Seasonal dynamics of the scyphomedusae, Aurelia aurita, was investigated twice a month from January 2010 to June 2011 and related to environmental conditions in Thau lagoon, southwestern Mediterranean Sea. Strobilation, indicated by the presence of 1 mm ephyrae, occurred from November to the middle of April. Maximum abundances up to 330 ind.100 m-3 were reached in May 2011, few weeks after the last ephyrae release. The population declines afterwards steadily until disappearing from the water column. Concurrent with increased water temperature and mesozooplankton predation during May, growth rates increased from 0.04 mm.day-1 to a peak of 4.5 mm.day-1, with a maximum bell diameter of 11.3 cm reached on the 7th May. During the study period, there was no advection of A. aurita between Thau lagoon and the coastal waters

Daly Yahia

Delphine Bonnet

Juan Carlos Molinero

Mohamed Néjib

Thomas Schohn

English

CiteSeerX

Reçu le 21 septembre 2011 ; accepté après révision le 9 février 2012.
Received 21 September 2011; accepted in revised form 9 February 2012.
Cah. Biol. Mar. (2012) 53 : 343-347
Seasonal changes in the population dynamics of Aurelia aurita
in Thau lagoon
Delphine BONNET1, Juan Carlos MOLINERO2, Thomas SCHOHN1 and Mohamed Néjib DALY YAHIA3
(1) Laboratoire ECOSYM, UMR 5119, Université Montpellier 2, CC093, Place Eugène Bataillon, 
34095 Montpellier Cedex 05, France
Fax: 04.67.14.37.19, E-mail: delphine.bonnet@univ-montp2.fr
(2) Helmholtz Centre for Ocean Research Kiel (GEOMAR), Marine Ecology/Food Webs, Duesternbrooker Weg 20, 
D-24105 Kiel, Germany
(3) Faculty of Sciences of Bizerte, Department of Life Sciences, Laboratory of Aquatic Systems Biodiversity and
Functionning, 7021, Zarzouna, Bizerte, Tunisia
Abstract: Seasonal dynamics of the scyphomedusae, Aurelia aurita, was investigated twice a month from January 2010 to
June 2011 and related to environmental conditions in Thau lagoon, southwestern Mediterranean Sea. Strobilation, indicated
by the presence of 1 mm ephyrae, occurred from November to the middle of April. Maximum abundances up to 330 ind.100
m-3 were reached in May 2011, few weeks after the last ephyrae release. The population declines afterwards steadily until
disappearing from the water column. Concurrent with increased water temperature and mesozooplankton predation during
May, growth rates increased from 0.04 mm.day-1 to a peak of 4.5 mm.day-1, with a maximum bell diameter of 11.3 cm
reached on the 7th May. During the study period, there was no advection of A. aurita between Thau lagoon and the coastal
waters.
Résumé : Etude des changements saisonniers de la dynamique de population d’Aurelia aurita dans la lagune de Thau. La
dynamique de population de la scyphoméduse Aurelia aurita a été étudiée en relation avec des variables environnementales
via des prélèvements bimensuels de janvier 2010 à juin 2011 dans la lagune de Thau, en Méditerranée Orientale. La
strobilation, indiquée par la présence d’éphyrules de taille d’environ 1mm, se déroule de Novembre à la mi-avril. Les
abondances maximales de 330 ind.100 m-3, sont observées en mai 2011, quelques semaines après les dernières émissions
d’éphyrules. Les effectifs diminuent ensuite très rapidement jusqu’en juin où A. aurita disparaît de la colonne d’eau. Les
faibles taux de croissance de 0,04 mm.j-1 observés de janvier à avril augmentent jusqu’à 4,5 mm.j-1 lorsque la température
augmente et qu’une forte pression de prédation est exercée sur le zooplancton en Mai. Un diamètre d’ombrelle maximum
de 11,3 cm est observé le 7 mai. Il n’y a pas eu d’advection d’A. aurita entre les eaux de la lagunes de Thau et la zone
côtière pendant la période de l’étude. 
Keywords: Aurelia aurita l Lagoon l Mediterranean Sea
344 AURELIA AURITA POPULATION DYNAMICS
Introduction
The scyphomedusa Aurelia aurita (Linnaeus, 1758) is
widespread in coastal waters around the world and it is
probably the most studied jellyfish (Lucas, 2001; Lo &
Chen, 2008). Although a number of studies on the life cycle
of A. aurita have been conducted, the multi-site compar-
isons of the seasonal patterns of A. aurita remain scarce
(i.e. Miyake et al., 1997), particularly in the Mediterranean
Sea where only one publication refers to the population
dynamics of this species (Chakroun & Aloui-Bejaouin,
1995).
Lagoons are semi enclosed ecosystems that offer an
ideal place to study the life histories of bentho-planktonic
populations such as A. aurita, since both benthic and
pelagic phases coexist. Furthermore, mass reproduction of
A. aurita occurs in inshore waters that warm up much
earlier than offshore waters (Vinogradov et al., 1992). Thau
