P.S.Z.N.1: Marine Ecology, 8 (1): 49-58 (1987) 0 1987 Paul Parey Scientific Publishers, Berlin and Hamburg

Accepted: September 1, 1986

ISSN 0173-9565

Notes on the Biology and Ecology of the Jellyfish Aurelia aurifa LAM. in Elefsis Bay (Saronikos Gulf, Greece) E. PXPATHANASSIOU, P. PANAYOTIDIS & K. ANAGNOSTAKI

National Centre for Marine Research, Agios Kosmas, GR-166 04 Hellinikon, Athens, Greece.

With 6 figures and 2 tables

Key words: Aurelia aurila, biology, ecology, Saronikos Gulf, Mediterranean Sea.

Abstract. The occurrence of planktonic stages of the scyphomedusa Aurelia auriro LAM. in monthly samples, from May 1983 to July 1985, was studied in Elefsis Bay (Saronikos Gulf, Greece). Results showed that the medusae biomass had its maximum value during summer, followed by a sharp drop during fall and winter. The major peak for the ephyrae liberation was during January-February, when zooplankton biomass reached its maximum. The vertical distribution of A. aurita in relation to light intensity is discussed.

Problem

The jellyfish Aurelia aurita LAM. is a cosmopolitan species found in a wide range of environmental conditions (MAYER, 1910). Abundant scyphomedusae of this species can reduce the zooplankton population and have great impact on the ecosystem (MOLLER, 1978; HERNROTH & GRONDAHL, 1983). In spite of its world- wide distribution only few reports deal with the biology of this species and the mechanisms which are responsible for large year-to-year variation (YASUDA, 1969, 1970; MOLLER, 1980 a; HERNROTH, 1984). The biology, die1 vertical migration, vertical distribution pattern, and ecology of this species have been thoroughly studied in Japan (YASUDA, 1968, 1969, 1970, 1973).

The present study is part of a project of the Center for Marine Research in Athens and describes aspects of the biology, ecology, and vertical distribution of A . aurita in Elefsis Bay, an area for which many hydrological data have been collected (HOPKINS et af . , 1974). Such a study is likely to provide interesting information on the relation of this species with zooplankton, which is its main food source (MOLLER, 1983). It should be pointed out that the occurrence of A.aurita in such numbers in Elefsis Bay is a unique phenomenon in the Mediterranean Sea, and similar abundances of this species have been only reported in other seas like Kiel Bight, W. Germany (MOLLER, 1980a), Urazoko

U.S. Copyright Clearance Center Code Statement: 0173-9565/87/0801-0049$02.50/0

50 PAPATHANASSIOU, PANAYOTIDIS & ANAGNOSTAKI

Bay, Japan (YASUDA, 1968), and Western Sweden (HERNROTH & GRONDAHL, 1983).

Material and Methods

Elefsis Bay is situated in the northern part of Saronikos Gulf (Fig. 1) and is one of the most eutrophic areas in Greece. It is relatively small (about 67 kmz) and shallow (maximum depth 33 m) and it is subjected to urban and industrial pollution (FRILIGOS, 1974; IGNATIADES & MimcoS, 1976). Elefsis Bay is an anoxic basin; in June, between Om and 20m, oxygen decreases with depth from 5.00 ml . I-’ to 0.371111 . I-’; in September, oxygen remains constant between Om and 10m (5.00ml . 1-‘) and below 1 0 m there is a rapid decrease of oxygen to almost zero at 20m (FRILIGOS, 1982).

Diatoms constitute more than 60 % of the total phytoplankton population throughout the year, with peak values of 98 % in December (GOTSIS-SKRETAS, 1985). Copepods and cladocerans are the main zooplankton groups, representing 78 % to 99.5 % of the zooplankton population (SIOKOU- FRANCOW & ANAGNOSTAKI, 1985).

Monthly sampling of the medusae was carried out from May 1983 to July 1985. The hauls were double oblique, from Om to 10m, and the ship speed was 3 knots. A 2.5m long plankron net with l m mouth opening and 1.5mm mesh site was used. This type of net proved to be the best in avoiding clogging due to the plankton blooms. An “Hydrobios” flowmeter was used to assess the water volume filtered through the net. All measurements of the bell diameter (It: 0.5cm) were carried out on board while the medusae were still alive.

