abundance, growth and food demand of the scyphomedusa aurelia aurita in the western wadden sea

Netherlands Journal of Sea Research 19 (1): 38-44 (1985)

ABUNDANCE, GROWTH AND FOOD DEMAND OF THE SCYPHOMEDUSA AURELIA AURITA IN THE WESTERN WADDEN SEA

H.W. VAN DER VEER and W. OORTHUYSEN

Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg Texel, The Netherlands

ABSTRACT

Medusae of Aurelia aurita are found in the western Wadden Sea from the beginning of May till August with maximum numbers of 250 to 500 individuals per 103 m 3 during May.June. The exis- tence of a continuous ebb surplus suggests an origin from polyps living in the inner parts of the estuary and a transport or migration of the released medusae towards the North Sea. Growth is fast; a bell size of 20 cm diameter is reached within 3 to 4 months. The species is important as a predator from May to July, reaching maximum carbon biomass values of 12 to 18 g C.103 m -3. Predation by A. aurita may affect the recruitment of one of its food sources, v/z. fish larvae.

1. INTRODUCTION

For a long time medusae have been recognized as important predators in the pelagic food chain. However, quantitative data about their seasonal occurrence and biomass are scarce. Most of the knowledge concerns the scyphomedusa Aurel ia aur i ta which population dynamics have been studied in the North Sea (HAY & HISLOP, 1979; M£)LLER, 1980a), in the Baltic (M(~LLER, 1980a) and in Japanese waters (YASUDA, 1968, 1969).

The potential growth of Aurel ia auri ta appears to be fast. This means that a high food demand must be expected, possibly resulting in a heavy predation pressure on zooplankton and fish larvae (M(3LLER, 1980b).

For a number of fish species entering the Wad- den Sea during their larval stage (CREUTZBERG et al., 1978; CORTEN & VAN DE KAMP, 1979) this estuary acts as an important nursery area (ZIJLSTRA, 1972, 1978). Therefore, a heavy predation by Aure-

l ia aur i ta on larval fish in the Wadden Sea could significantly influence the recruitment of certain fish species, as observed for herring in Kiel Bight (MOLLER, 1984).

This paper describes the seasonal occurrence and growth of Aurel ia auri ta in the western Wad- den Sea, while an attempt is made to evaluate its impact as a predator of zooplankton and larval fish.

Acknowledgements.--Thanks are due to C.F.M. Sadee, H. van Garderen and the crews of ms "Griend" and ms "Navicula" for assistance during sampling, and to G.P. Baerends, Mrs. G. van der Wolf, P. de Wolf and J.J. Zijlstra for critical reading of the manuscript.

These investigations were supported in part by the Foundation for Fundamental Biological Research (BION), which is subsidized by the Netherlands Organization for the Advancement of Pure Research (ZWO).

2. MATERIAL AND METHODS

From February to August 1981 and from February to December 1982 series of samples were collected in tidal channels ranging from 5 to 10 m in depth near the main entrance of the western part of the Dutch Wadden Sea (Fig. 1). Usually 10 to 20 hauls were made every week, both during ebb and flood tide.

All samples were collected by making double oblique hauls using a plankton net with an opening of 0.7 m 2 and a length of 5 m. The net was constructed of polyamid plankton gauze (Monodur 2000) and had a mesh size of 2.0 ram. The amount of water passing through the net was measured with a Savonius flow meter, placed in the opening of the net. The duration of

AURELIA IN THE DUTCH WADDEN SEA 39

1 NORTH SE~ I ~ ~ "

Fig. 1. Location of the sampling stations (ll) in the western Wadden Sea.

the hauls varied between 10 and 45 minutes, de- pending on the strength of current and density of the coelenterates. Per haul a volume of at least 300 m 3 (up to 2000 m 3) water was fi ltered. In a previous paper (VAN DER VEER & SADI=!:E, 1984) it was demonstrated, on the basis of current- measurements inside and outside the net, that c logging and overf low hardly occurred even during and after a heavy bloom of Phaeocystis poucheti i .