lagoon is the widest lagoon of the Languedoc Roussillon
area with 7500 ha. It is a shallow environment with a main
depth of 8 m. In Thau, oyster and mussel productions are of
high importance as they represent 10% of the French
national production. The wood structures supporting
bivalve productions are probably very suitable substrata for
the reproductive sessile stage of A. aurita (Uye et al., 2003;
Uye & Ueta, 2004). Our objectives were (1) to provide the
first description of Aurelia aurita pelagic life cycle in the
Western Mediterranean Sea, (2) to compare its population
dynamics with other lagoons worldwide and (3) finally to
determine the temperature and salinity roles in Aurelia
aurita life cycle.
Material and Methods
Plankton samples are collected twice a month at 4
monitoring sampling stations from January 2010 to June
2011. Three of these stations are located inside the lagoon:
in a harbour (43°19’082N-3°33’532E, 3 m depth), between
oyster production tables (43°24’644N-3°36’016E, 4.5 m
depth) and nearby a disused shipyard (43°25’329N-
3°40’862E, 4.5 m depth). A coastal station (43°21’925N-
3°41’453E, 23 m depth) was also monitored in waters
surrounding the lagoon to assess possible links between
changes in A. aurita abundance and inflow/outflow water
masses exchanges. Gelatinous plankton was collected by
horizontal tows near surface with a Hansen net (700 µm
mesh aperture). A large plexiglass collector was used to
keep the organisms in good shape. Samples were then
preserved in 4% buffered formaldehyde. We considered A.
aurita ephyrae i.e. < 10 mm, while immature organisms
were determined from the absence of developed gonads.
Bell diameter, sex and the presence of planula larvae were
determined. As bell size measurements were done on
preserved individuals, size was corrected for formalin
shrinkage according to Möller (1980). Temperature and
salinity were recorded in sub-surface (< 0.5 m)
simultaneously during sampling with a probe (EC 300
VWR international/ WTW model 350i).
To detect associations between the abundance of Aurelia
life stages and environmental variables (i.e. temperature,
salinity) we used a cumulative frequency method and a
Monte Carlo randomization, as in Perry & Smith (1994)
and Paramo et al. (2003). First, the relative cumulative
frequency distribution (CFD) was calculated for
temperature and salinity. Afterwards, we weighted the
CFDs for each environmental variable by multiplying the
curve by the abundance of Aurelia life stages, as in Paramo
et al. (2003). The absolute difference between the two CFD
curves, unweighted and weighted, shows whether the life
stages were associated with a certain environmental
window or whether they were randomly distributed. In the
latter case two curves will accrue similarly and will not be
significantly different. However, when life stages were
associated with a particular thermal or salinity window, the
slope of the weighted CFD was steeper than that of the
unweighted environmental variable. 
The test of significance of the difference between the
curves was performed by a Monte Carlo test (1000
randomizations) under the hypothesis of a random
distribution. 
Results and Discussion
Seasonal occurrence
Ephyrae production occurs mainly from November until
April (Fig. 1A). Similar patterns have been observed in
temperate ecosystems of the Northern hemisphere where
ephyrae production usually occurs in winter through spring,
although in some fjords, autumn and spring productions
have been also observed (Lucas & Williams, 1994).
Maximum abundances of ephyrae were observed in April
2010 and February 2011 (50 and 200 ind.100 m-3
respectively), whereas the maximum abundances of adults
were found in June 2010 and May 2011 (100 and 330
ind.100 m-3, respectively). Similar values have been
observed in the Bay of Tokyo (153 ind.100m-3) (Lucas,
2001). Immature organisms are observed in March-April,
and sexual adults from April to June (Fig. 1B). Abundance
decreased drastically from April to June allowing the
population decline, both ephyrae and adults. This sharp
decrease in abundance, likely due to natural mortality, is
well known and has been repeatedly observed, for example
in mid-summer, June-July, in the Black Sea (Vinogradov &
D. BONNET, J.C. MOLINERO, T. SCHOHN, M.N. DALY YAHIA 345
Shushkina, 1992; Mutlu, 2001) soon after the gametes
release from the mature adults. From the two-year survey in
Thau lagoon, it appeared that only one generation per year
occurs, while in the Black Sea for example, one or two
generations (winter and spring) might occur (Shushkina &
Musayeva, 1983;  Mutlu, 2001). Our observations suggest
