltic clanrs. fi 'our Norlvay, the North Sea, thc Wadclen Sea anclFrattce. shou,cd tltat po;lulations do difÈr in terrlleratllre tolcrance bLrt also thatthese cliÍJèreltces \\'ere not casy to cor"r;llc to tlie original environment. Interestingly.indivic lLrals stntr l lcr  than 5 mur had much higher levels of HSPs than larger ones.This diÍÏèrencc nray be ascribed to telrperatr-u'e diÍÈreuces bctween the placeswhere Balt ic c lans of di Í ïerent sizes tcncl  to be located. Smal ler individuals are bur-rowed closc to 1hc sediment surfacc. while larger ones are burrowed ntore cleeply.Close to the surfàce telxperatllres Í-luctuate nrore and will also reach lti_shcr levels.especially wlrcn a ntuclÍ' lat is exposed around noon. Moreover, small Baltic clarnsare on avera-qc locatecl Irieher up in the tidal zone than larger ones. wltich scrvesrnainly to escape prcdatot's like crabs and shrinp. Thesc places are subnter-{ed lesslong and can thercÍore heat up more dr"rrin-e low ticle than the lowcr ticlal zone. Itwould thcrcfore appear tltat as a Baltic clarl grows, the tcmperature of its sun'olurd-ings f luctuates less and also reaches lor. ler maxima. I t  wi l l  then necd HSPs lessoÍlen to react to proteiu dama-{e caused by hi-uh terrperatures and ther-eÍbrc haveless of thcse proteins in stock.t 7 lt72 Stt t t t t t t t t t ' t 'After  Íèr t i l izat ion of  the eg-9s.  larvac devclop.  The larvae remain in  the vvatercolutnu Íbr a l 'cw lr,eeks. This is a tinre that is charactcrized by high nrurtality ratesand could be a l iniit ing fàctor Íor thc occurrcr.rcc oÍ'Baltic clams. Thc cl}èct of tem-peratl lre on tltc clcvclopmcnt of larvae of Baltic clanrs Íl 'cln.r the Balsljorcl iu Norwayaud fron-t the Ciironde in Francc r,vas studied in a rcalinq experintent in the labora-tory. Larvae of both populations -9rcr.r,and dcvclopccl f irster at high tentperatures butalso clisplayccl a hi-qher nortality latc. Frcnch larvae developccl fhster thanNorwegiitn larvae. Rernarkably. thc Nonvcgian lalvae sti l l  developecl successfullyat  a water  tcr lperature of  20 'C' .  This  is  approximatc ly  l5 'C h igher  than normal  inthe Balsf-1orcl! Lalvac clearly tolcrate a wide range of tentperatures.All three cx;lcl ir.nents lvit lr rcared Baltic clanrs uncovered -eenctically deter-ntined clifÍèrcnces betu,een populations. but thc nrost ltotable observatiorr rvas thatin  a l l  cases thc indi l ' iduals u 'erc capable of  rcs l - roncl ing ef tèct i r . 'e ly  to  var iable c i r -cun-rstances. The Baltic clanr is -gcnctically aclalrtccl to local circumstances. but.fbremost .  reacts pheuotypical ly  p last ic  to  val ia t ions in  i ts  sun'or-urc l ings.The eÍ1'ccts of temperature on the Baltic clam wcrc studied furthcr among indi-v iduals Í l 'ont  thc Wadden Sea usiu-9 l ie ld observat ions and laborato ly  cxpcl i r rents.Temperature has a strolrs inl ' lLrcncc on bioloqical 1-rrocesses. ln colcl-blooded ani-nrals l ike i l ' lut 'orttu hultlt ic'u elrcfgy clcnrand incrcascs steeply as it beconres \\ 'anrer.E,arlier research had already pointccl ont that a hisher energy clemancl in rri ld u'in-ters reduces thc uumber of proclLrcecl eggs courparccl to cold winters. In a starvatiouexperiment. subjccting Mut'ottru hulthicu to diÍïèrcnt terxperatl lres. thc Baltic clanswere Íbuncl to bc capable of allocating their energy effectively tou,arcls synthesis ofeggs even at u,inter tenrpelatrrlcs rbor e a\ erage.Besides har ing an inÍ' lucncc on process rates. tcurperature nray also le_uulatethe breeding scasou. Baltic clanrs spa\v11 after the \\,ater temperature cxcccds a cer-ta in thresholc l .  and i t  is  assunrcc l  that  th is  represents a d i rect  react iorr .  I la l t ic  c larrsthat  l ive in  t ic la l  Í la ts  woulc l  thcrr  possib ly  spawn car l ier  than thei r  consl - rcc i Í ics indeeper water. bccausc at low ticle the sun warms thc Ílats and nruch lcss so thc bot-tot r  of  thc gul l ies.  Whi le t rack ing the reproduct ivc s tatus of  Bal t ic  c lants no d i f fer-ence it.r t irring u,as obsen'ed bctu'een the tidal Í ' lats arrd the gull ics. It r,vas hotvet'eruoted that the timc durirrg u'hich spau'nirr_e takes place is long. at least tu'o nonths.IndividLrals spawn scveral t iurcs cluring this periocl. We suppose that in this u'ay thcyenlarge their chauccs to proclucc oÍl.spring at a l irvolrlable time, a l<incl of bet-hed-ging.To get an idca of thc actual temperature cliÍï 'crences betu,een populations. acot.nparison u,as utade between lleld sites alorrg thc European coasÍ. Baltic claursrvere collcctccl at a lar-ee nunrbcl of sites and tlrc gro\\rth ratc cstiutated from thegrorvth rit 'rgs on thc shells. Thc cxpectation lvas that I lalt ic claurs clf ntorc norther-ly popLrlatiolts grow more slor,vly cll le to the lower tcmperatures. The obscrved pat-tem, however', is not uncquivocal. Thrs points to tlre importance of otltcr Íàctolssuch as Íbocl  avai labi l i ty .  01 ' thc th lee populat ions BalsÍ jord.  Waclc lcn Sea andSummaryGironde the seasonal cycle of body mass and gonad development was estimated.From this, an energy budget could be reconstructed and it appeared that Balticclams from the Balsflord devote more energy to reproduction than the moresoutherly clams from the Wadden Sea and the Gironde. This can mainly be ascribedto the lower temperature, which leaves more energy for body maintenance.Furthermore, it could be deduced that food availability differs between the popula-tions.The results of the separate parts of this study all point to a high level of tole-rance to differences in environmental variables. At the same time genetic adapta-tions to local circumstances exist. It seems that genetic adaptations exist for certainconsistent differences between populations, while superimposed on this, flexibletraits enable the individuals to respond to their variable environment.Do these results imply that the Baltic clam is safeguarded against changes in itsenvironment due to its phenotypic plasticity? The answer is no. Long-term researchof the Royal Netherlands Institute for Sea Research at Íhe Balgzand and other areasin the western Wadden Sea shows that since the end of the nineteen eighties the pop-ulation of Macoma balthica is declining. While average density used to be around100 adults per square meter, now it is merely 10 per square meter. The causes of thisdecline are still uncertain. On the basis of the results in this thesis it is not probablethat direct effects of climate on the physiology of the Baltic clam are responsible.Indirect effects through interactions with other species that respond differently toclimate or other environmental factors are more likely. For example, the spatfall ofBaltic clams may have severely decreased due to predation by shrimp. Shrimp areparticularly numerous on the tidal flats at the time of spatfall after mild winters.However, when conducting research into effects of human-induced changes inecosystems, in this case climate, it must be realised that human influences are var-ied. Pollution, eutrofication and not forgetting fisheries all have undoubtedly influ-enced the current development of Baltic clams and other benthic fauna of theWadden Sea.t73

University of Groningen

