The invasive freshwater medusa Craspedacusta sowerbii Lankester , 1880 ( Hydrozoa : Olindiidae ) in Israel

The freshwater jellyfish Craspedacusta sowerbii, native to China, has been introduced to lentic and lotic habitats in artificial and natural bodies of water, in subtropical to temperate regions worldwide. In the Middle East it has been recorded from an artificial lake in the Nile Delta, Egypt, a recreational lake in Iraq, a reservoir and a dammed lake in Turkey. The first record in Israel dates back half a century – to an aquarium at the Hebrew University in Jerusalem. Since 2003 medusae of C. sowerbii have been observed in a perennial stream flowing into Sea of Galilee – Israel’s principal freshwater storage and supply reservoir. The possible impacts of a substantial bloom are discussed.


Introduction
The freshwater jellyfish Craspedacusta sowerbii Lankester, 1880, native to China, has colonized since the end of the 19th century water bodies in subtropical to temperate regions worldwide (Kramp 1950;Dumont 1994).It is frequently found in disturbed or artificial bodies of water e.g.quarry ponds and gravel pits, reservoirs, aquaria and even wastewater treatment facilities, in addition to natural lentic and lotic habitats (Tattersall 1933;Augustin et al. 1987;Fritz 2007).Its chitin-covered drought resistant resting stage enables it to withstand long periods of food shortage and tolerate extreme environmental conditions, as well as serve as a convenient life-stage for anthropogenic transport (Jankowski 2001).The species reproduces asexually, via a budding polyp and a motile frustule, and produces sexually reproducing free swimming medusae, which bud off from the polyp (Reisinger 1957).
A considerable number of reports on new C. sowerbii occurrences have been published in the past twenty years, greatly increasing our knowledge of the species spatial extent.However, our knowledge of its occurrence in the Middle East is limited to four records (Figure 1, Appendix 1).Dumont (2009) reported that a 1956 record (Elster et al. 1960) of "a brief mass occurrence of medusae in the Nozha hydrodrome", near Lake Mariut, Alexandria, Egypt, previously ascribed to Limnocnida, may have been in fact C. sowerbii.Saadalla (2006) recorded its presence in June-September 2002 in an artificial recreational lake north of Baghdad, Iraq.In 1999 it was found in the Topcam Reservoir in the Büyük Menderes River Basin in SW Turkey, and in August 2008 in the Kralkizi Dam Lake in SE Turkey (Balik et al 2001;Bekleyen et al 2011).
In this paper we report the presence of an established population of Craspedacusta sowerbii from one of the perennial streams flowing into Lake Kinneret, Israel.

Study area
Lake Kinneret (Sea of Galilee), in the northern part of the Jordan Rift Valley, is a warm monomictic lake, stratified from mid-May to November, with plankton restricted to the   epilimnic and metalimnic layers.The lake plays a vital role in Israel's freshwater balance, serving as the principal storage and supply reservoir (Serruya 1978).The lake surface is 166 km², its average volume 4100 Mm³ (million cubic meters), of which ~300 Mm³ are used annually for drinking and irrigation (Rimmer 2009).The salinity (190-280 ppm Cl -) is higher than the salinity of the streams in the lake's catchment area (20-30 ppm Cl -).Zavitan stream is a tributary of the Meshushim stream, one of five perennial streams flowing into Lake Kinneret.The streams cut deep ravines through the basalt rock, forming a series of waterfalls and ponds used as popular swimming holes.One of these sites is the Iris pond (32°59′58.50″N35°44′22.20″E),a natural pool with a surface area of 300 m² and a maximal depth of 3 m (Figure 2).In mid September 2011 the surface water temperature at the Iris pond was 25°C, salinity 0.2 psu, D.O. concentration 8.6 mg/l (102% saturation) and Secchi transparency 84 cm (greenish coloration).The results of the plankton and macroinvertebrates analysis are given in Table 1.