a A. aurita life span of 7-8 months in Thau lagoon, which
is concurrent with former results in other European
ecosystems (Lucas & Williams, 1994) while Miyake et al.
(1997) suggest that its life span in some bays of Japan could
reach up to two years.
The spatial survey here used suggests that A. aurita
originated from the lagoon as it was not found in the coastal
station. As shown elsewhere, the semi-enclosed system of
the lagoon might provide refuge, food and substratum to
polyp colonies. Indeed, Tokyokawa et al. (2011) indicated
that in semi enclosed ecosystems, such as harbours protected
by breakwaters, if ephyrae were encountered then it could be
concluded that the polyp colonies could be found nearby. 
Breeding and growth
A. aurita presented a slow growth rate from January to mid-
April around 0.04 mm.d-1. Then during the next 2-3 weeks,
the growth rate suddenly increased of 100 times up to
4.5mm.d-1 leading to sexual adults with an average size of
8.42 cm and a maximum observed size of 11.33 cm. Similar
size ranges have been observed in Tomales Bay, USA
(Miyake et al., 1997). While growth rates in the range of 3.4
cm.week-1 and 4 cm.week-1 were observed respectively by
Möller (1980) in Kiel Bight and Lucas & Williams (1994)
off Southampton, larger A. aurita are usually found in
coastal waters (references in Lucas & William, 1994;
Miyake et al., 1997).
The period of intense growth coincides with a sharp
increase in temperature (from 11 to 20°C in April) and an
intense predation on the mesozooplankton stock (decrease
in abundance of ~ 81%). It is not clear however, why
medusae in Thau lagoon do not attain the sizes found in
other areas where organisms can reach > 20 cm. A possible,
non-exclusive explanation is a food limitation constrain as
a result of either their own predatory impact, or through
competition for food resources with bivalves which are
very abundant in the lagoon.
Between May and June 2010, the bell size decreased
from 19% at the rate of 1.4 mm.d-1, and the population
started disappearing from the water column. While the
reasons for this decrease in bell size are still discussed
(food limitation, decrease in the metabolic activity), the
size reduction is in the range of those usually observed,
between 10 and 20% (Möller, 1980; Lucas & Williams,
1994). The sex ratio was well balanced with 49% of males
and 51% of females. Light microscopic examination of the
organisms showed that up to 1528 eggs were carried in the
brood sacs of a single individual. 
Temperature and salinity
The time of occurrence of A. aurelia in the pelagic plankton
is highly variable among localities because of environmen-
tal differences (reviewed by Miyake et al., 1997). In Thau,
maximum abundances of A. aurita ephyrae occurred at
16°C, although they could be observed within a large
temperature window, between 5 and 26°C (Fig. 2B). The
presence of ephyrae was continuous from November to
April, and afterwards disappeared from the water column
as temperature increased over 16°C. Lo & Chen (2008)
have recorded an inverse correlation between A. aurelia
ephyrae abundance and temperature in Tapong Bay. They
indicated that ephyrae were almost absent during the warm
and wet season (May-October). In Thau, the mature adults
were preferentially encountered around 19°C (Fig. 2A).
When temperature increased above this optimum, there was
a total disappearance of the adults from the water column.
However, the review in Lo & Chen (2008) indicated that A.
auritamedusae were repeatedly encountered at temperature
> 20°C.
Salinity optima are similar for both A. aurita ephyrae
and adults (34.8 and 35.3 respectively, Fig. 2C & D).
Figure 1. Aurelia aurita. A.Abundance  (ind.100 m-3) in Thau
lagoon. B. Pelagic life cycle.
Figure 1. Aurelia aurita. A. Abondance (ind.100m-3) dans la
lagune de Thau. B. Partie pélagique du cycle de vie d’Aurelia
aurita.
Supporting Lo & Chen’s results (2008), no significant
correlation between salinity and A. aurelia abundance
(ephyra or adult) was found. This is probably due to
extreme inflow events which can affect abruptly the
salinity. Indeed, intense precipitations often occur in fall.
Salinity conditions strongly vary as the freshwater inflows
into the lagoon vanish in summer which leads to an
increase of salinity up to 39.3. The contrasting hydrological
seasons characterized by strong temperature and salinity
changes are hypothesized as a further reason why A. aurita
disappears from the water column in Thau lagoon from July
to November.
Figure 3 synthesises the occurrence of A. aurita in 3
Western Mediterranean lagoons. The medusae occur
roughly at the same salinity in Bizerte and in Thau (32.5 to
