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 D E M A N D OF THE S C Y P H O M E D U S A 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 existence 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 Aurelia aurita 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 Aurelia aurita 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 Wadden Sea during their larval stage (CREUTZBERGet al., 1978; CORTEN & VAN DE KAMP, 1979) this estuary acts as an important nursery area (ZIJLSTRA, 1972, 1978). Therefore, a heavy predation by Aure-

lia aurita 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 Aurelia aurita in the western Wadden 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

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39

and dry weight was estimated as 0.018 W (KERSTAN, 1977). Organic carbon content equals about 8.4% of the dry weight, according to MOLLER (1980b), w h i c h factor was used in calculating carbon biomass. 3. RESULTS 3.1. ABUNDANCE

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

the hauls varied between 10 and 45 minutes, depending 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 filtered. In a previous paper (VAN DER VEER & SADI=!:E, 1984) it was demonstrated, on the basis of currentmeasurements inside and outside the net, that clogging and overflow hardly occurred even during and after a heavy bloom of Phaeocystis pouchetii. In this study, Aurelia ephyrae (diameter <1 cm) were not taken into consideration; only medusae larger than 1 cm were sorted and counted. Medusae smaller than 4 cm were preserved in 4% formaldehyde, measured in the laboratory to the mm below w i t h i n 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. All densities 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 D2.8

Aurelia aurita showed a similar pattern of occurrence in both years (Fig. 2). The first medusae appeared in April, be it in densities of only a few individuals per 103 m 3. in May the numbers increased 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 resulting in a complete absence of the species from the end of August onwards. The somewhat less regular pattern in 1982 will partly be caused by the lower sampling intensity. In 1981 sampling intensity was high enough to permit a division of the hauls into ebb and flood (Table 1). A significant ebb surplus did exist (p <0.01; n = 7 ) i n d i c a t i n g a net transport of medusae from the Wadden Sea towards the North Sea. The mean coefficient of variation of all the catches per week fluctuated from 66% to 259%, with a mean value of 142%. When ebb and flood samples were separated, this mean coefficient was 124% for the flood 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 April 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

11-5 18-5 25-5 2-6 9-6 23-6 7-7

Numbers caught Flood

Ebb

4 165 196 38 10 2 6

59 252 247 78 25 7 18

40

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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 till JulyAugust 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 August 1982. The size distributions in the course of the season (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. Therefore, the observed increase in diameter is probably a fair reflection of the growth of the individual specimens. 3.3. BIOMASS The Aurelia aurita 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. aurita is likely 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 Aurelia aurita is relatively well known (SARS, 1841; THIEL, 1962; HAMNER & JENSSEN, 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 follows 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 system, the medusae usually die in autumn. The planktonic larvae disperse and finally settle on suitable substrates. Here they metamorphose

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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 A u r e l i a a u r i t a fits 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 accordance with the observations by VERWEY (1942) and VAN ERP (1958). Strobulation continues up to approximately the end of May, the time at which the last small medusae were caught. VAN ERP (1958) showed that strobulation depends on water temperature: under laboratory conditions

41

it occurred within the range 6 to 10°C. Although s t r o b u l a t i o n h a p p e n s a s a rule in s p r i n g (for a review see VERWEY, 1942), in certain areas, as for instance the Danish fjords (HORTENSIUS, personal communication), small medusae are also found in October-November. For The Netherlands one such observation has. been reported by KORRINGA & BURGERS (1944; in: VAN ERe, 1958), be it not in the Wadden Sea. THIEL (1962) demonstrated that in the inner parts of Kiel Bight the sessile polyps are found at salinities of about 10.10 -3 , especially on sluice gates both on the estuarine and on the fresh water side. The significant ebb surplus of A. a u r i t a 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 ebb surplus of the medusae indicates a transport from the inner parts of the Wadden Sea towards 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 continuous 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. a u r i t a medusae are found on the beaches and edges of the tidal channels in summer. So far, predators of young A. a u r i t a are unknown (MOLLER, 1980C). This movement of A u r e l i a a u r i t a from the inner parts of the Wadden Sea towards the North Sea is the opposite of that observed in another coelenterate, 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 significant flood surplus was observed, indicating that P. p i l e u s is transported most likely passively from outside the area towards the inner parts where it will accumulate and most probably wash ashore in the intertidal area. Species 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 significant ebb surplus as observed for A. a u r i t a 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,