   University of GroningenLife history variation of a marine bivalve (Macoma balthica) in a changing worldDrent, JanIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2004Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Drent, J. (2004). Life history variation of a marine bivalve (Macoma balthica) in a changing world.Groningen: s.n.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-2019Summarl,l t  is  beconing inc leasinuly  c lear  that  the u 'or lc l 's  tenrpclature is  i r rcreasing.  ln  t l rcNetlrerlancls. too. tenrperature has beeu on the increase dLrring thc last clecaclcs anclth is  t rencl  is  expectec l  to  cont inue.  These changcs nray substant ia l l l ,  iu tpact  coul lu-n i t ies of  p lants ancl  ani rnals .  Cl inrate has a st ron-e in Íhrcnce on thc c l is t r ibut ion ofspecies.  What  c l ïècts r . r , i l l  a  chaugiug c l imate have or . r  arr  ccological ly  in tpor tantarea l ike thc Wadclen Sea'l The lescarch prcscnted here highlights a part of thisbroader c luest ion.Thc Wadden Sea is  an area of  t ic la l  Í - la ts  in te lsectcc l  by c leepcr  eul l ies.  Thc se-clirrcnt is inhabitccl by lar-ee nunrbcrs of bivalr, 'cs and lvorn-rs. Most l ivc burrowcclin the scclitnent. but sotle s1'rccics. the nrr.rsscl ancl at prescnt l lso thc PaciÍlc oystcr.are attachctl to the substrate ancl can in this way Íbnr largc banks. The benthic Íi iunaconsumes r rost ly  unicel lLr lar  a lgac Í i 'om the water  ancl  Í l -orr  the sedintent  sur Íàce.The bivalvcs and \l,c'rtt.t.ts irr their turn are eaten by lar-uc numbcls o1'bilds ancl l lsh.thus Íbnr ing arr  i rnpor tant  l ink in  the Waclden Sea ccosystcnr .  Wi thout  i ts  bcnth icÍ iu" tna the area would lack i ts  ro le as a nurscry for  Íbr  iustancc p la ice 'o l  l rs  u inte l ingancl stopor"cr arca Íbr r. laders.What  u ' i l l  happcn to the coururuni ty  of  thc t ida l  Í la t  sedinrent  i f  thc c l inratcc l ianges '? To tack le th is  cprest ion.  i t  . " r ,as chosen to lbcus ou one s in-u lc  species.  theBal t ic  c lan Mu<ot t tu ln l t l t icu (L. ) .  The Bal t ic  c lanr  is  a smal l  b i r ,a l r .c  nro l lusc thatri ' i l l  grow to about 25 nrnr. TIrc shells ale 'ul,hitc. pink. yellovn' or oran-ge. The anirrall ives burrou,cd a fèw ccnt inreters in to thc sedinrent .  p l inrar i ly  in  thc t ida l  l la ts .  butit can also bc Íbund ir.r sor.nenhat cleeper vr,atcfs, down to abor"rt 2,5 rrctcrs. TheBal t ic  c larr  sucks up sand and food par t ic les through i ts  long and Í lex ib le s iphon.In the shci l  cav i ty  the animal  sor ts  thc col lectec l  rnater ia l .  in_9csts thc edib le pal t i -c les Íbr  fur ther  c l igcst ion and excrctcs thc rest  as pseucioÍaeces.  Thc Bal t ic  c lanr  isa u,idesprcad and cornuron species that is Íbund Íl 'om the Cii lonclc at thc Freuclrcoast  uear  Bordeaur r " rp to Nclva Zernbla in  the h igh nor th.  For  thc l la l t ic  c lanr  theWaddcn Sea is thcrclt lre a fi i ir lv soLrtlrerly klcation r.r, ithin its _rrco_uraphic clistl ibLr-t ion.  On the basis  of  th is  i t  r ray be expected that  th is  s1 ' rcc ics i le lat ive ly  seusi t iveto u'artrinq of t l ie Wirclclen Sea, lvhich \\ 'as eur irrportant rcasou 1br choosine theBaltic clanr as rcscarch objcct. Morcover, a long rcsearch traclit ion exists Ítrrl, lucortttt hultlt ic'u. as a rcsult ol rvhich prior knou,lecl-9e of its biolo-uy \\ 'as cxtcn-s ivc.  Notably.  a successf i r l  rcar ing techniquc had been dcveloped.  Íac i l i ta t ing bree-ding crperirrclrts ir.r the laboratory.What  happens to the Bal t ic  c lanr  u 'hen the tempcratu le r iscs ' l  Thcrc a le rcal lyonly two possib i l i t ics:  ad-yust  to  the neu,c i rcur .nstanccs or ' lcavc ' lbr  cooler  p lacest10 S' t t  t t t  t t t r t  t ' t 'to thc north, which implies local ertinctior, . Adjr,rstrneuts to thc surroundings opc-ratc at severarl levels. within an inclivicLral or belll 'een individuals. Within ar-r ir-rcli-v idual the capacity to adjust o the euvironment is cal led phcnotypic plast ic i ty.  Thephenotype is the entire indiviclr.ral nd comprises morphological. physiological andbehavioural traits. Phenotypic plasticity can bc diviclecl into phenotypic Ílexibilityancl cleveloprnental p asticity. Phenotypic Ílcribility inrplies reversible changes inbehaviour,  physiology or even morphology: the changes tend to be rapid ar,d cnablcrcsponciing to short tenn alterations of the enviror.rlnent. Devcloprncntal plasticity isthc capacity of an inclividual to ttrhe on a particular phenotype, depending on thcenviror.rr .nent,  dul ing dcvelopment.  The lat ter is l loïe per\asire aud part icular lysuited Íbr respondirrg to environrncntal variation that is relatively srnall during anincliviclual's lifetin-re. The difïèrent Íbru,s of plasticity u'il l cornc into play ciepen-ding on the spat ial  and temporal scales involved.At some point the possibi l i t ies fbr arr  i rTdividLral  to respclnd to changcs r,r ' i l l  bccrhaustecl .  I t  is then possible that at  the populat ion level there are somc individualsthat are stil l capable of coping, while others succumb. If thcsc diÍï-crcnccs arc gcuc-t ical ly deterrnined, the gcnct ic corrst i tut i t rn of the populat ion wi l l  change. Thisprocess is called natural selection ancl takcs place over several -ÍIenerations.Phenotypic plasticity and genetic adaptations compler.nent each other in achanging world.  For the Balt ic c larr  i t  was eran.r ined to what cxtcut phenotypicplasticity and genetic adaptatior-rs arc ilrvolved. ln the most extrelne cases. alladjustments coLrld be either plastic or gcnctic. Molecular r"narkers indicated that atthe European scale genetic diÍï-crcnccs bct'uveen populations are very pronounced.Baltic clarns fi-om the Baltic Seti are even so ertrenrely diÍïèrent that they mi-eht beconsidered to bc a separate species. But other populat ions. too, display clear di Í ïè-rences thal point to a strongly restricted exchauge of individLrals and thus genesbetll,een locations. As a conscqucucc. Baltic cleuls rnay well be c|ftèrentially adap-ted at locations with diÍïcrcnt circurnstances. This result was unexpected because itwas always assnrncd that tsaltic clams eÍÏèctil 'ely tlavel large distances i,vhen theyclriÍi fieely in the water colun-tr as larvae. Unhanrpcrcd larval clispersal lvor"rldguarantcc substantial rnirin-e betw'een populations. inhibiting genetic diÍ1èrenccs{r'orn arising and resr"rlting in the inevitability that adjLrstments to diÍïèrences in envi-ronllelrt carr happen only thror.rgh phcnotypic plasticity.The resnlts of thc moleculerr merrker study indicate that genetic adaptations tocircur.nstanccs spcciÍic to local popr,rlartions are indeed possible. By rearing Balticclams of diÍï-crcnt origins r-urder ider-rtical circurnstances in the laboratory it is pos-sible to detennine to urhat ertent pafticLrlaf traits arc hcritablc (gcnctic). Aclults werecollccted at various localities (Balsfjord. norlhern Norrvay; Wadden Sea and NorthSea, thc Nctherleu-rds: Gironde. France) and indr-rced to spawn in the laboratory. Thclarvae werc rcarcd to the juvenile stage nnder controlled conditions. Any subse-qLrent difÍèrclrccs bctrvcen oflipring of pareuts Íï'om difÍèrent origins are uost like-ly genetically dctcrminecl. Three aspects were str-rdied: Íirst. the Íbraging apparatus,Stt t t t  t t t t  t ' tsccond. scnsit iv i ty of Balt ic c lams to high tcmperatures ancl.  