38 and 33 to 39.3, respectively), however, they are present
at temperatures (between 12.5 and 27.5°C) higher than in
Thau (between 7 and 24°C). In Berre, with contrasting
environmental conditions in comparison with the two first
lagoons, i.e. brackish waters, A. aurita is encountered
between 16.5 and 26.5. In addition, the thermal niche
overlaps with those of Bizerte and Thau with a range of 4
to 27°C.
These results highlight the plasticity of the species to
live and reproduce in contrasted environments. They
further support former observations showing that A. aurita
might inhabits a broad window of environmental conditions
ranging from salinity from 10 to 32 and temperature from -
2 up to 30°C (Miyake et al., 1997). The variability in the
environmental ranges A. aurita inhabits in the western
Mediterranean raise several questions to thoroughly assess
its life history, in particular the plasticity of the species to
346 AURELIA AURITA POPULATION DYNAMICS
Figure 2. Aurelia aurita. Cumulative frequency distribution (CFD) of the habitat variables. A & B. Temperature. C & D. Salinity.
unweighted CFD: thick line, weighted CFD: dotted line, absolute difference between unweighted and weighted: filled areas.
Figure 2. Aurelia aurita. Distribution des fréquences cumulées (CFD) des variables définissant l’habitat. A & B. Température. C &
D. Salinité. Les CFD non pondérés sont représentés par une ligne épaisse, les CFD pondérés par une ligne pointillée, les aires grises
représentent la différence absolue entre les CFD non pondérés et pondérés.
evolve in changing environments and whether cryptic
species exist in the Mediterranean lagoons. This first study
could be used as baseline for further investigations on the
A. aurita dynamics in the Mediterranean Sea with some
molecular biology approaches urgently needed.
References
Chakroun F. & Aloui-Bejaouin N. 1995. Invasion d’Aurelia
aurita (Cnidaria, Scyphomédusa) dans le lac de Bizerte
(Tunisie) au cours de l’été 1994. Annales de l’Institut
océanographique, 71: 67-69.
Lo W.T. & Chen I.L. 2008. Population succession and feeding of
the scyphomedusae, Aurelia aurita, in a eutophic tropical
lagoon in Taiwan. Estuarine, Coastal and Shelf Science, 76:
227-238.
Lucas C.H. 2001. Reproduction and life strategies of the common
jellyfish Aurelia aurita in relation to its ambient environment.
Hydrobiologia, 451: 229-246.
Lucas C.H. & Williams J.A. 1994. Population dynamics of the
scyphomedusae Aurelia aurita in Southampton water. Journal
of Plankton Research, 16: 879-895.
Miyake H., Iwao K. & Kakinuma Y. 1997. Life history and
environment of Aurelia aurita. South Pacific Study, 17: 273-
285.
Möller H. 1980. Population dynamics of Aurelia Aurelia medusa
in Kiel Bight, Germany. Marine Biology, 60: 123-128.
Mutlu E. 2001. Distribution and abundance of moon jellyfish
(Aurelia aurita) and its zooplankton food in the Black Sea.
Marine Biology, 138: 329-339.
Paramo J., Quiñones R.A., Ramirez A. & Wiff R. 2003.
Relationship between abundance of small pelagic fishes and
environmental factors in the Colombian Caribbean Sea: an
analysis based on hydroacoustic information. Aquatic Living
Resources, 16: 239-245.
Perry R.I. & Smith S.J. 1994. Identifying habitat associations of
marine fishes using survey data: an application in the
Northwest Atlantic. Canadian Journal of Fisheries and
Aquatic Sciences, 51: 589-602.
Shushkina, E.A. & Musayeva, E.I. 1983. The role in the energy
system of the Black Sea plankton communities. Oceanology,
23: 92-96.
Toyokawa M., Aoki K., Yamada S., Yasuda A., Murata Y. &
Kikuchi T. 2011. Distribution of ephyrae and polyps of jelly-
fish Aurelia aurita (Linnaes 1758) sensu lato in Mikawa Bay,
Japan. Journal of Oceanography, 67: 209-218.
Uye S. & Ueta Y. 2004. Recent increase of Jellyfish populations
and their nuisance to fisheries in the Inland Sea of Japan.
Bulletin of the Japanese Society of Fisheries Oceanography,
68: 9-19 (in Japanese with English Abstract).
Uye S., Fujii N. & Takeoka H. 2003. Unusual aggregations of the
scyphomedusa Aurelia aurita in coastal waters along western
Shikoku. Plankton Biology and Ecology, 50: 14-21.
Vinogradov M.Ye & Shushkina E.A. 1992. Temporal changes in
community structure in the open Black Sea. Oceanology, 32:
485-491.
D. BONNET, J.C. MOLINERO, T. SCHOHN, M.N. DALY YAHIA 347
Figure 3. Aurelia aurita. Temperature and salinity windows of
occurrence in 3 Mediterranean lagoons: Berre (M), Bizerte (B)
and Thau (T).
Figure 3. Aurelia aurita. Fenêtres de distribution en fonction
de la température et de la salinité dans 3 lagunes
Méditerranéennes : Berre (M), Bizerte (B) et Thau (T).


