Walrus (Odobenus rosmarus) prey extensively on the bivalve Mya truncata, removing the siphons. We performed a simple experiment whereby the siphons from 27 M. truncata were removed, 33 controls were left intact, and the clams left on the sea bottom for a year. All the damaged Mya died; all but 2 controls lived. We conclude that M. truncata whose siphons have been grazed by walrus die, leaving over half the clam to predators or scavengers.

Martin-Bergmann, Kathleen

Welch, Harold E.

English

ABSTRACT. Walrus (Odobenus rosmurus) prey extensively on the bivalve Myu truncutu, removing the siphons. We performed a simple experiment whereby the siphons from 2 7 M. huncata were removed, 33 controls were left intact,  and the clams left on the sea bottom for a year. All the damaged Myu died;  all but 2 controls lived. We conclude that M. truncutu whosesiphons have been grazed by walrus die, leaving over half the clam to predators or scavengers. Key words:  Arctic,  benthos,  bivalves,  walrus, Mya truncuta, energetics RkXJM6. Les morses (Odobenus rosmrus) se nourrissent en grande partie d bivalves de l&apos;eswe Myu truncatu, en lui enlevant ses siphons. On s&apos;est livré B une expérience relativement simple dans laquelle on a enlevé les siphons de 27 Mya truncatu, et laissé intacts 33 spécimens de contrôle. On a ensuite dépose les myes sur le fond marin. Au bout d&apos;un an,  aucune des Mya endommagées n&apos;avait survécu alors que toutes les myes de contrôle sauf deux étaient encore vivantes. O n en conclut que les Mya truncutu dont les siphons ont étte broutés par les morses ne survivent pas et que plus de la moitié de coquillage est laissée en pâture aux prédateurs et aux animaux nécrophages

Harold E. Welch

Kathleen Martin-bergmann

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ARCTIC Does the Clam Mya truncata Regenerate Its Siphon after Predation by Walrus?  An Experimental Approach