Í inal ly.  srowth auddcvelclprnent clf lalr,ae at diÍfcrent tentperaturcs.Thc u'ater curreltt with which particlcs tl 'onr the seclilnent surfàce are suckecl upt l t rough the siphon is gencrated by ci l ia on thc gi l ls in thc shel l  cavi ty.  Part ic les areÍlltered frou the water curreltt, also by the gills, and tlren transported to the so-cal led palps. On the palps. l ike the gi l ls covered with ci l ia.  the par-t ic lcs arc sortecl .aÍter which thc ediblc Íl 'action is ingcstecl by thc mouth and thc rest is excreted aspsetrdoÍàcces. Strong diÍ1èrences erist betr.veen populations in the ratio betwecn thegi l ls (col lcct ing) ancl  the palps (sort ing).  In the reared Muc'onru hulthi t 'n the di f--fcrences betwcerr populations turned out to be heritable and thercÍbre genetic. Anadditional experitnent den-tonstrated that thc ratio betrveen gills and palps changeswith cl i f tèrent seci iment types. ln course sand the,ei l ls ale larqe ancl the palps srral l .and this is revcrsecl in si l t .  This is probably rclated to thc antount of part ic les thateltters at a particular pr,trnping capacity. A volunre of sancl contains Íar Í-cwcr parti-c les t l iat t  the sanre tolut .uc of s i l t  and therefbre sort in-e out a volume is muclr  lesswork i f  i t  consists of sancl than i f  i t  contains a lot  of  s i l t .  In contrast to the di Í ïe-rel tces betweclt  populat ions. c l i f Ïèrent gi l l /palp rat ios within populat ions arc to alar'-ee ertent flexiblc and lvill respond to the local circur.nstances.tf Baltic clartrs in the nortlt and in the south are both adapted to thcir local cir-cltr.nstances. thc sot-tthern bivalves should be bettcr able to cope u,ith Itigh tcrrpela-tures. This rvas studied by Lrtil izing thc lact that proteins denatule when exposed tohighert l tan opt imal tcr.npet 'atures. I f  this happens inside a cel l  a spccial  type of pro-teins. the hcat shocl< proteins (HSPs). is synthcsized to ci ther repai l  the danrage orre trdcr the danagcd proteius harurle ss and deport thern. The concentration of HSPsis thereforc a lleasure of tentperature stress. A comparison betwcen Íbr.rr groups oflaboratoly reared Baltic clanrs. fi 'our Norlvay, the North Sea, thc Wadclen Sea anclFrattce. shou,cd tltat po;lulations do difÈr in terrlleratllre tolcrance bLrt also thatthese cliÍJèreltces \\'ere not casy to cor"r;llc to tlie original environment. Interestingly.indivic lLrals stntr l lcr  than 5 mur had much higher levels of HSPs than larger ones.This diÍÏèrencc nray be ascribed to telrperatr-u'e diÍÈreuces bctween the placeswhere Balt ic c lans of di Í ïerent sizes tcncl  to be located. Smal ler individuals are bur-rowed closc to 1hc sediment surfacc. while larger ones are burrowed ntore cleeply.Close to the surfàce telxperatllres Í-luctuate nrore and will also reach lti_shcr levels.especially wlrcn a ntuclÍ' lat is exposed around noon. Moreover, small Baltic clarnsare on avera-qc locatecl Irieher up in the tidal zone than larger ones. wltich scrvesrnainly to escape prcdatot's like crabs and shrinp. Thesc places are subnter-{ed lesslong and can thercÍore heat up more dr"rrin-e low ticle than the lowcr ticlal zone. Itwould thcrcfore appear tltat as a Baltic clarl grows, the tcmperature of its sun'olurd-ings f luctuates less and also reaches lor. ler maxima. I t  wi l l  then necd HSPs lessoÍlen to react to proteiu dama-{e caused by hi-uh terrperatures and ther-eÍbrc haveless of thcse proteins in stock.t 7 lt72 Stt t t t t t t t t t ' t 'After  Íèr t i l izat ion of  the eg-9s.  larvac devclop.  The larvae remain in  the vvatercolutnu Íbr a l 'cw lr,eeks. This is a tinre that is charactcrized by high nrurtality ratesand could be a l iniit ing fàctor Íor thc occurrcr.rcc oÍ'Baltic clams. Thc cl}èct of tem-peratl lre on tltc clcvclopmcnt of larvae of Baltic clanrs Íl 'cln.r the Balsljorcl iu Norwayaud fron-t the Ciironde in Francc r,vas studied in a rcalinq experintent in the labora-tory. Larvae of both populations -9rcr.r,and dcvclopccl f irster at high tentperatures butalso clisplayccl a hi-qher nortality latc. Frcnch larvae developccl fhster thanNorwegiitn larvae. Rernarkably. thc Nonvcgian lalvae sti l l  developecl successfullyat  a water  tcr lperature of  20 'C' .  This  is  approximatc ly  l5 'C h igher  than normal  inthe Balsf-1orcl! Lalvac clearly tolcrate a wide range of tentperatures.All three cx;lcl ir.nents lvit lr rcared Baltic clanrs uncovered -eenctically deter-ntined clifÍèrcnces betu,een populations. but thc nrost ltotable observatiorr rvas thatin  a l l  cases thc indi l ' iduals u 'erc capable of  rcs l - roncl ing ef tèct i r . 'e ly  to  var iable c i r -cun-rstances. The Baltic clanr is -gcnctically aclalrtccl to local circumstances. but.fbremost .  reacts pheuotypical ly  p last ic  to  val ia t ions in  i ts  sun'or-urc l ings.The eÍ1'ccts of temperature on the Baltic clam wcrc studied furthcr among indi-v iduals Í l 'ont  thc Wadden Sea usiu-9 l ie ld observat ions and laborato ly  cxpcl i r rents.Temperature has a strolrs inl ' lLrcncc on bioloqical 1-rrocesses. ln colcl-blooded ani-nrals l ike i l ' lut 'orttu hultlt ic'u elrcfgy clcnrand incrcascs steeply as it beconres \\ 'anrer.E,arlier research had already pointccl ont that a hisher energy clemancl in rri ld u'in-ters reduces thc uumber of proclLrcecl eggs courparccl to cold winters. In a starvatiouexperiment. subjccting Mut'ottru hulthicu to diÍïèrcnt terxperatl lres. thc Baltic clanswere Íbuncl to bc capable of allocating their energy effectively tou,arcls synthesis ofeggs even at u,inter tenrpelatrrlcs rbor e a\ erage.Besides har ing an inÍ' lucncc on process rates. tcurperature nray also le_uulatethe breeding scasou. Baltic clanrs spa\v11 after the \\,ater temperature cxcccds a cer-ta in thresholc l .  and i t  is  assunrcc l  that  th is  represents a d i rect  react iorr .  I la l t ic  c larrsthat  l ive in  t ic la l  Í la ts  woulc l  thcrr  possib ly  spawn car l ier  than thei r  consl - rcc i Í ics indeeper water. bccausc at low ticle the sun warms thc Ílats and nruch lcss so thc bot-tot r  of  thc gul l ies.  Whi le t rack ing the reproduct ivc s tatus of  Bal t ic  c lants no d i f fer-ence it.r t irring u,as obsen'ed bctu'een the tidal Í ' lats arrd the gull ics. It r,vas hotvet'eruoted that the timc durirrg u'hich spau'nirr_e takes place is long. at least tu'o nonths.IndividLrals spawn scveral t iurcs cluring this periocl. We suppose that in this u'ay thcyenlarge their chauccs to proclucc oÍl.spring at a l irvolrlable time, a l<incl of bet-hed-ging.To get an idca of thc actual temperature cliÍï 'crences betu,een populations. acot.nparison u,as utade between lleld sites alorrg thc European coasÍ. Baltic claursrvere collcctccl at a lar-ee nunrbcl of sites and tlrc gro\\rth ratc cstiutated from thegrorvth rit 'rgs on thc shells. Thc cxpectation lvas that I lalt ic claurs clf ntorc norther-ly popLrlatiolts grow more slor,vly cll le to the lower tcmperatures. The obscrved pat-tem, however', is not uncquivocal. Thrs points to tlre importance of otltcr Íàctolssuch as Íbocl  avai labi l i ty .  