Seasonal changes in the population dynamics of Aurelia aurita in Thau lagoon

Bonnet, Delphine

Molinero, Juan Carlos

Schohn, Thomas

Daly Yahia, Mohamed Nejib

OceanRep

Reçu le 21 septembre 2011 ; accepté après révision le 9 février 2012.Received 21 September 2011; accepted in revised form 9 February 2012.Cah. Biol. Mar. (2012) 53 : 343-347Seasonal changes in the population dynamics of Aurelia auritain Thau lagoonDelphine BONNET1, Juan Carlos MOLINERO2, Thomas SCHOHN1 and Mohamed Néjib DALY YAHIA3(1) Laboratoire ECOSYM, UMR 5119, Université Montpellier 2, CC093, Place Eugène Bataillon, 34095 Montpellier Cedex 05, FranceFax: 04.67.14.37.19, E-mail: delphine.bonnet@univ-montp2.fr(2) Helmholtz Centre for Ocean Research Kiel (GEOMAR), Marine Ecology/Food Webs, Duesternbrooker Weg 20, D-24105 Kiel, Germany(3) Faculty of Sciences of Bizerte, Department of Life Sciences, Laboratory of Aquatic Systems Biodiversity andFunctionning, 7021, Zarzouna, Bizerte, TunisiaAbstract: Seasonal dynamics of the scyphomedusae, Aurelia aurita, was investigated twice a month from January 2010 toJune 2011 and related to environmental conditions in Thau lagoon, southwestern Mediterranean Sea. Strobilation, indicatedby the presence of 1 mm ephyrae, occurred from November to the middle of April. Maximum abundances up to 330 ind.100m-3 were reached in May 2011, few weeks after the last ephyrae release. The population declines afterwards steadily untildisappearing from the water column. Concurrent with increased water temperature and mesozooplankton predation duringMay, growth rates increased from 0.04 mm.day-1 to a peak of 4.5 mm.day-1, with a maximum bell diameter of 11.3 cmreached on the 7th May. During the study period, there was no advection of A. aurita between Thau lagoon and the coastalwaters.Résumé : Etude des changements saisonniers de la dynamique de population d’Aurelia aurita dans la lagune de Thau. Ladynamique de population de la scyphoméduse Aurelia aurita a été étudiée en relation avec des variables environnementalesvia des prélèvements bimensuels de janvier 2010 à juin 2011 dans la lagune de Thau, en Méditerranée Orientale. Lastrobilation, indiquée par la présence d’éphyrules de taille d’environ 1mm, se déroule de Novembre à la mi-avril. Lesabondances maximales de 330 ind.100 m-3, sont observées en mai 2011, quelques semaines après les dernières émissionsd’éphyrules. Les effectifs diminuent ensuite très rapidement jusqu’en juin où A. aurita disparaît de la colonne d’eau. Lesfaibles taux de croissance de 0,04 mm.j-1 observés de janvier à avril augmentent jusqu’à 4,5 mm.j-1 lorsque la températureaugmente et qu’une forte pression de prédation est exercée sur le zooplancton en Mai. Un diamètre d’ombrelle maximumde 11,3 cm est observé le 7 mai. Il n’y a pas eu d’advection d’A. aurita entre les eaux de la lagunes de Thau et la zonecôtière pendant la période de l’étude. Keywords: Aurelia aurita l Lagoon l Mediterranean Sea344 AURELIA AURITA POPULATION DYNAMICSIntroductionThe scyphomedusa Aurelia aurita (Linnaeus, 1758) iswidespread in coastal waters around the world and it isprobably the most studied jellyfish (Lucas, 2001; Lo &Chen, 2008). Although a number of studies on the life cycleof A. aurita have been conducted, the multi-site compar-isons of the seasonal patterns of A. aurita remain scarce(i.e. Miyake et al., 1997), particularly in the MediterraneanSea where only one publication refers to the populationdynamics of this species (Chakroun & Aloui-Bejaouin,1995).Lagoons are semi enclosed ecosystems that offer anideal place to study the life histories of bentho-planktonicpopulations such as A. aurita, since both benthic andpelagic phases coexist. Furthermore, mass reproduction ofA. aurita occurs in inshore waters that warm up muchearlier than offshore waters (Vinogradov et al., 1992). Thaulagoon is the widest lagoon of the Languedoc Roussillonarea with 7500 ha. It is a shallow environment with a maindepth of 8 m. In Thau, oyster and mussel productions are ofhigh importance as they represent 10% of the Frenchnational production. The wood structures supportingbivalve productions are probably very suitable substrata forthe reproductive sessile stage of A. aurita (Uye et al., 2003;Uye & Ueta, 2004). Our objectives were (1) to provide thefirst description of Aurelia aurita pelagic life cycle in theWestern Mediterranean Sea, (2) to compare its populationdynamics with other lagoons worldwide and (3) finally todetermine the temperature and salinity roles in Aureliaaurita life cycle.Material and MethodsPlankton samples are collected twice a month at 4monitoring sampling stations from January 2010 to June2011. Three of these stations are located inside the lagoon:in a harbour (43°19’082N-3°33’532E, 3 m depth), betweenoyster production tables (43°24’644N-3°36’016E, 4.5 mdepth) and nearby a disused shipyard (43°25’329N-3°40’862E, 4.5 m depth). A coastal station (43°21’925N-3°41’453E, 23 m depth) was also monitored in waterssurrounding the lagoon to assess possible links betweenchanges in A. aurita abundance and inflow/outflow watermasses exchanges. Gelatinous plankton was collected byhorizontal tows near surface with a Hansen net (700 µmmesh aperture). A large plexiglass collector was used tokeep the organisms in good shape. Samples were thenpreserved in 4% buffered formaldehyde. We considered A.aurita ephyrae i.e. < 10 mm, while immature organismswere determined from the absence of developed gonads.Bell diameter, sex and the presence of planula larvae weredetermined. As bell size measurements were done onpreserved individuals, size was corrected for formalinshrinkage according to Möller (1980). Temperature andsalinity were recorded in sub-surface (< 0.5 m)simultaneously during sampling with a probe (EC 300VWR international/ WTW model 350i).To detect associations between the abundance of Aurelialife stages and environmental variables (i.e. temperature,salinity) we used a cumulative frequency method and aMonte Carlo randomization, as in Perry & Smith (1994)and Paramo et al. (2003). First, the relative cumulativefrequency distribution (CFD) was calculated fortemperature and salinity. Afterwards, we weighted theCFDs for each environmental variable by multiplying thecurve by the abundance of Aurelia life stages, as in Paramoet al. (2003). The absolute difference between the two CFDcurves, unweighted and weighted, shows whether the lifestages were associated with a certain environmentalwindow or whether they were randomly distributed. In thelatter case two curves will accrue similarly and will not besignificantly different. However, when life stages wereassociated with a particular thermal or salinity window, theslope of the weighted CFD was steeper than that of theunweighted environmental variable. The test of significance of the difference between thecurves was performed by a Monte Carlo test (1000randomizations) under the hypothesis of a randomdistribution. Results and DiscussionSeasonal occurrenceEphyrae production occurs mainly from November untilApril (Fig. 1A). Similar patterns have been observed intemperate ecosystems of the Northern hemisphere whereephyrae production usually occurs in winter through spring,although in some fjords, autumn and spring productionshave been also observed (Lucas & Williams, 1994).Maximum abundances of ephyrae were observed in April2010 and February 2011 (50 and 200 ind.100 m-3respectively), whereas the maximum abundances of adultswere found in June 2010 and May 2011 (100 and 330ind.100 m-3, respectively). Similar values have beenobserved in the Bay of Tokyo (153 ind.100m-3) (Lucas,2001). Immature organisms are observed in March-April,and sexual adults from April to June (Fig. 1B). Abundancedecreased drastically from April to June allowing thepopulation decline, both ephyrae and adults. This sharpdecrease in abundance, likely due to natural mortality, iswell known and has been repeatedly observed, for examplein mid-summer, June-July, in the Black Sea (Vinogradov &D. BONNET, J.C. MOLINERO, T. SCHOHN, M.N. DALY YAHIA 345Shushkina, 1992; Mutlu, 2001) soon after the gametesrelease from the mature adults. From the two-year survey inThau lagoon, it appeared that only one generation per yearoccurs, while in the Black Sea for example, one or twogenerations (winter and spring) might occur (Shushkina &Musayeva, 1983;  Mutlu, 2001). Our observations suggesta A. aurita life span of 7-8 months in Thau lagoon, whichis concurrent with former results in other Europeanecosystems (Lucas & Williams, 1994) while Miyake et al.