History of Craspedacusta sowerbii in the region
A photograph of a polyp of Craspedacusta sowerbii (as Calpasoma dactyloptera) found in a freshwater aquarium at the Department of Zoology, Hebrew University of Jerusalem, was the first record of the species in Israel (Rahat 1961, Figure 1).These polyps were propagated, cultured and used for studies on atentacular ('Microhydra') and tentacular ('Calpasoma') polyps at the Hebrew University of Jerusalem (Spira 1963;Rahat and Campbell 1974).Hydranth-type 'Calpasoma' is considered a polyp form of C. sowerbii, though molecular studies are required for substantiation (Jankowski 2001).
In 1994 polyps and medusae of C. sowerbii were discovered in a freshwater aquarium at the laboratory of the senior author (AG), Department of Zoology, Tel Aviv University.The aquarium was used to rear sponges collected on cobbles in Lake Kinneret littoral zone (Dembo 1996).There were no further records of the species from Lake Kinneret in subsequent routine plankton sampling (T.Zohary, head of the Kinneret Limnological Laboratory, Israel Oceanographic and Limnological Research, pers.comm.).During the summer months of 2003, 2006, 2009 medusae of C. sowerbii were observed in ponds along the Zavitan and Meshushim streams.On October 4th 2004, a specimen was collected at the Iris pond, Zavitan, and deposited in the National Collections at Tel Aviv University (TAU CO 32351).On August 21st 2009 medusae were photographed at the same location, and on September 28th 2009 a single specimen was collected from the pond (TAU CO 35310).In the summer of 2010 (August -October) that pond was surveyed several times and no medusae were found.However, during August and September 2011, C. sowerbii medusae were again reported in the Iris pond.In September 12th 2011 specimens were collected at the Iris pond by one of the authors (SG) (Figure 3), but a survey of other sites in the Zavitan and in Meshushim revealed no specimens.

Discussion
The numerous records of C. sowerbii in aquaria suggest that 'escape' from aquaria is an important human-mediated vector (Stadel 1961;Duggan 2010).Parent (1982) proposed that the American waterweed, Elodea canadensis (Michaux, 1803), a common aquarium plant, may have been a vehicle for Craspedacusta transport, with a time lag of ca 50 years to establishment in the natural environment.Indeed, C. sowerbii was first recorded in Israel from an aquarium used to raise the axolotl, Ambystoma mexicanum (Shaw & Nodder, 1798), and the polyp was attached to stems of E. canadensis (Rahat 1961).The author hastened to distance himself from the introduction: "Attempts to trace the origin of these polyps to that of the plants or Amphibia present in the aquaria, proved unsuccessful.Thus neither of these two species of polyps could be found in the aquaria that provided the stock of plants and Amphibia for the aquaria examined by us." (Rahat 1961: 172).Yet, the foreign origin of both its aquarium mates provides an interesting clue to the possible vector.Dumont (1994) proposed that ornithochory or aerial dispersal of the species' drought-resistant resting stages may serve as means of upstream and inter-valley dispersal.
More than 50 years have passed since C. sowerbii was first recorded in Israel, but its distribution seems to be restricted to few lentic habitats in one stream in the catchment area of Lake Kinneret, where relatively low abundances of its medusae were recorded in late summer (August-October).Although the species is a successful invader world-wide, its occurrence may be restricted by habitat conditions such as water temperature, flow velocity and specific food sources (e.g.eutrophic ponds, Jankowski 2001).
Little is known about the trophic interactions of zooplanktivorous freshwater hydromedusae and their role in aquatic food webs.Both polyps and medusae of Craspedacusta sowerbii are zooplanktivorous, and their prey is often the pivotal link between primary producers and higher trophic levels.It has been proposed that though its predation rates are too small to cause zooplankton population declines, the medusae could, if abundant, restructure zooplankton composition (Dodson and Cooper 1983;Spadinger and Maier 1999).However, microcosm experiments and observational evidence suggest a decrease in abundances of nauplii and adult copepods (Davis 1955;Jankowski and Ratte 2002;Jankowski 2004;Jankowski et al. 2005;Smith and Alexander 2008).Thus, while blooms of C. sowerbii are infrequent, they may significantly impact aquatic food webs: a related shift in the composition of the algal grazers may result in higher algal biomass with cascading effects on primary productivity and dissolved oxygen (Smith and Alexander 2008).
It has been suggested that C. sowerbii blooms may be associated with rising temperatures or increased nutrient inputs (Rayner 1988;Boothroyd et al. 2002).Were Lake Kinneret to experience a substantial bloom of C. sowerbii, its ecosystem could be significantly altered through direct (predation) and indirect (incidental mortality) effects.Cyclopoid copepods dominate the zooplankton in Lake Kinneret, followed by cladocerans and rotifers (Gophen 1978;Pinel-Alloul et al. 2004).Predation by C. sowerbii may depress abundances of the bosminids and cyclopoid copepods, which constitute the diet of the numerically dominant fish, the endemic cyprinid, Kinneret bleak, Acanthobrama terraesanctae Steinitz, 1952.

Figure 1 .
Figure 1.Distribution records of Craspedacusta sowerbii Lankester, 1880 in the Middle East, shown on Google map.Numbers corresponding to sites in Annex1.