01 ' thc th lee populat ions BalsÍ jord.  Waclc lcn Sea andSummaryGironde the seasonal cycle of body mass and gonad development was estimated.From this, an energy budget could be reconstructed and it appeared that Balticclams from the Balsflord devote more energy to reproduction than the moresoutherly clams from the Wadden Sea and the Gironde. This can mainly be ascribedto the lower temperature, which leaves more energy for body maintenance.Furthermore, it could be deduced that food availability differs between the popula-tions.The results of the separate parts of this study all point to a high level of tole-rance to differences in environmental variables. At the same time genetic adapta-tions to local circumstances exist. It seems that genetic adaptations exist for certainconsistent differences between populations, while superimposed on this, flexibletraits enable the individuals to respond to their variable environment.Do these results imply that the Baltic clam is safeguarded against changes in itsenvironment due to its phenotypic plasticity? The answer is no. Long-term researchof the Royal Netherlands Institute for Sea Research at Íhe Balgzand and other areasin the western Wadden Sea shows that since the end of the nineteen eighties the pop-ulation of Macoma balthica is declining. While average density used to be around100 adults per square meter, now it is merely 10 per square meter. The causes of thisdecline are still uncertain. On the basis of the results in this thesis it is not probablethat direct effects of climate on the physiology of the Baltic clam are responsible.Indirect effects through interactions with other species that respond differently toclimate or other environmental factors are more likely. For example, the spatfall ofBaltic clams may have severely decreased due to predation by shrimp. Shrimp areparticularly numerous on the tidal flats at the time of spatfall after mild winters.However, when conducting research into effects of human-induced changes inecosystems, in this case climate, it must be realised that human influences are var-ied. Pollution, eutrofication and not forgetting fisheries all have undoubtedly influ-enced the current development of Baltic clams and other benthic fauna of theWadden Sea.t73

   University of GroningenLife history variation of a marine bivalve (Macoma balthica) in a changing worldDrent, JanIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2004Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Drent, J. (2004). Life history variation of a marine bivalve (Macoma balthica) in a changing world. s.n.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 02-06-2022Summarl,l t  is  beconing inc leasinuly  c lear  that  the u 'or lc l 's  tenrpclature is  i r rcreasing.  ln  t l rcNetlrerlancls. too. tenrperature has beeu on the increase dLrring thc last clecaclcs anclth is  t rencl  is  expectec l  to  cont inue.  These changcs nray substant ia l l l ,  iu tpact  coul lu-n i t ies of  p lants ancl  ani rnals .  Cl inrate has a st ron-e in Íhrcnce on thc c l is t r ibut ion ofspecies.  What  c l ïècts r . r , i l l  a  chaugiug c l imate have or . r  arr  ccological ly  in tpor tantarea l ike thc Wadclen Sea'l The lescarch prcscnted here highlights a part of thisbroader c luest ion.Thc Wadden Sea is  an area of  t ic la l  Í - la ts  in te lsectcc l  by c leepcr  eul l ies.  Thc se-clirrcnt is inhabitccl by lar-ee nunrbcrs of bivalr, 'cs and lvorn-rs. Most l ivc burrowcclin the scclitnent. but sotle s1'rccics. the nrr.rsscl ancl at prescnt l lso thc PaciÍlc oystcr.are attachctl to the substrate ancl can in this way Íbnr largc banks. The benthic Íi iunaconsumes r rost ly  unicel lLr lar  a lgac Í i 'om the water  ancl  Í l -orr  the sedintent  sur Íàce.The bivalvcs and \l,c'rtt.t.ts irr their turn are eaten by lar-uc numbcls o1'bilds ancl l lsh.thus Íbnr ing arr  i rnpor tant  l ink in  the Waclden Sea ccosystcnr .  Wi thout  i ts  bcnth icÍ iu" tna the area would lack i ts  ro le as a nurscry for  Íbr  iustancc p la ice 'o l  l rs  u inte l ingancl stopor"cr arca Íbr r. laders.What  u ' i l l  happcn to the coururuni ty  of  thc t ida l  Í la t  sedinrent  i f  thc c l inratcc l ianges '? To tack le th is  cprest ion.  i t  . " r ,as chosen to lbcus ou one s in-u lc  species.  theBal t ic  c lan Mu<ot t tu ln l t l t icu (L. ) .  The Bal t ic  c lanr  is  a smal l  b i r ,a l r .c  nro l lusc thatri ' i l l  grow to about 25 nrnr. TIrc shells ale 'ul,hitc. pink. yellovn' or oran-ge. The anirrall ives burrou,cd a fèw ccnt inreters in to thc sedinrent .  p l inrar i ly  in  thc t ida l  l la ts .  butit can also bc Íbund ir.r sor.nenhat cleeper vr,atcfs, down to abor"rt 2,5 rrctcrs. TheBal t ic  c larr  sucks up sand and food par t ic les through i ts  long and Í lex ib le s iphon.In the shci l  cav i ty  the animal  sor ts  thc col lectec l  rnater ia l .  in_9csts thc edib le pal t i -c les Íbr  fur ther  c l igcst ion and excrctcs thc rest  as pseucioÍaeces.  Thc Bal t ic  c lanr  isa u,idesprcad and cornuron species that is Íbund Íl 'om the Cii lonclc at thc Freuclrcoast  uear  Bordeaur r " rp to Nclva Zernbla in  the h igh nor th.  For  thc l la l t ic  c lanr  theWaddcn Sea is thcrclt lre a fi i ir lv soLrtlrerly klcation r.r, ithin its _rrco_uraphic clistl ibLr-t ion.  On the basis  of  th is  i t  r ray be expected that  th is  s1 ' rcc ics i le lat ive ly  seusi t iveto u'artrinq of t l ie Wirclclen Sea, lvhich \\ 'as eur irrportant rcasou 1br choosine theBaltic clanr as rcscarch objcct. Morcover, a long rcsearch traclit ion exists Ítrrl, lucortttt hultlt ic'u. as a rcsult ol rvhich prior knou,lecl-9e of its biolo-uy \\ 'as cxtcn-s ivc.  Notably.  a successf i r l  rcar ing techniquc had been dcveloped.  Íac i l i ta t ing bree-ding crperirrclrts ir.r the laboratory.What  happens to the Bal t ic  c lanr  u 'hen the tempcratu le r iscs ' l  Thcrc a le rcal lyonly two possib i l i t ics:  ad-yust  to  the neu,c i rcur .nstanccs or ' lcavc ' lbr  cooler  p lacest10 S' t t  t t t  t t t r t  t ' t 'to thc north, which implies local ertinctior, . Adjr,rstrneuts to thc surroundings opc-ratc at severarl levels. within an inclivicLral or belll 'een individuals. Within ar-r ir-rcli-v idual the capacity to adjust o the euvironment is cal led phcnotypic plast ic i ty.  Thephenotype is the entire indiviclr.ral nd comprises morphological. physiological andbehavioural traits. Phenotypic plasticity can bc diviclecl into phenotypic Ílexibilityancl cleveloprnental p asticity. Phenotypic Ílcribility inrplies reversible changes inbehaviour,  physiology or even morphology: the changes tend to be rapid ar,d cnablcrcsponciing to short tenn alterations of the enviror.rlnent. Devcloprncntal plasticity isthc capacity of an inclividual to ttrhe on a particular phenotype, depending on thcenviror.rr .nent,  dul ing dcvelopment.  The lat ter is l loïe per\asire aud part icular lysuited Íbr respondirrg to environrncntal variation that is relatively srnall during anincliviclual's lifetin-re. The difïèrent Íbru,s of plasticity u'il l cornc into play ciepen-ding on the spat ial  and temporal scales involved.At some point the possibi l i t ies fbr arr  i rTdividLral  to respclnd to changcs r,r ' i l l  bccrhaustecl .  