(1997) suggest that its life span in some bays of Japan couldreach up to two years.The spatial survey here used suggests that A. auritaoriginated from the lagoon as it was not found in the coastalstation. As shown elsewhere, the semi-enclosed system ofthe lagoon might provide refuge, food and substratum topolyp colonies. Indeed, Tokyokawa et al. (2011) indicatedthat in semi enclosed ecosystems, such as harbours protectedby breakwaters, if ephyrae were encountered then it could beconcluded that the polyp colonies could be found nearby. Breeding and growthA. aurita presented a slow growth rate from January to mid-April around 0.04 mm.d-1. Then during the next 2-3 weeks,the growth rate suddenly increased of 100 times up to4.5mm.d-1 leading to sexual adults with an average size of8.42 cm and a maximum observed size of 11.33 cm. Similarsize ranges have been observed in Tomales Bay, USA(Miyake et al., 1997). While growth rates in the range of 3.4cm.week-1 and 4 cm.week-1 were observed respectively byMöller (1980) in Kiel Bight and Lucas & Williams (1994)off Southampton, larger A. aurita are usually found incoastal waters (references in Lucas & William, 1994;Miyake et al., 1997).The period of intense growth coincides with a sharpincrease in temperature (from 11 to 20°C in April) and anintense predation on the mesozooplankton stock (decreasein abundance of ~ 81%). It is not clear however, whymedusae in Thau lagoon do not attain the sizes found inother areas where organisms can reach > 20 cm. A possible,non-exclusive explanation is a food limitation constrain asa result of either their own predatory impact, or throughcompetition for food resources with bivalves which arevery abundant in the lagoon.Between May and June 2010, the bell size decreasedfrom 19% at the rate of 1.4 mm.d-1, and the populationstarted disappearing from the water column. While thereasons for this decrease in bell size are still discussed(food limitation, decrease in the metabolic activity), thesize reduction is in the range of those usually observed,between 10 and 20% (Möller, 1980; Lucas & Williams,1994). The sex ratio was well balanced with 49% of malesand 51% of females. Light microscopic examination of theorganisms showed that up to 1528 eggs were carried in thebrood sacs of a single individual. Temperature and salinityThe time of occurrence of A. aurelia in the pelagic planktonis highly variable among localities because of environmen-tal differences (reviewed by Miyake et al., 1997). In Thau,maximum abundances of A. aurita ephyrae occurred at16°C, although they could be observed within a largetemperature window, between 5 and 26°C (Fig. 2B). Thepresence of ephyrae was continuous from November toApril, and afterwards disappeared from the water columnas temperature increased over 16°C. Lo & Chen (2008)have recorded an inverse correlation between A. aureliaephyrae abundance and temperature in Tapong Bay. Theyindicated that ephyrae were almost absent during the warmand wet season (May-October). In Thau, the mature adultswere preferentially encountered around 19°C (Fig. 2A).When temperature increased above this optimum, there wasa total disappearance of the adults from the water column.However, the review in Lo & Chen (2008) indicated that A.auritamedusae were repeatedly encountered at temperature> 20°C.Salinity optima are similar for both A. aurita ephyraeand adults (34.8 and 35.3 respectively, Fig. 2C & D).Figure 1. Aurelia aurita. A.Abundance  (ind.100 m-3) in Thaulagoon. B. Pelagic life cycle.Figure 1. Aurelia aurita. A. Abondance (ind.100m-3) dans lalagune de Thau. B. Partie pélagique du cycle de vie d’Aureliaaurita.Supporting Lo & Chen’s results (2008), no significantcorrelation between salinity and A. aurelia abundance(ephyra or adult) was found. This is probably due toextreme inflow events which can affect abruptly thesalinity. Indeed, intense precipitations often occur in fall.Salinity conditions strongly vary as the freshwater inflowsinto the lagoon vanish in summer which leads to anincrease of salinity up to 39.3. The contrasting hydrologicalseasons characterized by strong temperature and salinitychanges are hypothesized as a further reason why A. auritadisappears from the water column in Thau lagoon from Julyto November.Figure 3 synthesises the occurrence of A. aurita in 3Western Mediterranean lagoons. The medusae occurroughly at the same salinity in Bizerte and in Thau (32.5 to38 and 33 to 39.3, respectively), however, they are presentat temperatures (between 12.5 and 27.5°C) higher than inThau (between 7 and 24°C). In Berre, with contrastingenvironmental conditions in comparison with the two firstlagoons, i.e. brackish waters, A. aurita is encounteredbetween 16.5 and 26.5. In addition, the thermal nicheoverlaps with those of Bizerte and Thau