Sampling of the ephyrae was carried out by double oblique hauls (from 0 to lorn) using a WP-2 plankton net of 200pm mesh size, equipped with an “Hydrobios” flowmeter. Specimens smaller than lOmm were defined as ephyrae and those between l 0 m m and 40mm as young medusae ( MOLLER, 1980 a). Both the ephyrae and young medusae were fixed in 4 % neutralize0 formalin.

The total volume of the medusae was determined in a 21 cylinder, into which the specimens were transfered after their diameter was measured. Some medusae were also transported to the

SARONIKOS GULF

B

0, 0

Fig. I. Map of Elefsis Bay in Saronikos Gulf.

Biology and ecology of Aurelia aurita 51

laboratory for biomass measurements. Based on our measurements the relation between bell diameter and the jellyfish wet weight follows the formula:

G = 0.0971 x D2* where G is the wet weight in g and D is the bell diameter in cm. This is similar to the relation described by KERSTAN (1977). The above formula was used to calculate biomass of A. aurita based on diameter measurements. The relation between wet weight and dry weight was found dwt = 0.04 x wwt.

Vertical distribution was studied by sampling medusae every two hours from 13:OO on August 22, 1983 to 12:OO on August 23,1983. Horizontal hauls at 1 m and 10m depth were made simultaneously using the net described above. These depths were considered to be representative since the depth at this site varied between 14m and 22 m.

During the medusae sampling procedure, physical, chemical, and biological data were collected; sampling with NIO bottles was carried out for salinity, sea-water temperature, phytoplankton, and nutrient measurements. The sampling concerned the surface layer during 1983 cruises and the whole water column during 198685 cruises. Underwater illumination was measured using a "Centralen" photometer. A SECCHI disk was used to estimate water transparency.

Zooplankton samples were also taken every month with a WP-2 plankton net (200 ym mesh size) using both horizontal and double oblique hauls from 1 m to 10 m depth.

Results

1. Environmental conditions

During the study period (May 1983-July 1985) the salinity of Elefsis Bay showed small fluctuations in the water column (= 38%0). The sea water temperature in the whole column was almost constant during winter, while during summer differences of more than 5 "C were observed between surface and bottom layers (Table 1).

The transparency of the sea water in the study area decreased rapidly with depth; SECCHI disc readings varied from 5 m to 7 m only, while the light intensity decreased to 10 % of the total at 10m depth (Table 2). The phytoplankton cell density varied from 100,000 to 10,000,000 cells 1-'. The zooplankton dry weight biomass varied between 100 mg . m-3 and less than 10 mg . m-I, with the highest value in January when the dominance of the copepod Acarfia clausi (GIES- BRECHT) was 99.5 %.

Table 1. Sea-water temperature ("C) from May 1983 to July 1985 in Elefsis Bay.

year depth Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

1983 I m ~~~~ ~~~~~

23.1 23.4 24.7 21.3 16.1

I m 12.1 12.4 16.2 23.4 23.7 24.6 23.6 21.2 18.3 14.3 1984 10m 12.2 12.4 14.2 21.6 23.7 24.3 23.6 21.2 18.3 14.2

20 rn 12.2 12.7 13.3 15.6 15.7 17.0 19.8 21.2 18.3 14.1

l m 11.6 10.3 12.9 16.4 21.6 25.0 1985 IOm 11.6 10.1 11.4 15.2 20.3 21.3

20m 11.6 10.1 11.2 13.4 13.3 14.3

52 PAPATHANASSIOU, PANAYOTIDIS & ANAGNOSTAKI

Table 2. Light relative intensity (surface = 100) in Elefsis Bay. _ _ ~ -~ ~

Light relative intensity (%I

May 84 Aug 84 Feb 85 Apr 85 Jun 85 Dec 85

0 100 100 100 100 100 100 5 39 47 11 25 17 26

10 10 20 3 2 3 9 1 3 15 2 9 -

20 - 2 - 25 - -

- - - -

- - - -

2. Biology of Aurelia aurita

The number of medusae larger than 40 mm showed a remarkable increase from spring to summer. After that a sharp drop in numbers was noted (Fig. 2). The abundance of young medusae (bell diameter < 40 mm) and ephyrae is shown in Fig. 3. Very few specimens were present from May to December. By the end of January and the beginning of February there was a remarkable increase in the number of the ephyrae, attaining a maximum of 44 indiv. . rn-) on February 1984. The ephyrae abundance remained high during March, followed by a sharp drop during April. A second much smaller peak in May was observed. After this period no ephyrae were observed and young medusae were found in the samples in diminishing numbers.