I t  is then possible that at  the populat ion level there are somc individualsthat are stil l capable of coping, while others succumb. If thcsc diÍï-crcnccs arc gcuc-t ical ly deterrnined, the gcnct ic corrst i tut i t rn of the populat ion wi l l  change. Thisprocess is called natural selection ancl takcs place over several -ÍIenerations.Phenotypic plasticity and genetic adaptations compler.nent each other in achanging world.  For the Balt ic c larr  i t  was eran.r ined to what cxtcut phenotypicplasticity and genetic adaptatior-rs arc ilrvolved. ln the most extrelne cases. alladjustments coLrld be either plastic or gcnctic. Molecular r"narkers indicated that atthe European scale genetic diÍï-crcnccs bct'uveen populations are very pronounced.Baltic clarns fi-om the Baltic Seti are even so ertrenrely diÍïèrent that they mi-eht beconsidered to bc a separate species. But other populat ions. too, display clear di Í ïè-rences thal point to a strongly restricted exchauge of individLrals and thus genesbetll,een locations. As a conscqucucc. Baltic cleuls rnay well be c|ftèrentially adap-ted at locations with diÍïcrcnt circurnstances. This result was unexpected because itwas always assnrncd that tsaltic clams eÍÏèctil 'ely tlavel large distances i,vhen theyclriÍi fieely in the water colun-tr as larvae. Unhanrpcrcd larval clispersal lvor"rldguarantcc substantial rnirin-e betw'een populations. inhibiting genetic diÍ1èrenccs{r'orn arising and resr"rlting in the inevitability that adjLrstments to diÍïèrences in envi-ronllelrt carr happen only thror.rgh phcnotypic plasticity.The resnlts of thc moleculerr merrker study indicate that genetic adaptations tocircur.nstanccs spcciÍic to local popr,rlartions are indeed possible. By rearing Balticclams of diÍï-crcnt origins r-urder ider-rtical circurnstances in the laboratory it is pos-sible to detennine to urhat ertent pafticLrlaf traits arc hcritablc (gcnctic). Aclults werecollccted at various localities (Balsfjord. norlhern Norrvay; Wadden Sea and NorthSea, thc Nctherleu-rds: Gironde. France) and indr-rced to spawn in the laboratory. Thclarvae werc rcarcd to the juvenile stage nnder controlled conditions. Any subse-qLrent difÍèrclrccs bctrvcen oflipring of pareuts Íï'om difÍèrent origins are uost like-ly genetically dctcrminecl. Three aspects were str-rdied: Íirst. the Íbraging apparatus,Stt t t t  t t t t  t ' tsccond. scnsit iv i ty of Balt ic c lams to high tcmperatures ancl.  Í inal ly.  srowth auddcvelclprnent clf lalr,ae at diÍfcrent tentperaturcs.Thc u'ater curreltt with which particlcs tl 'onr the seclilnent surfàce are suckecl upt l t rough the siphon is gencrated by ci l ia on thc gi l ls in thc shel l  cavi ty.  Part ic les areÍlltered frou the water curreltt, also by the gills, and tlren transported to the so-cal led palps. On the palps. l ike the gi l ls covered with ci l ia.  the par-t ic lcs arc sortecl .aÍter which thc ediblc Íl 'action is ingcstecl by thc mouth and thc rest is excreted aspsetrdoÍàcces. Strong diÍ1èrences erist betr.veen populations in the ratio betwecn thegi l ls (col lcct ing) ancl  the palps (sort ing).  In the reared Muc'onru hulthi t 'n the di f--fcrences betwcerr populations turned out to be heritable and thercÍbre genetic. Anadditional experitnent den-tonstrated that thc ratio betrveen gills and palps changeswith cl i f tèrent seci iment types. ln course sand the,ei l ls ale larqe ancl the palps srral l .and this is revcrsecl in si l t .  This is probably rclated to thc antount of part ic les thateltters at a particular pr,trnping capacity. A volunre of sancl contains Íar Í-cwcr parti-c les t l iat t  the sanre tolut .uc of s i l t  and therefbre sort in-e out a volume is muclr  lesswork i f  i t  consists of sancl than i f  i t  contains a lot  of  s i l t .  In contrast to the di Í ïe-rel tces betweclt  populat ions. c l i f Ïèrent gi l l /palp rat ios within populat ions arc to alar'-ee ertent flexiblc and lvill respond to the local circur.nstances.tf Baltic clartrs in the nortlt and in the south are both adapted to thcir local cir-cltr.nstances. thc sot-tthern bivalves should be bettcr able to cope u,ith Itigh tcrrpela-tures. This rvas studied by Lrtil izing thc lact that proteins denatule when exposed tohighert l tan opt imal tcr.npet 'atures. I f  this happens inside a cel l  a spccial  type of pro-teins. the hcat shocl< proteins (HSPs). is synthcsized to ci ther repai l  the danrage orre trdcr the danagcd proteius harurle ss and deport thern. The concentration of HSPsis thereforc a lleasure of tentperature stress. A comparison betwcen Íbr.rr groups oflaboratoly reared Baltic clanrs. fi 'our Norlvay, the North Sea, thc Wadclen Sea anclFrattce. shou,cd tltat po;lulations do difÈr in terrlleratllre tolcrance bLrt also thatthese cliÍJèreltces \\'ere not casy to cor"r;llc to tlie original environment. Interestingly.indivic lLrals stntr l lcr  than 5 mur had much higher levels of HSPs than larger ones.This diÍÏèrencc nray be ascribed to telrperatr-u'e diÍÈreuces bctween the placeswhere Balt ic c lans of di Í ïerent sizes tcncl  to be located. Smal ler individuals are bur-rowed closc to 1hc sediment surfacc. while larger ones are burrowed ntore cleeply.Close to the surfàce telxperatllres Í-luctuate nrore and will also reach lti_shcr levels.especially wlrcn a ntuclÍ' lat is exposed around noon. Moreover, small Baltic clarnsare on avera-qc locatecl Irieher up in the tidal zone than larger ones. wltich scrvesrnainly to escape prcdatot's like crabs and shrinp. Thesc places are subnter-{ed lesslong and can thercÍore heat up more dr"rrin-e low ticle than the lowcr ticlal zone. Itwould thcrcfore appear tltat as a Baltic clarl grows, the tcmperature of its sun'olurd-ings f luctuates less and also reaches lor. ler maxima. I t  wi l l  then necd HSPs lessoÍlen to react to proteiu dama-{e caused by hi-uh terrperatures and ther-eÍbrc haveless of thcse proteins in stock.t 7 lt72 Stt t t t t t t t t t ' t 'After  Íèr t i l izat ion of  the eg-9s.  larvac devclop.  The larvae remain in  the vvatercolutnu Íbr a l 'cw lr,eeks. This is a tinre that is charactcrized by high nrurtality ratesand could be a l iniit ing fàctor Íor thc occurrcr.rcc oÍ'Baltic clams. Thc cl}èct of tem-peratl lre on tltc clcvclopmcnt of larvae of Baltic clanrs Íl 'cln.r the Balsljorcl iu Norwayaud fron-t the Ciironde in Francc r,vas studied in a rcalinq experintent in the labora-tory. Larvae of both populations -9rcr.r,and dcvclopccl f irster at high tentperatures butalso clisplayccl a hi-qher nortality latc. Frcnch larvae developccl fhster thanNorwegiitn larvae. Rernarkably. thc Nonvcgian lalvae sti l l  developecl successfullyat  a water  tcr lperature of  20 'C' .  This  is  approximatc ly  l5 'C h igher  than normal  inthe Balsf-1orcl! Lalvac clearly tolcrate a wide range of tentperatures.All three cx;lcl ir.nents lvit lr rcared Baltic clanrs uncovered -eenctically deter-ntined clifÍèrcnces betu,een populations. but thc nrost ltotable observatiorr rvas thatin  a l l  cases thc indi l ' iduals u 'erc capable of  rcs l - roncl ing ef tèct i r . 'e ly  to  var iable c i r -cun-rstances. The Baltic clanr is -gcnctically aclalrtccl to local circumstances. but.fbremost .  