with a range of 4to 27°C.These results highlight the plasticity of the species tolive and reproduce in contrasted environments. Theyfurther support former observations showing that A. auritamight inhabits a broad window of environmental conditionsranging from salinity from 10 to 32 and temperature from -2 up to 30°C (Miyake et al., 1997). The variability in theenvironmental ranges A. aurita inhabits in the westernMediterranean raise several questions to thoroughly assessits life history, in particular the plasticity of the species to346 AURELIA AURITA POPULATION DYNAMICSFigure 2. Aurelia aurita. Cumulative frequency distribution (CFD) of the habitat variables. A & B. Temperature. C & D. Salinity.unweighted CFD: thick line, weighted CFD: dotted line, absolute difference between unweighted and weighted: filled areas.Figure 2. Aurelia aurita. Distribution des fréquences cumulées (CFD) des variables définissant l’habitat. A & B. Température. C &D. Salinité. Les CFD non pondérés sont représentés par une ligne épaisse, les CFD pondérés par une ligne pointillée, les aires grisesreprésentent la différence absolue entre les CFD non pondérés et pondérés.evolve in changing environments and whether crypticspecies exist in the Mediterranean lagoons. This first studycould be used as baseline for further investigations on theA. aurita dynamics in the Mediterranean Sea with somemolecular biology approaches urgently needed.ReferencesChakroun F. & Aloui-Bejaouin N. 1995. Invasion d’Aureliaaurita (Cnidaria, Scyphomédusa) dans le lac de Bizerte(Tunisie) au cours de l’été 1994. Annales de l’Institutocéanographique, 71: 67-69.Lo W.T. & Chen I.L. 2008. Population succession and feeding ofthe scyphomedusae, Aurelia aurita, in a eutophic tropicallagoon in Taiwan. Estuarine, Coastal and Shelf Science, 76:227-238.Lucas C.H. 2001. Reproduction and life strategies of the commonjellyfish Aurelia aurita in relation to its ambient environment.Hydrobiologia, 451: 229-246.Lucas C.H. & Williams J.A. 1994. Population dynamics of thescyphomedusae Aurelia aurita in Southampton water. Journalof Plankton Research, 16: 879-895.Miyake H., Iwao K. & Kakinuma Y. 1997. Life history andenvironment of Aurelia aurita. South Pacific Study, 17: 273-285.Möller H. 1980. Population dynamics of Aurelia Aurelia medusain Kiel Bight, Germany. Marine Biology, 60: 123-128.Mutlu E. 2001. Distribution and abundance of moon jellyfish(Aurelia aurita) and its zooplankton food in the Black Sea.Marine Biology, 138: 329-339.Paramo J., Quiñones R.A., Ramirez A. & Wiff R. 2003.Relationship between abundance of small pelagic fishes andenvironmental factors in the Colombian Caribbean Sea: ananalysis based on hydroacoustic information. Aquatic LivingResources, 16: 239-245.Perry R.I. & Smith S.J. 1994. Identifying habitat associations ofmarine fishes using survey data: an application in theNorthwest Atlantic. Canadian Journal of Fisheries andAquatic Sciences, 51: 589-602.Shushkina, E.A. & Musayeva, E.I. 1983. The role in the energysystem of the Black Sea plankton communities. Oceanology,23: 92-96.Toyokawa M., Aoki K., Yamada S., Yasuda A., Murata Y. &Kikuchi T. 2011. Distribution of ephyrae and polyps of jelly-fish Aurelia aurita (Linnaes 1758) sensu lato in Mikawa Bay,Japan. Journal of Oceanography, 67: 209-218.Uye S. & Ueta Y. 2004. Recent increase of Jellyfish populationsand their nuisance to fisheries in the Inland Sea of Japan.Bulletin of the Japanese Society of Fisheries Oceanography,68: 9-19 (in Japanese with English Abstract).Uye S., Fujii N. & Takeoka H. 2003. Unusual aggregations of thescyphomedusa Aurelia aurita in coastal waters along westernShikoku. Plankton Biology and Ecology, 50: 14-21.Vinogradov M.Ye & Shushkina E.A. 1992. Temporal changes incommunity structure in the open Black Sea. Oceanology, 32:485-491.D. BONNET, J.C. MOLINERO, T. SCHOHN, M.N. DALY YAHIA 347Figure 3. Aurelia aurita. Temperature and salinity windows ofoccurrence in 3 Mediterranean lagoons: Berre (M), Bizerte (B)and Thau (T).Figure 3. Aurelia aurita. Fenêtres de distribution en fonctionde la température et de la salinité dans 3 lagunesMéditerranéennes : Berre (M), Bizerte (B) et Thau (T).

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=19B97B08DE09ADF7B6EDFA0BAE1015B5?doi=10.1.1.1039.9106&rep=rep1&type=pdf

Seasonal changes in the population dynamics of Aurelia aurita in Thau lagoon

Abstract

Similar works

Full text

Available Versions

CiteSeerX

OceanRep