In this study, Aurelia ephyrae (diameter <1 cm) were not taken into considerat ion; only medusae larger than 1 cm were sorted and count- ed. Medusae smaller than 4 cm were preserved in 4% formaldehyde, measured in the laboratory to the mm below wi th in a few weeks and their sizes corrected for shrinkage (10% for all sizes (OORTHUYSEN & SADEE, 1982)). Specimens larger than 4 cm were freshly measured to the 0.5 cm below.

Al l densi t ies were expressed as numbers per 103 m 3 (n-103 m-3). Wet weight W (g) was calculated from diameter D (cm) according to:

W = 0.07 D 2.8

and dry weight was est imated as 0.018 W (KER- STAN, 1977). Organic carbon content equals about 8.4% of the dry weight, according to MOLLER (1980b), which factor was used in calculat ing carbon biomass.

3. RESULTS

3.1. ABUNDANCE

Aurelia aurita showed a simi lar pattern of occurrence in both years (Fig. 2). The first medusae ap- peared in April, be it in densit ies of only a few indiv iduals per 103 m 3. in May the numbers in- creased rapidly up to a maximum of 210 individuals per 103 m 3 in 1981 and 490 per 103 m 3 in 1982. Then, a fast decrease started result ing in a complete absence of the species from the end of Au- gust onwards. The somewhat less regular pattern in 1982 wi l l partly be caused by the lower sampl ing intensity.

In 1981 sampl ing intensi ty was high enough to permit a division of the hauls into ebb and flood (Table 1). A s igni f icant ebb surplus did exist (p <0.01; n = 7 ) ind ica t ing a net transport of medusae from the Wadden Sea towards the North Sea. The mean coeff ic ient of variat ion of all the catches per week f luctuated from 66% to 259%, with a mean value of 142%. When ebb and flood samples were separated, this mean coeffi- cient was 124% for the f lood and 100% for the ebb.

3.2. GROWTH

The mean diameter of Aurelia aurita progressed at approximately equal rates in 1981 and 1982 (Fig. 2). At the end of Apri l the mean diameter of the medusae was a few cm. From May on there

TABLE 1 Weekly mean densities (n.103 m -3) of Aurelia aurita

during flood and ebb tides in 1981.

Date Numbers caught

Flood Ebb

11-5 4 59 18-5 165 252 25-5 196 247 2-6 38 78 9-6 10 25

23-6 2 7 7-7 6 18

40 H.W. VAN DER VEER & W. OORTHUYSEN

obundance (n. ~o 3 m -3)

3(30"

Z50

200

150

.............. / \ v .

1981

mean diomeler (cm)

24

2o

16

0

.

.7 / /

t981

5O0

450

400

350

300

250

200

150

I00

50-

O-

20-

1982 16 •

t2-

e ,

/.U / /

/

1982

F M A M J J A S 0 N D F M A M J J A S 0 N D

Fig. 2. Abundance (n.103 m -3) and mean diameter (cm) of Aurelia aurita in te western Wadden Sea in 1981 and t982.

was an almost continuous increase ti l l July- August when they disappeared completely from the Wadden Sea. This growth resulted in a mean diameter of 24 cm in July 1981 and 20 cm in Au- gust 1982.

The size distr ibut ions in the course of the sea- son (Fig. 3) showed a regular pattern of increase in the larger size groups, which pointed to the presence of a single cohort, a relatively short period of strobulation and regular growth. There- fore, the observed increase in diameter is probably a fair reflection of the growth of the individual specimens.

3.3. BIOMASS

The Aurel ia auri ta population built up a large biomass amounting to 12.5 g C-103 m -3 in May 1981 and to 17.5 g C.103 m -3 in May 1982 (Fig. 4a). Thereafter the biomass declined gradually, reaching low values by the middle of August. In

terms of food intake and predation pressure, A. auri ta is l ikely to be of major importance in the period of high abundance and biomass, i.e. during the months May and June.

