FRESHWATER JELLYFISH CRASPEDACUSTA SOWERBYI LANKESTER , 1880 ( HYDROZOA , OLINDIIDAE ) – 50 YEARS ’ OBSERVATIONS IN SERBIA

Detailed and relevant limnological investigations of Serbian waters were initiated in 1958 and have continued to the present. During the period 1971-2008 we monitored biological elements as a part of working studies/projects, including the distribution of the freshwater jellyfish Craspedacusta sowerbyi Lankester, 1880. We observed over 500 sampling sites in running and standing waters. Specimens of this hydro-medusa were found in five of them. Throughout the period of investigation, only the medusae stages were observed. Our purpose in this paper was to provide data of the records and distribution of this limnomedusa during the period 1958-2008 in inland waters of Serbia. These observations should contribute to knowledge on the limnofauna not only of the Balkan Peninsula but Europe as a whole.


INTRODUCTION
All Craspedacusta species inhabit freshwater bodies of Eastern Asia (China and Japan).However, one species -Craspedacusta sowerbyi Lankester, 1880, has expanded its home-range and currently has a cosmopolitan distribution.The hydromedusae appear frequently in shallow pools alongside rivers.These pools experience the fluctuating plankton populations, temperatures and water levels caused by conditions in the main river system (Slobodkin and Bossert, 1991).
The life cycle of C. sowerbyi includes polyp stage and a free-swimming, sexually reproducing medusa stage.Both medusa and polyp feed on various zooplankton taxa (De Vries, 1992;Dumont, 1994).The polyp form has a much wider distribution than the hydromedusa form (Kato and Hirabayashi, 1991;Angradi, 1998).
This paper presents the occurrence and distribution of Craspedacusta sowerbyi in the inland waters of Serbia over the 50 years of investigations.

MATERIALS AND METHODS
Specimens of C. sowerbyi were collected from five localities of the Danube, Sava and Velika Morava River Basins in Serbia (Fig. 1).These specimens were stored in the collection of biological material of the Benthological Section, of the Department of Hydroecology and Water Protection, Institute for Biological Research, Belgrade.The biological material was collected by many researchers for several different research projects in the period 1958-2008(WQMP, 1981-2008;;Datebase -Faculty of Science, Kragujevac).
Samples were collected by sweeping through aquatic macrophytes with a planktonic net (mesh size 20 -50 μm), from the sediment using different bottom-samplers (Van-Veen -370cm 2 , Ekman -225cm 2 , Peterson grab -400cm 2 ) and with scubadiving activities.Specimens were preserved in 70% alcohol and measured, while a few were maintained live in an aquarium and photographed using a digital camera, with 6 x magnification.

RESULTS
Specimens of Craspedacusta sowerbyi were collected at five sampling sites during the hydrobiological study of the inland waters of Serbia.In 1958 this freshwater jellyfish was registered in Serbia for the first time.All observed individuals were found only in standing waters (natural pools and artificial water bodies) at the free-swimming medusa stage (Fig. 1 and Table 1).Measurements of the collected limnomedusae by stereomicroscope show that the average length of the umbrella was from 12 to 20 mm.A permanent monitoring program with the aim of health protection was performed on the Sava Lake, near Belgrade (locality 2, Table 1) (WQMP, 1981(WQMP, -2008)).
The sampling sites where Craspedacusta sowerbyi were found are presented in Fig. 1.Some of captured individuals were kept in an aquarium for a few days and photographed (Fig. 2).

DISCUSSION
The native range of Craspedacusta sowerbyi is freshwater bodies of Eastern Asia (China and Japan) and it has been spread to all continents except Antarctica (D u m o n t , 1994).
We stress that the first findings of C. sowerbyi for the Balkan area were in 1958 in Serbia (Grozdanić and Manojlović, 1958) and in Montenegro in the River Crnojevića (Milovanović and Živković, 1965).In Croatia, medusae were recorded in 1992 and in Bulgaria in 1994 (Jaslovská andStloukal, 2004).
The local occurrence of Craspedacusta sowerbyi depends to a great extent on environmental conditions such as temperature.Hydromedusae blooms are most common in summer and fall, from July to October, in still or slow-moving fresh water bodies, when the water temperature rises to at least 25°C.Craspedacusta sowerbyi does not advance from the polyp to the more readily identifiable hydromedusa stage in the absence of warm temperatures (Pennak, 1989;Kato and Hirabayashi, 1991;Angradi, 1998).
In the 50 years of biological monitoring of the inland waters in Serbia the polyp stage of Craspedacusta sowerbyi has not been found and the medusae stage is scarcely registered.The appearance of this cnidarian in five sites of Serbia was rare, in the range of 1 to 50 individuals.It is interesting to note that the mass appearance of hydromedusae occurs during summer, from July to September, when the temperatures of lake water are increased (20-26 o C).This limnomedusa only inhabits standing, natural and artificial water ecosystems in Serbia.
Craspedacusta sowerbyi has expanded its homerange mainly through unintentional introduction by humans (transported with ornamental aquatic plants) and currently has international distribution (Slobodkin and Bossert 1991).

Table 1 .
Distribution of Craspedacusta sowerbyi in Serbia