42

H.W. VAN DER VEER & W. OORTHUYSEN

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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) will be sufficient 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 accordance with other observations (SARS, 1841; DAVlDSON & HUNTSMAN, 1926) growth of the pelagic medusae appears to be fast in the Wadden Sea area and is also rather similar 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 Wadden 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 times stronger yearclass of 1982 may suggest food limitation. Growth rates in the 2 years, however, were equal in May although in 1982 the densities were 2 times higher in that month, whereas in June and July the growth differed between the years and densities were nearly equal. In order to calculate the food demand of the Aurelia population, growth curves at different temperatures and optimal food conditions 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

AURELIA IN THE DUTCH WADDEN SEA

food demand is not possible. Instead the actual production is estimated from the weekly increase of the mean body weight, and the mean numbers per 103 m 3 (to avoid negative values sometimes longer time intervals have been used for the calculation). Production is high only during May and June when carbon production values of 1 to 2 g C.103 m - 3 . d -1 are reached (Fig. 4b). The sharply decreasing production in June is deceptive. It was not caused by a smaller individual growth rate but by low numbers in the Wadden Sea due to mortality and emigration. To estimate the food intake of the population the production figures have to be converted into the amounts of food actually consumed. The only available data for this conversion are those of FRASER (1969) who calculated a growth efficiency of 37%, based on only one observation. According to this conversion, the c o n s u m p t i o n would be roughly 3 times the production, leading to carbon c o n s u m p t i o n 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 population biomass similar 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 t h o u g h both are known as consumers of zooplankton (copepods) and fish larvae, A. aurita prefers fish larvae, as LEBOUR (1923) already found in experiments and in the field. FRASER (1969) used copepods, and larvae of cod and American flounder as normal food in his experiments with 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 t h o u g h stomach content analyses are lacking, 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 literature data, an average carbon biomass of fish larvae of about 0.7 g C.103 m -3 in the Wadden Sea in May-June, mainly consisting of herring, sprat, flounder and plaice larvae. In comparison with the c o n s u m p t i o n rates estimated it is clear that these fish 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 predation influences the recruitment to the fish stocks

43

in question is unknown, but an effect seems plausible. Flatfish species will, in contrast to herring and sprat, profit from their rapid disappearance from the plankton by settling in the intertidal zone (CREUTZBERG et al., 1978; BERGMAN et al., 1980). Zooplankton in the Wadden Sea, mainly consisting 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 continuous increase of the zooplankton stock till July suggests an only moderate effect of coelenterate predation on the zooplankton population in this estuarine area. Besides, it seems likely that these zooplankton populations are replenished c o n t i n u o u s l y from the coastal area where predation 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 Onderzoek 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, Texel, 1958-1: 1-37.

44

H.W. VAN DER VEER & W. OORTHUYSEN

FRANSZ, H.G., 1981. Quantitative 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 distribution 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 differentiation in Aurelia aurit a . - - A m . Zoologist. 14: 833- 849. KERSTAN, M., 1977. Untersuchungen zur Nahrungs5kologie 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. Significance of coelenterates in relation to other plankton organisms.--Ber, dt. wiss. Kommn Meeresforsch. 27: 1-18. - - - - , 1980a. A summer sur:vey of large zooplankton, particularly scyphomedusae in North Sea and Baltic.--Ber, dt. wiss. Kommn Meeresforsch. 28: 61-68. - - - - , 1980b. Scyphomedusae as predators and food competitors of larval fish.--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 population by jellyfish 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 westelijke Waddenzee en hun mogelijke rol als predator van platvislarven. Interne Verslagen Nederlands Instituut voor Onderzoek der Zee, Texel (unpublished report): 1-28.

SARS, M., 1841.0ber die Entwicklung der Medusa Aurelia aurita und der Cyanea capillata.--Arch. Naturgesch. 7: 9-34. SPANGENBERG, D.B., 1965. Cultivation of the life stages of Aurelia aurita under controlled conditions.--J. exp. Zool. 159: 303-318. TH~EL, H., 1962. 0ntersuchungen Liber die Strobulation 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 Wadden Sea.--Mar. Biol. 79: 219-227. VENEMA. S.C. & F. CREUTZBERG, 1973. Seasonal migration of the swimming crab Macropipus holsatus in an estuarine area controlled by tidal streams.-Neth. J. Sea Res. 7: 94- 102. VERWEY, J., 1942. Die periodizit&t im Auftreten und die aktieven und passieven Bewegungen der Quallen.--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 settling. --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.--Bull. Jap. Soc. scient. Fish. 34: 983-987. - - - - , 1969. Ecological studies on the jelly-fish Aurelia aurita in Urazoka Bay, Fukui Prefecture. I. Occurrence pattern of the medusa.--Bull. 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 conservation 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.