4. DISCUSSION

The life cycle of Aurel ia auri ta is relatively well known (SARS, 1841; THIEL, 1962; HAMNER & JENS- SEN, 1974). In spring the ephyrae develop by asexual reproduction from the scyphistomae, the transformed polyps. After detaching, these pelagic ephyrae transform rapidly into medusae, after which a fast increase of the bell-diameter fol lows to - 1 0 to 15 cm within a few months. In summer the medusae become sexually mature. After spawning and the release of planula larvae, which developed inside the gastrodermal sys- tem, the medusae usually die in autumn. The planktonic larvae disperse and f inal ly settle on suitable substrates. Here they metamorphose


. . . . . . . . 0~o,00o~, it occurred within the range 6 to 10°C. Although (n io s m ~

. . . . . . . s t robu la t i on h a p p e n s a s a rule in spr ing (for a ......... ~ . . . . . . . . . . . . . . . . . . . . . . . . . . review see VERWEY, 1942), in certain areas, as for , ..... .,, instance the Danish fjords (HORTENSIUS, per- ii ~ ~ '~!~_ ~ sonal communication), small medusae are also

. . . . . . . . . . . . . . . . . . . . . . . . . . ;~ilF, . . . . . . . . . . . . . . . . . . found in October-November. For The Nether- . . . . . . . . . °~°" ~!~ l~ . . . . lands one such observation has. been reported by 0 ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . KORRINGA & BURGERS (1944; in: VAN ERe, 1958),

1915

be it not in the Wadden Sea. THIEL (1962) demonstrated that in the inner parts of Kiel Bight the sessile polyps are found at sal ini t ies of about 10.10 -3 , especial ly on sluice gates both on the estuarine and on the fresh water side. The signifi- cant ebb surplus of A. aur i ta medusae in the western Wadden Sea also suggests that the polyps live in the inner parts of the estuary.

Once released as ephyrae in the plankton the ~o,o,~ ~,~ ~ . . . . . ebb surplus of the medusae indicates a transport

, o ,

. . . . . . . . . ~ - . . . . . . . . . . . ~ - ~ - ~ ~ . . . . . . . . . from the inner parts of the Wadden Sea towards

20 J Z 5 / 6 2 0 - 2 3 / 6

C) • > ,

the outer parts and the coastal zone of the North Sea. This transport results in a decreasing density seawards from the source as well as in a con- t inuous decrease of numbers within the Wadden Sea itself. However, part of the observed reduction in numbers in the Wadden Sea will be caused by mortality. Many stranded and dead A.

2~t , . ~oJ 7J7 aur i ta medusae are found on the beaches and '~l- Z, o.-o...,~.~S~h~ , ~ , ~ 'i~ . . . . . . . . . . . . . . . . . . . . . . edges of the tidal channels in summer. So far,

dlomete, (£ml

Fig. 3. Size histograms (n.103 m -3) of Aurelia aurita in the western Wadden Sea in 1981 and 1982.

into scyphistomae, the asexual stage of the life cycle. These small sessile polyps form ephyrae in the next year. As a rule the species may be considered as annual (HAMNER & JENSSEN, 1974; SPANGENBERG, 1965; MOLLER, 1980C), although sometimes overwintering specimens have been recorded (HAMNER & JENSSEN, 1974).

In the western Wadden Sea the seasonal occurrence of Aure l i a au r i t a f i ts in with the life cycle described. The appearance of the first medusae in April indicates strobulation as early as March and even February, which is in accor- dance with the observations by VERWEY (1942) and VAN ERP (1958). Strobulation continues up to approximately the end of May, the t ime at which the last small medusae were caught. VAN ERP (1958) showed that strobulation depends on water temperature: under laboratory condit ions

predators of young A. au r i t a are unknown (MOLLER, 1980C).

This movement of Aure l i a aur i ta from the inner parts of the Wadden Sea towards the North Sea is the opposite of that observed in another coe- lenterate, the ctenophore P l e u r o b r a c h i a p i l e u s

(VAN DER VEER & SADI~E, 1984). For the latter species a signif icant flood surplus was observed, in- dicating that P. p i l e u s is transported most l ikely passively from outside the area towards the inner parts where it will accumulate and most probably wash ashore in the intertidal area. Spe- cies like the eel, A n g u i l l a a n g u i l l a (CREUTZBERG, 1961) and the crab M a c r o p i p u s h o l s a t u s (VENEMA & CREUTZBERG, 1973) show an active behaviour for either an inward or a seaward migration. The signif icant ebb surplus as observed for A. aur i ta

is most probably not only caused by diffusion, but also by an active behaviour of the medusae, as e.g. selective swimming or vertical migration during certain stages of the tide.