reacts pheuotypical ly  p last ic  to  val ia t ions in  i ts  sun'or-urc l ings.The eÍ1'ccts of temperature on the Baltic clam wcrc studied furthcr among indi-v iduals Í l 'ont  thc Wadden Sea usiu-9 l ie ld observat ions and laborato ly  cxpcl i r rents.Temperature has a strolrs inl ' lLrcncc on bioloqical 1-rrocesses. ln colcl-blooded ani-nrals l ike i l ' lut 'orttu hultlt ic'u elrcfgy clcnrand incrcascs steeply as it beconres \\ 'anrer.E,arlier research had already pointccl ont that a hisher energy clemancl in rri ld u'in-ters reduces thc uumber of proclLrcecl eggs courparccl to cold winters. In a starvatiouexperiment. subjccting Mut'ottru hulthicu to diÍïèrcnt terxperatl lres. thc Baltic clanswere Íbuncl to bc capable of allocating their energy effectively tou,arcls synthesis ofeggs even at u,inter tenrpelatrrlcs rbor e a\ erage.Besides har ing an inÍ' lucncc on process rates. tcurperature nray also le_uulatethe breeding scasou. Baltic clanrs spa\v11 after the \\,ater temperature cxcccds a cer-ta in thresholc l .  and i t  is  assunrcc l  that  th is  represents a d i rect  react iorr .  I la l t ic  c larrsthat  l ive in  t ic la l  Í la ts  woulc l  thcrr  possib ly  spawn car l ier  than thei r  consl - rcc i Í ics indeeper water. bccausc at low ticle the sun warms thc Ílats and nruch lcss so thc bot-tot r  of  thc gul l ies.  Whi le t rack ing the reproduct ivc s tatus of  Bal t ic  c lants no d i f fer-ence it.r t irring u,as obsen'ed bctu'een the tidal Í ' lats arrd the gull ics. It r,vas hotvet'eruoted that the timc durirrg u'hich spau'nirr_e takes place is long. at least tu'o nonths.IndividLrals spawn scveral t iurcs cluring this periocl. We suppose that in this u'ay thcyenlarge their chauccs to proclucc oÍl.spring at a l irvolrlable time, a l<incl of bet-hed-ging.To get an idca of thc actual temperature cliÍï 'crences betu,een populations. acot.nparison u,as utade between lleld sites alorrg thc European coasÍ. Baltic claursrvere collcctccl at a lar-ee nunrbcl of sites and tlrc gro\\rth ratc cstiutated from thegrorvth rit 'rgs on thc shells. Thc cxpectation lvas that I lalt ic claurs clf ntorc norther-ly popLrlatiolts grow more slor,vly cll le to the lower tcmperatures. The obscrved pat-tem, however', is not uncquivocal. Thrs points to tlre importance of otltcr Íàctolssuch as Íbocl  avai labi l i ty .  01 ' thc th lee populat ions BalsÍ jord.  Waclc lcn Sea andSummaryGironde the seasonal cycle of body mass and gonad development was estimated.From this, an energy budget could be reconstructed and it appeared that Balticclams from the Balsflord devote more energy to reproduction than the moresoutherly clams from the Wadden Sea and the Gironde. This can mainly be ascribedto the lower temperature, which leaves more energy for body maintenance.Furthermore, it could be deduced that food availability differs between the popula-tions.The results of the separate parts of this study all point to a high level of tole-rance to differences in environmental variables. At the same time genetic adapta-tions to local circumstances exist. It seems that genetic adaptations exist for certainconsistent differences between populations, while superimposed on this, flexibletraits enable the individuals to respond to their variable environment.Do these results imply that the Baltic clam is safeguarded against changes in itsenvironment due to its phenotypic plasticity? The answer is no. Long-term researchof the Royal Netherlands Institute for Sea Research at Íhe Balgzand and other areasin the western Wadden Sea shows that since the end of the nineteen eighties the pop-ulation of Macoma balthica is declining. While average density used to be around100 adults per square meter, now it is merely 10 per square meter. The causes of thisdecline are still uncertain. On the basis of the results in this thesis it is not probablethat direct effects of climate on the physiology of the Baltic clam are responsible.Indirect effects through interactions with other species that respond differently toclimate or other environmental factors are more likely. For example, the spatfall ofBaltic clams may have severely decreased due to predation by shrimp. Shrimp areparticularly numerous on the tidal flats at the time of spatfall after mild winters.However, when conducting research into effects of human-induced changes inecosystems, in this case climate, it must be realised that human influences are var-ied. Pollution, eutrofication and not forgetting fisheries all have undoubtedly influ-enced the current development of Baltic clams and other benthic fauna of theWadden Sea.t73

   University of GroningenLife history variation of a marine bivalve (Macoma balthica) in a changing worldDrent, JanIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2004Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Drent, J. (2004). Life history variation of a marine bivalve (Macoma balthica) in a changing world. [Thesisfully internal (DIV), University of Groningen]. s.n.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 05-12-2023Summarl,l t  is  beconing inc leasinuly  c lear  that  the u 'or lc l 's  tenrpclature is  i r rcreasing.  ln  t l rcNetlrerlancls. too. tenrperature has beeu on the increase dLrring thc last clecaclcs anclth is  t rencl  is  expectec l  to  cont inue.  These changcs nray substant ia l l l ,  iu tpact  coul lu-n i t ies of  p lants ancl  ani rnals .  Cl inrate has a st ron-e in Íhrcnce on thc c l is t r ibut ion ofspecies.  What  c l ïècts r . r , i l l  a  chaugiug c l imate have or . r  arr  ccological ly  in tpor tantarea l ike thc Wadclen Sea'l The lescarch prcscnted here highlights a part of thisbroader c luest ion.Thc Wadden Sea is  an area of  t ic la l  Í - la ts  in te lsectcc l  by c leepcr  eul l ies.  Thc se-clirrcnt is inhabitccl by lar-ee nunrbcrs of bivalr, 'cs and lvorn-rs. Most l ivc burrowcclin the scclitnent. but sotle s1'rccics. the nrr.rsscl ancl at prescnt l lso thc PaciÍlc oystcr.are attachctl to the substrate ancl can in this way Íbnr largc banks. The benthic Íi iunaconsumes r rost ly  unicel lLr lar  a lgac Í i 'om the water  ancl  Í l -orr  the sedintent  sur Íàce.The bivalvcs and \l,c'rtt.t.ts irr their turn are eaten by lar-uc numbcls o1'bilds ancl l lsh.thus Íbnr ing arr  i rnpor tant  l ink in  the Waclden Sea ccosystcnr .  Wi thout  i ts  bcnth icÍ iu" tna the area would lack i ts  ro le as a nurscry for  Íbr  iustancc p la ice 'o l  l rs  u inte l ingancl stopor"cr arca Íbr r. laders.What  u ' i l l  happcn to the coururuni ty  of  thc t ida l  Í la t  sedinrent  i f  thc c l inratcc l ianges '? To tack le th is  cprest ion.  i t  . " r ,as chosen to lbcus ou one s in-u lc  species.  theBal t ic  c lan Mu<ot t tu ln l t l t icu (L. ) .  The Bal t ic  c lanr  is  a smal l  b i r ,a l r .c  nro l lusc thatri ' i l l  grow to about 25 nrnr. TIrc shells ale 'ul,hitc. pink. yellovn' or oran-ge. The anirrall ives burrou,cd a fèw ccnt inreters in to thc sedinrent .  p l inrar i ly  in  thc t ida l  l la ts .  butit can also bc Íbund ir.r sor.nenhat cleeper vr,atcfs, down to abor"rt 2,5 rrctcrs. TheBal t ic  c larr  sucks up sand and food par t ic les through i ts  long and Í lex ib le s iphon.In the shci l  cav i ty  the animal  sor ts  thc col lectec l  rnater ia l .  