The biomass of A . uuritu in Elefsis Bay varied between 0.01 g dwt to 2.80g dwt - m-I. The minimum value was attained in March and the maximum in July.

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

1983 1984 1985

Fig. 2 . Abundance of A . artriru (specimens larger than 40mm) in Elefsis Bay

Biology and ecology of Aurelia aurita 53

It should be pointed out that, in places where the medusae formed patches, biomass reached the exceptional value of 3.5 g dwt md3. The medusae biomass showed a sharp drop from late July. Young medusae and ephyrae made only a small contribution to the total biomass. During February, when their number was high, their biomass was less than 0.01 g dwt . ~ n - ~ . The relationship between chlorophyll a, zooplankton biomass and medusae biomass is shown in Fig. 4.

I

m

0 2.

5 20000.

I - - : 10000 L

2. c W a

0

E 0

40000, 9 l. - a

TJ D

z 30000.

E . .

J . . . . . , , * * . * , , . . : * ) M J J A S O N D J F M A M J J A S O N D J F M A M J J A S

1 9 8 3 1984 1985

t-12.3. Abundance of the ephyrae and the young medusae in Elefsis Bay.

1983 1984 1985

Fig. 4. Relationship between chlorophyll a, zooplankton biomass and medusae biomass, in Elefsis Bay.

54 PAPATHANASSOU, PANAYOTlDlS & ANAGNOSTAKI

3. Vertical distribution

The weather conditions were favorable to study the vertical migrations of A. aurita. The wind was moderate (northwest, 4 m - sec-I) during the day, while the night was windless. The sky was cloudless and the surface temperature was 24 rt 1°C.

The bell diameter for both sampling depths (1 m and 10 m) followed the same pattern of relative abundance, shown in Fig. 5 . The size of the medusae varied from 30 mm to 200 mm, with most of the specimens between 80 mm and 90mm.

An obvious migration of A . nurifa took place within the 24 hour survey; in the morning, between 9:OO and 13:00, more A. aurifa individuals were caught in the lOm horizontal hauls. After that time, most migrated to the surface; at 17:OO many medusae were found swimming near the surface (Fig. 6). Later they began to sink gradually to deeper layers; no medusae were caught near the surface at 3:OO. After sunrise the main distribution of the medusae shifted to the surface up to 9:00, after which the previously reported pattern was repeated. The same number of medusae was present at both depths at 21:OO hours.

% !

10

3 4 5

0 1 m depth sampling

10 rn depth samplirig

6 7 0 9 10 11 1 2 1 3 1 4 1 5 1 6 1 7 I 0 1 9 2 0 2 1

bell diamater (cm) Fig. 5. Size distribution of A. aurifu specimens in 1 rn and in 10 rn depth.

T i m e ( h )

2 2 . 8 * 83 2 3.8.8 3 I Fig. 6. Vertical distribution of A . aurita during the 24 h survey.

Biology and ecology of Aurelia aurita 55

Discussion

The release of the ephyrae of A. aurita in Elefsis Bay had a major peak during January-February and a much smaller one during April-May.

A late wintedearly spring peak for ephyrae liberation has been described before by many authors (RUSSEL, 1970; RASMUSSEN, 1973; MOLLER, 1980a); a detailed study on the liberation of the ephyrae was also prepared by THIEL (1962). HERNROTH & GRONDAHL (1983), in Sweden, found the major peak of the ephyrae during autumn; this was not observed in Elefsis Bay.

The peak of the ephyrae was almost 3 times higher than the maximum value reported in the literature. MOLLER (1984) reported 6-8 ephyrae - and HERNROTH & GRONDAHL (1983) 1496 indiv. * loom-’. In the same order ‘of magnitude YASUDA (1968) reported about 1000 ephyrae * 100 m-3. The sampling method cannot be responsible for this vast difference, since the present study was carried out by methods similar to those described in the literature (HERN- ROTH & GRONDAHL, 1983). However, it should be pointed out that the numbers reported by MOLLER (1980a) referred to an average of 25 stations inside and outside the main production area of the ephyrae. The high values that were observed in Elefsis Bay could be mainly explained by food availability.