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VOL. 43, NO. 2 (JUNE 1990) P. 157-158 ARCTIC Does  the  Clam Mya truncata Regenerate  Its Siphon  after  Predation by Walrus?  An  Experimental  Approach HAROLD  E.  WELCH' and KATHLEEN  MARTIN-BERGMANN' (Received 5 June 1989; accepted  in  revised form 15 November 2989) ABSTRACT.  Walrus (Odobenus rosmurus) prey  extensively  on  the  bivalve Myu truncutu, removing  the  siphons. We  performed a simple  experiment whereby  the  siphons  from 2 7 M .  huncata were  removed, 33 controls  were  left  intact,  and  the  clams  left on the  sea  bottom  for a year.  All  the  damaged Myu died;  all but 2 controls lived. We conclude that M .  truncutu whosesiphons have  been  grazed  by  walrus die, leaving  over half  the  clam  to  predators or scavengers. Key words:  Arctic,  benthos,  bivalves,  walrus, Mya truncuta, energetics RkXJM6. Les morses (Odobenus  rosmrus) se nourrissent  en  grande  partie d  bivalves de  l'eswe Myu truncatu, en  lui  enlevant ses siphons. On  s'est livré B une  expérience  relativement  simple  dans  laquelle on a enlevé les siphons de 27 Mya truncatu, et  laissé  intacts 33 spécimens  de  contrôle.  On a ensuite  dépose  les  myes  sur  le  fond  marin.  Au  bout  d'un  an,  aucune des Mya endommagées n'avait survécu alors  que  toutes  les  myes  de  contrôle sauf  deux  étaient  encore  vivantes. O n  en conclut  que les Mya truncutu dont  les  siphons  ont étte  broutés  par les  morses  ne  survivent  pas  et  que  plus de la moitié  de  coquillage  est  laissée  en  pâture  aux  prédateurs  et  aux  animaux  nécrophages. Mots  clés:  Arctique,  benthos,  bivalve,  morse, Myu truncutu, énergétique Traduit  pour le journal  par Nbida Loyer. INTRODUCTION Walrus (Odobenus rosmurus) feed primarily on the bivalves Myu truncutu and Serripes groenlundicus in the Western  Arctic (Fay, 19821, north Baffin Bay (Vibe, 1950), and the eastern Canadian Arctic  (Mansfield, 1958; Fisher, 1989). M. truncutu lives deep in  the sediment, its siphon is  very  long, and it is this organ that is found in walrus stomachs.  The actual feeding process  in the sea  has  not  been  observed an  is  in  some doubt. Fay (1982) and Vibe (1950) both suggested that siphons could be  sucked  in and broken off by  walrus, and  our  own observa- tions indicate that this  could be the case.  The in  situ siphons of M. truncutu take 1-2 s to retract when touched;  they are easily grasped in the gloved hand of a diver, and  when pulled the siphons almost always break at the mantle  unless the sediment  is  very  soft, when occasionally the entire clam  can be  removed.  It should be noted that Oliver et al. (1983) found that 83% of  Myu shells  associated with Pacific walrus feeding tracks had  the periostracum (siphon sheath) still intact  with the  shell, suggesting that the soft parts had been  sucked  from the shell and the sheath left.  This  is  inconsistent with theMyu truncutu remains we find in eastern arctic walrus stomachs, which  contain  both siphons and sheaths but not the rest of the clam. "If a walrus removes the siphon alone  from a clam,  can the clam  regenerate its siphon?" (Fay, 1982). Other bivalves with relatively small siphons (a few percent of body weight) readily  regenerate siphons that have been grazed by  fish - for  example, Tellinu tenuis grazed by plaice  (Trevallion, 1971), Mucomu  bulthicu grazed by plaice and other fishes (de Vlas, 1985), and Protothacu stamina and Mucomu spp. grazed by several  fish  species  (Peterson and Quammen, 1982). How- ever,  because  the siphon and sheath constitute 45% of the soft body  dry weight of  M. truncutu, regeneration in this species  is less  likely. To  test  the  hypothesis that M. truncutu may  regen- erate siphons after  removal  by  walrus, we conducted a simple experiment by removing siphons from living clams and returning them  to the sea  floor  for  recuperation.  The  experi- ment  took  place at Resolute, 74"42'N,  94'50'W in the Cana- dian Arctic. EXPERIMENT On 31 July 1985, about 220 M. truncutu were  collected  by hand from  soft sediment at 10-15 m  depth in  Resolute Bay. The  clams were held  in a stainless  steel tank with running sea water at ambient temperature (0°C) for 48 h,  then  placed  in the holding array on 2 August and returned to  the  tank. The  holding array (Fig. 1) consisted of 60 PVC cylinders 12.7cm highby5.1 cm1.D. glued toastainless steel tray. Clean quartz sand was  placed in the bottom of each  cylinder  to a depth of 4.2 cm, a single  clam  placed upright inside, and a 50:50 mixture of sand  and silty mud placed around the clam, the mud coming  from the same habitat  originally  occupied  by the  clams. A range in  clam  sizes was selected (mean length 5.20 cm, range 3.52-6.70 cm; mean width 3.96 cm, range 2.72-4.77 cm), but placement  in  the array was random. On 17 August, 15 days after  placement  in  the  holding array, the array was taken  from the tank and every  second  clam  was artificially "grazed by  holding  the  shell down in the cylinder with one  hand  and pinching off the siphon between thumb and forefinger  with  the  other.  