in_9csts thc edib le pal t i -c les Íbr  fur ther  c l igcst ion and excrctcs thc rest  as pseucioÍaeces.  Thc Bal t ic  c lanr  isa u,idesprcad and cornuron species that is Íbund Íl 'om the Cii lonclc at thc Freuclrcoast  uear  Bordeaur r " rp to Nclva Zernbla in  the h igh nor th.  For  thc l la l t ic  c lanr  theWaddcn Sea is thcrclt lre a fi i ir lv soLrtlrerly klcation r.r, ithin its _rrco_uraphic clistl ibLr-t ion.  On the basis  of  th is  i t  r ray be expected that  th is  s1 ' rcc ics i le lat ive ly  seusi t iveto u'artrinq of t l ie Wirclclen Sea, lvhich \\ 'as eur irrportant rcasou 1br choosine theBaltic clanr as rcscarch objcct. Morcover, a long rcsearch traclit ion exists Ítrrl, lucortttt hultlt ic'u. as a rcsult ol rvhich prior knou,lecl-9e of its biolo-uy \\ 'as cxtcn-s ivc.  Notably.  a successf i r l  rcar ing techniquc had been dcveloped.  Íac i l i ta t ing bree-ding crperirrclrts ir.r the laboratory.What  happens to the Bal t ic  c lanr  u 'hen the tempcratu le r iscs ' l  Thcrc a le rcal lyonly two possib i l i t ics:  ad-yust  to  the neu,c i rcur .nstanccs or ' lcavc ' lbr  cooler  p lacest10 S' t t  t t t  t t t r t  t ' t 'to thc north, which implies local ertinctior, . Adjr,rstrneuts to thc surroundings opc-ratc at severarl levels. within an inclivicLral or belll 'een individuals. Within ar-r ir-rcli-v idual the capacity to adjust o the euvironment is cal led phcnotypic plast ic i ty.  Thephenotype is the entire indiviclr.ral nd comprises morphological. physiological andbehavioural traits. Phenotypic plasticity can bc diviclecl into phenotypic Ílexibilityancl cleveloprnental p asticity. Phenotypic Ílcribility inrplies reversible changes inbehaviour,  physiology or even morphology: the changes tend to be rapid ar,d cnablcrcsponciing to short tenn alterations of the enviror.rlnent. Devcloprncntal plasticity isthc capacity of an inclividual to ttrhe on a particular phenotype, depending on thcenviror.rr .nent,  dul ing dcvelopment.  The lat ter is l loïe per\asire aud part icular lysuited Íbr respondirrg to environrncntal variation that is relatively srnall during anincliviclual's lifetin-re. The difïèrent Íbru,s of plasticity u'il l cornc into play ciepen-ding on the spat ial  and temporal scales involved.At some point the possibi l i t ies fbr arr  i rTdividLral  to respclnd to changcs r,r ' i l l  bccrhaustecl .  I t  is then possible that at  the populat ion level there are somc individualsthat are stil l capable of coping, while others succumb. If thcsc diÍï-crcnccs arc gcuc-t ical ly deterrnined, the gcnct ic corrst i tut i t rn of the populat ion wi l l  change. Thisprocess is called natural selection ancl takcs place over several -ÍIenerations.Phenotypic plasticity and genetic adaptations compler.nent each other in achanging world.  For the Balt ic c larr  i t  was eran.r ined to what cxtcut phenotypicplasticity and genetic adaptatior-rs arc ilrvolved. ln the most extrelne cases. alladjustments coLrld be either plastic or gcnctic. Molecular r"narkers indicated that atthe European scale genetic diÍï-crcnccs bct'uveen populations are very pronounced.Baltic clarns fi-om the Baltic Seti are even so ertrenrely diÍïèrent that they mi-eht beconsidered to bc a separate species. But other populat ions. too, display clear di Í ïè-rences thal point to a strongly restricted exchauge of individLrals and thus genesbetll,een locations. As a conscqucucc. Baltic cleuls rnay well be c|ftèrentially adap-ted at locations with diÍïcrcnt circurnstances. This result was unexpected because itwas always assnrncd that tsaltic clams eÍÏèctil 'ely tlavel large distances i,vhen theyclriÍi fieely in the water colun-tr as larvae. Unhanrpcrcd larval clispersal lvor"rldguarantcc substantial rnirin-e betw'een populations. inhibiting genetic diÍ1èrenccs{r'orn arising and resr"rlting in the inevitability that adjLrstments to diÍïèrences in envi-ronllelrt carr happen only thror.rgh phcnotypic plasticity.The resnlts of thc moleculerr merrker study indicate that genetic adaptations tocircur.nstanccs spcciÍic to local popr,rlartions are indeed possible. By rearing Balticclams of diÍï-crcnt origins r-urder ider-rtical circurnstances in the laboratory it is pos-sible to detennine to urhat ertent pafticLrlaf traits arc hcritablc (gcnctic). Aclults werecollccted at various localities (Balsfjord. norlhern Norrvay; Wadden Sea and NorthSea, thc Nctherleu-rds: Gironde. France) and indr-rced to spawn in the laboratory. Thclarvae werc rcarcd to the juvenile stage nnder controlled conditions. Any subse-qLrent difÍèrclrccs bctrvcen oflipring of pareuts Íï'om difÍèrent origins are uost like-ly genetically dctcrminecl. Three aspects were str-rdied: Íirst. the Íbraging apparatus,Stt t t t  t t t t  t ' tsccond. scnsit iv i ty of Balt ic c lams to high tcmperatures ancl.  Í inal ly.  srowth auddcvelclprnent clf lalr,ae at diÍfcrent tentperaturcs.Thc u'ater curreltt with which particlcs tl 'onr the seclilnent surfàce are suckecl upt l t rough the siphon is gencrated by ci l ia on thc gi l ls in thc shel l  cavi ty.  Part ic les areÍlltered frou the water curreltt, also by the gills, and tlren transported to the so-cal led palps. On the palps. l ike the gi l ls covered with ci l ia.  the par-t ic lcs arc sortecl .aÍter which thc ediblc Íl 'action is ingcstecl by thc mouth and thc rest is excreted aspsetrdoÍàcces. Strong diÍ1èrences erist betr.veen populations in the ratio betwecn thegi l ls (col lcct ing) ancl  the palps (sort ing).  In the reared Muc'onru hulthi t 'n the di f--fcrences betwcerr populations turned out to be heritable and thercÍbre genetic. Anadditional experitnent den-tonstrated that thc ratio betrveen gills and palps changeswith cl i f tèrent seci iment types. ln course sand the,ei l ls ale larqe ancl the palps srral l .and this is revcrsecl in si l t .  This is probably rclated to thc antount of part ic les thateltters at a particular pr,trnping capacity. A volunre of sancl contains Íar Í-cwcr parti-c les t l iat t  the sanre tolut .uc of s i l t  and therefbre sort in-e out a volume is muclr  lesswork i f  i t  consists of sancl than i f  i t  contains a lot  of  s i l t .  