In order to complete the life cycle, the planula larvae have to reach the locations of the old polyps in the inner parts of the Wadden Sea area. Once released by the medusae a passive transport as observed for barnacle larvae (DE WOLF,


biomer~

( 9 0 , 1 ~ -3)

,21 ,oi

.i 61 41

oi

,81

12

. . . . . . . . . . . . . . . . . /

A B

1981

/

1982

. . . . . . . . . . . . . . . . . . . . ,

production (gC IO~-& d -~ )

Z.O-

LS-

I.O-

O5

O

25

20

15

I0

05

1981

1982

F M A M J J A S O N D F M A M J J A S O N D

Fig. 4. Aure/ia aurita in the western Wadden Sea in 1981 and 1982. a. Carbonbiomass(gC.103m 3) b. Estimated daily production rates (g C-103 m-3.d-1).

1973) wil l be suff icient to result in accumulation in the inner parts of the estuary.

The strobulation in the Wadden Sea in spring coincides with that in Kiel Bight (M(3LLER, 1980C). THIEL (1962) found that both water temperature and food supply can affect strobulation. In accor- dance with other observations (SARS, 1841; DAVlDSON & HUNTSMAN, 1926) growth of the pelagic medusae appears to be fast in the Wad- den Sea area and is also rather simi lar with growth rates observed in Kiel Bight (MOLLER, 1980C), where an increase in bell diameter was found from a few cm up to - 2 0 cm during the period of May to September. Size reduction in September and October as observed by MOLLER (1980C) in Kiel Bight was not found in the Wad- den Sea. During that period all Aurelia medusae have already disappeared from the Wadden Sea and are found in the coastal zone of the North Sea, from where at present only scarce growth data are available.

Although growth in Aurelia is always fast, the final bell size differs from year to year. M(3LLER (1980C) suggests that either differences in water temperature or in food supply are responsible. The difference in final bell diameter found between the 1981 and the 2 t imes stronger year- class of 1982 may suggest food l imitat ion. Growth rates in the 2 years, however, were equal in May although in 1982 the densit ies were 2 t imes higher in that month, whereas in June and July the growth differed between the years and densit ies were nearly equal.

In order to calculate the food demand of the Aurelia population, growth curves at different temperatures and optimal food condit ions are necessary. The only data available are those of HAMNER & JENSSEN (1974) who measured a rate of bell size increase of 0.1 cm.d -1 during 80 days at 16 to 18°C. As the observed diameter increase of the Wadden Sea population is higher in almost all cases, a calculation of the potential


food demand is not possible. Instead the actual product ion is est imated from the weekly increase of the mean body weight, and the mean numbers per 103 m 3 (to avoid negative values sometimes longer t ime intervals have been used for the calculat ion). Product ion is high only during May and June when carbon product ion values of 1 to 2 g C.103 m-3.d -1 are reached (Fig. 4b). The sharply decreasing product ion in June is deceptive. It was not caused by a smaller individual growth rate but by low numbers in the Wadden Sea due to mortal i ty and emigration.

To est imate the food intake of the populat ion the product ion f igures have to be converted into the amounts of food actual ly consumed. The only avai lable data for this conversion are those of FRASER (1969) who calculated a growth effi- ciency of 37%, based on only one observation. According to this conversion, the consumpt ion would be roughly 3 t imes the product ion, leading to carbon consumpt ion rates of 2.5 to 6 g C.103 m-3 .d -1 in May and June. During this period the ctenophore Pleurobrachia pileus with a popula- t ion biomass simi lar to that of Aurelia is also important as a predator of zooplankton (VAN DER VEER & S,~,DEE, 1984). With respect to their food spectrum the two species may differ. A l though both are known as consumers of zooplankton (copepods) and fish larvae, A. aurita prefers f ish larvae, as LEBOUR (1923) already found in experiments and in the field. FRASER (1969) used copepods, and larvae of cod and American f lounder as normal food in his experiments wi th Aurelia. Strong predation of Aurelia on larval herring is reported, both in the field (MOLLER, 1980b, 1984) and in experiments (ARAI & HAY, 1982; BAILEY & BATTY, 1983).