The amount of siphon removed ranged from about half to  all.  There were 27 experimentals and 33 controls.  The array was  then returned to the tank. Five days later  on 22 August the clams  were  assessed.  All 33 controls had their siphons exposed and reacted  to  touch. Of the 27 experimentals, 14 had the damaged siphon end exposed and reactive to touch; 6 were exposed and not reactive; 2 had the end of the shell  exposed  with  no  flesh visible; and in 5 cylinders there was  no  sign of the  clam  in  the 'Department of Fisheries  and  Oceans,  Central  and  Arctic  Region,  Freshwater  Institute, 501 University  Crescent,  Winnipeg,  Manitoba,  Canada R3T ZN6 @The  Arctic  Institute  of  North  America 158 / H.E. WELCH and K. MARTIN-BERGMANN n \ I , , , , !  0 scale, cm 5 RG. I. Experimental holding  array for Mya truncata. Sixty  plastic ylinders 12.7 cm high by 5.1 cm  I.D. were  glued  toa  stainless  steel  tray  and  a  single clam  held in  each. unbroken sediment surface. The array was then lowered slowly to the bottom at 10 m depth at the collection  site, where it remained until removed a year later, on 15 August 1986. When removed, the sediment in the cylinders was in place. All 27 of the experimentals were dead; only the shells were left.  All but 2 of the 33 controls were alive and appeared to be healthy and normal. DISCUSSION From this simple experiment we conclude that M .  truncata whose siphons are grazed by walrus  die shortly after preda- tion.  It is possible that clams in this experiment were stressed by handling and habitat change and  that M .  truncatu pre- dated by walrus live and regenerate their siphons, but  we consider it highly unlikely. The siphon of M .  truncata constitutes 45% of the soft body dry weight, or 38% of the total energy content (Table 1). TABLE 1. Weights and energy  contents of Mya  t runca ta  from Barrow Strait" Siphon Body Shell  Total Wet wt. (g) (N=100) 6.422 10.86 10.44 27.72 Dry wt.  (g) (N=100) 1.199 1.459 9.2M 11.94 Kilojoules-g" (N) 13.13(10) 17.94(27) - - aClams  collected  from south coast of Comwallis Island and  north coast of Somerset Island. Mean  valve length 4.48 mm. Assuming walrus ingest only the siphons, they consume about 38% of the clam's energy, with the remainder going to feed scavengers such as lysianassid amphipods of the genus Onisimus and whelks (Buccinum spp.), both abundant in Barrow Strait. The impact on the bivalve population may be considerable, depending upon the density of walrus. The daily intake of Myu siphons by an  adult walrus is on the order of 4500 to 6500 per day (Fisher, 1989, and calculated from Fay, 1982), leaving 50-70 kg clam flesh per day available for immediate consumption by other species  in the food web. REFERENCES de VLAS, J. 1985. Secondary production by siphon  regeneration in a  tidal flat population of Macoma balthica. Netherlands  Journal of Sea Research 1 9  FAY, F.H. 1982. Ecology and biology of the Pacific walrus Odobenus rosmrus divergens Illiger.  Washington, D.C.: United States  Department of the Inte- rior, Fish and Wildlife  Service, North American  Fauna  No. 74. vi + 279 p. FISHER, K.I. 1989. Food habits  and  digestive efficiency in  walrus, Odobenus rosmarus. M.Sc. thesis,  University of Manitoba. 85 p. MANSFIELD,  A.W. 1958. Thebiologyof the Atlantic  walrus, Odobenus rosmnrus rosmarus (Linnaeus)  in  the  eastern  Canadian Arctic. Fisheries Research Board of Canada, Manuscript  Report  Series  (Biology) 653.146 p. OLIVER,  J.S.,  SLATTERY,  P.N., OCONNOR, E.F., and LOWRY, L.F. 1983. Walrus, Odobenus  rosmarus, feeding in the Bering  Sea: abenthic perspective. Fishery  Bulletin 81:501-512. PETERSON,  C.H., and QUAMMEN,  M.L. 1982. Siphon  nipping: its impor- tance to small  fishes and its impact on  growth of the  bivalve Protothaca staminea (Conrad).  Journal of Experimental  Marine Biology and Ecology TREVALLION, A. 1971. Studies  on Tellina tenuis da Costa. 111. Aspects of general biology and  energy flow. Journal of Experimental  Marine Biology and Ecology 7:95-122. VIBE, C. 1950. The marine mammals and  the  marine  fauna  in  the  Thule district (Northwest  Greenland)  with  observations  on ice conditions in 1939-41. Meddelelser om Grernland 150(6):1-115. 147-164. 63249-268. 

Does the Clam Mya truncata Regenerate Its Siphon after Predation by Walrus? An Experimental Approach

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Does the Clam Mya truncata Regenerate Its Siphon after Predation by Walrus? An Experimental Approach

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