In contrast to the di Í ïe-rel tces betweclt  populat ions. c l i f Ïèrent gi l l /palp rat ios within populat ions arc to alar'-ee ertent flexiblc and lvill respond to the local circur.nstances.tf Baltic clartrs in the nortlt and in the south are both adapted to thcir local cir-cltr.nstances. thc sot-tthern bivalves should be bettcr able to cope u,ith Itigh tcrrpela-tures. This rvas studied by Lrtil izing thc lact that proteins denatule when exposed tohighert l tan opt imal tcr.npet 'atures. I f  this happens inside a cel l  a spccial  type of pro-teins. the hcat shocl< proteins (HSPs). is synthcsized to ci ther repai l  the danrage orre trdcr the danagcd proteius harurle ss and deport thern. The concentration of HSPsis thereforc a lleasure of tentperature stress. A comparison betwcen Íbr.rr groups oflaboratoly reared Baltic clanrs. fi 'our Norlvay, the North Sea, thc Wadclen Sea anclFrattce. shou,cd tltat po;lulations do difÈr in terrlleratllre tolcrance bLrt also thatthese cliÍJèreltces \\'ere not casy to cor"r;llc to tlie original environment. Interestingly.indivic lLrals stntr l lcr  than 5 mur had much higher levels of HSPs than larger ones.This diÍÏèrencc nray be ascribed to telrperatr-u'e diÍÈreuces bctween the placeswhere Balt ic c lans of di Í ïerent sizes tcncl  to be located. Smal ler individuals are bur-rowed closc to 1hc sediment surfacc. while larger ones are burrowed ntore cleeply.Close to the surfàce telxperatllres Í-luctuate nrore and will also reach lti_shcr levels.especially wlrcn a ntuclÍ' lat is exposed around noon. Moreover, small Baltic clarnsare on avera-qc locatecl Irieher up in the tidal zone than larger ones. wltich scrvesrnainly to escape prcdatot's like crabs and shrinp. Thesc places are subnter-{ed lesslong and can thercÍore heat up more dr"rrin-e low ticle than the lowcr ticlal zone. Itwould thcrcfore appear tltat as a Baltic clarl grows, the tcmperature of its sun'olurd-ings f luctuates less and also reaches lor. ler maxima. I t  wi l l  then necd HSPs lessoÍlen to react to proteiu dama-{e caused by hi-uh terrperatures and ther-eÍbrc haveless of thcse proteins in stock.t 7 lt72 Stt t t t t t t t t t ' t 'After  Íèr t i l izat ion of  the eg-9s.  larvac devclop.  The larvae remain in  the vvatercolutnu Íbr a l 'cw lr,eeks. This is a tinre that is charactcrized by high nrurtality ratesand could be a l iniit ing fàctor Íor thc occurrcr.rcc oÍ'Baltic clams. Thc cl}èct of tem-peratl lre on tltc clcvclopmcnt of larvae of Baltic clanrs Íl 'cln.r the Balsljorcl iu Norwayaud fron-t the Ciironde in Francc r,vas studied in a rcalinq experintent in the labora-tory. Larvae of both populations -9rcr.r,and dcvclopccl f irster at high tentperatures butalso clisplayccl a hi-qher nortality latc. Frcnch larvae developccl fhster thanNorwegiitn larvae. Rernarkably. thc Nonvcgian lalvae sti l l  developecl successfullyat  a water  tcr lperature of  20 'C' .  This  is  approximatc ly  l5 'C h igher  than normal  inthe Balsf-1orcl! Lalvac clearly tolcrate a wide range of tentperatures.All three cx;lcl ir.nents lvit lr rcared Baltic clanrs uncovered -eenctically deter-ntined clifÍèrcnces betu,een populations. but thc nrost ltotable observatiorr rvas thatin  a l l  cases thc indi l ' iduals u 'erc capable of  rcs l - roncl ing ef tèct i r . 'e ly  to  var iable c i r -cun-rstances. The Baltic clanr is -gcnctically aclalrtccl to local circumstances. but.fbremost .  reacts pheuotypical ly  p last ic  to  val ia t ions in  i ts  sun'or-urc l ings.The eÍ1'ccts of temperature on the Baltic clam wcrc studied furthcr among indi-v iduals Í l 'ont  thc Wadden Sea usiu-9 l ie ld observat ions and laborato ly  cxpcl i r rents.Temperature has a strolrs inl ' lLrcncc on bioloqical 1-rrocesses. ln colcl-blooded ani-nrals l ike i l ' lut 'orttu hultlt ic'u elrcfgy clcnrand incrcascs steeply as it beconres \\ 'anrer.E,arlier research had already pointccl ont that a hisher energy clemancl in rri ld u'in-ters reduces thc uumber of proclLrcecl eggs courparccl to cold winters. In a starvatiouexperiment. subjccting Mut'ottru hulthicu to diÍïèrcnt terxperatl lres. thc Baltic clanswere Íbuncl to bc capable of allocating their energy effectively tou,arcls synthesis ofeggs even at u,inter tenrpelatrrlcs rbor e a\ erage.Besides har ing an inÍ' lucncc on process rates. tcurperature nray also le_uulatethe breeding scasou. Baltic clanrs spa\v11 after the \\,ater temperature cxcccds a cer-ta in thresholc l .  and i t  is  assunrcc l  that  th is  represents a d i rect  react iorr .  I la l t ic  c larrsthat  l ive in  t ic la l  Í la ts  woulc l  thcrr  possib ly  spawn car l ier  than thei r  consl - rcc i Í ics indeeper water. bccausc at low ticle the sun warms thc Ílats and nruch lcss so thc bot-tot r  of  thc gul l ies.  Whi le t rack ing the reproduct ivc s tatus of  Bal t ic  c lants no d i f fer-ence it.r t irring u,as obsen'ed bctu'een the tidal Í ' lats arrd the gull ics. It r,vas hotvet'eruoted that the timc durirrg u'hich spau'nirr_e takes place is long. at least tu'o nonths.IndividLrals spawn scveral t iurcs cluring this periocl. We suppose that in this u'ay thcyenlarge their chauccs to proclucc oÍl.spring at a l irvolrlable time, a l<incl of bet-hed-ging.To get an idca of thc actual temperature cliÍï 'crences betu,een populations. acot.nparison u,as utade between lleld sites alorrg thc European coasÍ. Baltic claursrvere collcctccl at a lar-ee nunrbcl of sites and tlrc gro\\rth ratc cstiutated from thegrorvth rit 'rgs on thc shells. Thc cxpectation lvas that I lalt ic claurs clf ntorc norther-ly popLrlatiolts grow more slor,vly cll le to the lower tcmperatures. The obscrved pat-tem, however', is not uncquivocal. Thrs points to tlre importance of otltcr Íàctolssuch as Íbocl  avai labi l i ty .  01 ' thc th lee populat ions BalsÍ jord.  Waclc lcn Sea andSummaryGironde the seasonal cycle of body mass and gonad development was estimated.From this, an energy budget could be reconstructed and it appeared that Balticclams from the Balsflord devote more energy to reproduction than the moresoutherly clams from the Wadden Sea and the Gironde. This can mainly be ascribedto the lower temperature, which leaves more energy for body maintenance.Furthermore, it could be deduced that food availability differs between the popula-tions.The results of the separate parts of this study all point to a high level of tole-rance to differences in environmental variables. At the same time genetic adapta-tions to local circumstances exist. It seems that genetic adaptations exist for certainconsistent differences between populations, while superimposed on this, flexibletraits enable the individuals to respond to their variable environment.Do these results imply that the Baltic clam is safeguarded against changes in itsenvironment due to its phenotypic plasticity? The answer is no. Long-term researchof the Royal Netherlands Institute for Sea Research at Íhe Balgzand and other areasin the western Wadden Sea shows that since the end of the nineteen eighties the pop-ulation of Macoma balthica is declining. While average density used to be around100 adults per square meter, now it is merely 10 per square meter. The causes of thisdecline are still uncertain. On the basis of the results in this thesis it is not probablethat direct effects of climate on the physiology of the Baltic clam are responsible.Indirect effects through interactions with other species that respond differently toclimate or other environmental factors are more likely. For example, the spatfall ofBaltic clams may have severely decreased due to predation by shrimp. Shrimp areparticularly numerous on the tidal flats at the time of spatfall after mild winters.However, when conducting research into effects of human-induced changes inecosystems, in this case climate, it must be realised that human influences are var-ied. Pollution, eutrofication and not forgetting fisheries all have undoubtedly influ-enced the current development of Baltic clams and other benthic fauna of theWadden Sea.t73

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Life history variation of a marine bivalve (<i>Macoma balthica</i>) in a changing world

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