A l though stomach content analyses are lack- ing, predation of Aurelia aurita on fish larvae can also be expected in the Wadden Sea. VAN DER VEER & SADI~E (1984) calculated, partly based on l i terature data, an average carbon biomass of f ish larvae of about 0.7 g C.103 m -3 in the Wad- den Sea in May-June, mainly consist ing of herring, sprat, f lounder and plaice larvae. In comparison with the consumpt ion rates estimated it is clear that these f ish larvae cannot cover the total food demand of A. aurita. This means, that in the Wadden Sea, that acts as a nursery area for these fish species (ZIJLSTRA, 1972, 1978), the pelagic larvae may suffer under a heavy predation pressure by mainly A. aurita and partly Pleurobrachia pileus. To what extent this preda- t ion inf luences the recruitment to the f ish stocks

in quest ion is unknown, but an effect seems plausible. Flatf ish species wil l , in contrast to herring and sprat, profit from their rapid disappear- ance from the plankton by sett l ing in the intert idal zone (CREUTZBERG et al., 1978; BERG- MAN et al., 1980).

Zooplankton in the Wadden Sea, mainly consist ing of copepods and larvae of benthic organisms in May-June, occurs with carbon biomass values of 40 to 60 g C.103 m -3 (FRANSZ, 1981) and probably covers the remaining part of the food requirements of Aurelia aurita. The cont inuous increase of the zooplankton stock ti l l July suggests an only moderate effect of coelenter- ate predation on the zooplankton populat ion in th is estuarine area. Besides, it seems likely that these zooplankton populat ions are replenished cont inuous ly from the coastal area where preda- t ion by coelenterates may be even less important.

5. REFERENCES

ARAI, M.N. & D.E. HAY, 1982. Predation by medusae on Pacific herring (Clupea harengus pallasi) larvae.--Can. J. Fish. Aq. Sci. 39: 1537- 1540.

BAILEY, K.M. & R.S. BATTY, 1983. A laboratory study of predation by Aurelia aurita on larval herring (Clupea harengus): Experimental observations compared with model predictions.--Mar. Biol. 72: 295-301.

BERGMAN, M.J.N., P.J. SPLIETHOFF & H.W. VAN DER VEER, 1980. Ecologie van O-groep schol (Pleuronectes platessa) op het Balgzand. Deel I. Aantalsverloop, verspreiding en getijdenmigratie. Interne Verslagen Nederlands Instituut voor On- derzoek der Zee, Texel, 1980-9: 1-40,

CORTEN, A. & G. VAN DE KAMP, 1979. Abundance of herring larvae in the Dutch Wadden Sea as a possible indication of recruitment strength. ICES C.M. 1966/C 3: 1-5.

CREUTZBERG, F., 1961. On the orientation of migrating elvers (Anguilla vulgaris Turt.) in a tidal area.-- Neth. J. Sea Res. 1: 257-338.

CREUTZBERG, F., A.T.G.W. ELTINK & G.J. VAN NOORT, 1978. The migration of plaice larvae Pleuronectes platessa into the western Wadden Sea. In: D.S. McLUSKY & A.J. BERRY. Proc. 12th Europ. Mar. Biol. Syrup. Pergamon Press, Oxford, New York: 243-251.

DAVlDSON, V.M. & A.G. HUNTSMAN, 1926. The causation of diatom maxima.--Trans. R. Soc. Can. Ser. 5: 119-125.

ERP, J. VAN, 1958. Enkele waarnemingen betreffende de ongeslachtelijke voortplanting van de Scyphozoa: Aurelia aurita (Lam.); Cyanea capillata (L.) en Chrysaora hysoscella (L.). Interne Verslagen Nederlands Instituut voor Onderzoek der Zee, Tex- el, 1958-1: 1-37.


FRANSZ, H.G., 1981. Quant i tat ive data on the plankton of the Wadden Sea proper. In: N. BANKERS, H. KOHL & W.J. WOLFF. Ecology of the Wadden Sea, 1. Balkema, Rotterdam: 125-133.

FRASER, J.H., 1969. Experimental feeding of some medusae and chaetognatha.- -J . Fish. Res. Bd Can. 26: 1743-1762.

HAY, S.J. & R.J. HISLOP, 1979. The distr ibut ion and abundance of scyphomedusae in the North Sea during the summer of 1978. ICES C.M. 1979/L 35: 1-4.