SPANGENBERG (1968) reports that the number of segments on the scyphistoma is dependent on food availability. In the case of Elefsis Bay, food availability could be the reason for ephyrae liberation during February, since in January zooplankton biomass reached its maximum values (100mg m-’ dry weight). MOLLER (1980a) reported a reduction of the bell diameter during autumn; we were unable to observe this phenomenon during the study period.

During summer the zooplankton biomass in Elefsis Bay drops to its minimum level. Nevertheless the bell diameter of A . aririta continued to increase. An explanation for this could be the presence of food sources other than plankton. The feeding of A . aurita on detritus is reported by GOMOIU (1980) from the Black Sea; in addition, DUGDALE et al. (1985) discuss the medusae feeding on dissolved organic matter. Both detritus and dissolved organic matter are abun- dant in Elefsis Bay (FRILIGOS, 1982).

An additional factor favoring the abundance of ephyrae was the presence of many suitable substrates for polyp fixation in the bay; indeed, about 600 cargo ships are anchored there, most of them for years,

The extremely high medusae biomass in Elefsis Bay during early summer could play an important role in the energetics of this particular ecosystem (PANAYOTIDIS et al., 1985). This hypothesis is supported by the fact that whenever high medusae biomass was observed, a lower zooplankton biomass was noted in the samples (Fig. 4). There is accumulating evidence that medusae as well as ctenophores may deplete zooplankton stocks by predation (MOLLER, 1980 b). The consumption of the zooplankton stock could be the reason for the sharp drop in medusae biomass after July. On the other hand the anoxic conditions in the bay during summer (FRILIGOS, 1982) might have an impact on A . aurita abundance, although the medusae are probably able to stay out of the anoxic water layers.

The decreasing number of A. aurita near the surface during the day seemed to be correlated with light intensity. IRISAWA et al. (1956) reported that A. aurita

56 PAPATHANASSIOU, PANAYOTIDIS & f~NAGNOSTAKI

has a sense organ which is very sensitive to decreases in light intensity. They also stated that the medusae reacted to the decrease by a conspicuous bell pulsation. In the present study the same number of medusae was found near the surface and at lorn at 21:OO. This observation supports the postulation of the sense organ in A. aurita, since at 21:OO it was dark and the medusae can no longer respond to the light stimulant. The gradual sinking of medusae from the upper to the deeper layers might be due to the inactivation of bell pulsation.

A similar phenomenon has been reported by YASUDA (1973) in Urazoko Bay, Japan, where an extensive study of the vertical distribution of this species has been made. Another possibility for the vertical distribution of A . aurifa could be the die1 migration of zooplankton, since it is well known (BOUGIS, 1974) that pranktonic groups move vertically within a 24 h period.

The scyphomedusae A . aurita seem to be successfully adapted 1.0 the eu- trophic conditions of Elefsis Bay, reaching very high biomass values and playing a significant role in the control of the zooplankton population. Studies are underway to determine the causes of the sharp decrease of A . aurita abundance in late summer, as well as the importance of detritus and dissolved organic matter in the planktonic ecosystem energetics in Elefsis Bay.

Sum ma ry

The occurrence of planktonic stages of the scyphomedusa A . aurita was studied in Elefsis Bay (Saronikos Gulf, Greece) from May 1983 to July 1985. Monthly samples of medusae were taken with simultaneous measurements of other biological, physical, and chemical parameters.

The major peak for ephyrae liberation was observed during January and February when zooplankton biomass in the bay reached its maximum value. The number of ephyrae (44 indiv. * m-’) measured in our samples was almost 3 times higher than the maximum values reported in the literature. The number of medusae with bell diameters larger than 40 mm increased from spring to summer, then showed a sharp drop in fall. Medusae biomass varied from 0.01 g dwt to 2.80g dwt rn-’; the maximum was observed in summer.

The vertical migration of A . aurita populations was studied in the bay, over 24 hours, at 1 m and 10m depth. At 21:OO the same number of medusae was present at both depths. A gradual sinking of the medusae was observed during the night. After sunrise the medusae shifted to the surface.

Acknowledgements

We would like to thank the biologists of the “medusae group’’ of the National Centre for Marine Research for the helpful criticism of the manuscript, Miss T. KATAPODI for her assisrance in the medusae sampling during the 24 h survey, and Mrs. LAMBROPOULOU-MAROUDA for the technical assistance in preparing this paper.

Biology and ecology of Aurelia aurita

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