HAMNER, W.M. & R.M. JENSSEN, 1974. Growth, degrowth and irreversible cell di f ferent iat ion in Aurelia aurita . - -Am. Zoologist. 14: 833- 849.

KERSTAN, M., 1977. Untersuchungen zur Nahrungs- 5kologie von Aurelia aurita Lain. Diplomarbeit Universit&t Kiel: 1-95.

LEBOUR, M., 1923. The food of plankton organisms II .-- J. mar. biol. Ass. U.K. 13: 70-92.

M(~LLER, H., 1979. Signif icance of coelenterates in relation to other plankton organisms.--Ber, dt. wiss. Kommn Meeresforsch. 27: 1-18.

- - - - , 1980a. A summer sur:vey of large zooplankton, part icularly scyphomedusae in North Sea and Bal- t ic . - -Ber, dt. wiss. Kommn Meeresforsch. 28: 61-68.

- - - - , 1980b. Scyphomedusae as predators and food competi tors of larval f ish.--Ber, dr. wiss. Kommn Meeresforsch. 28: 90-100.

- - - - , 1980c. Population dynamics of Aurelia aurita medusae in Kiel Bight, Germany (FRG).--Mar. Biol. 60: 123-128.

M(~LLER, H., 1984. Reduction of a larval herring popula- t ion by jel lyf ish predator.--Science, N.Y. 224: 621-622.

OORTHUYSEN, W. & C.F.M. SABINE, 1982. Voorkomen en groei van Pleurobrachia pileus en Aurelia aurita in de westel i jke Waddenzee en hun mogel i jke rol als predator van platvislarven. Interne Verslagen Nederlands Instituut voor Onderzoek der Zee, Tex- el (unpublished report): 1-28.

SARS, M., 1841.0ber die Entwicklung der Medusa Aure- lia aurita und der Cyanea capil lata.--Arch. Natur- gesch. 7: 9-34.

SPANGENBERG, D.B., 1965. Cult ivat ion of the life stages of Aurelia aurita under control led condi t ions.--J. exp. Zool. 159: 303-318.

TH~EL, H., 1962. 0ntersuchungen Liber die Strobulat ion von Aurelia aurita Lam. an einer Population der Kieler FSrde.--Kieler Meeresforsch. 13: 198-230.

VEER, H.W. VAN DER & C.F.M. SADISE, 1984. The seasonal occurrence of the ctenophore Pleurobrachia pileus in the western Dutch Wad- den Sea.--Mar. Biol. 79: 219-227.

VENEMA. S.C. & F. CREUTZBERG, 1973. Seasonal migra- t ion of the swimming crab Macropipus holsatus in an estuarine area control led by t idal s t reams.- - Neth. J. Sea Res. 7: 94- 102.

VERWEY, J., 1942. Die periodizit&t im Auftreten und die aktieven und passieven Bewegungen der Qual len.--Archs n~erl. Zool. 6: 365-468.

WOLF, P. DE, 1973. Ecological observations on the mechanisms of dispersal of barnacle larvae during planktonic life and settl ing. - -Neth. J. Sea Res. 6: 1-129.

YASUDA, T., 1968. Ecological studies on the jelly-fish Aurelia aurita in Urazoka Bay, Fukui Prefecture. I1. Occurrence pattern of the ephyra.--Bul l . Jap. Soc. scient. Fish. 34: 983-987.

- - - - , 1969. Ecological studies on the jelly-fish Aurelia aurita in Urazoka Bay, Fukui Prefecture. I. Occur- rence pattern of the medusa.--Bul l . Jap. Soc. scient. Fish. 35: 1-6.

ZIJLSTRA, J.J., 1972. On the importance of the Wadden Sea as a nursery area in relation to the conserva- t ion of the southern North Sea fishery resources.--Symp, zool. Soc. Lond. 29: 233-258.

- - - - , 1978. The function of the Wadden Sea for the members of its fish fauna. In: N. BANKERS, W.J. WOLFF & J.J. ZIJLSTRA. Ecology of the Wadden Sea, 2. Balkema, Rotterdam: 20-32.

abundance, growth and food demand of the scyphomedusa aurelia aurita in the western wadden sea

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