Photo by Cornelia Sellei (http://www.ecology.su.se/dbbm/var/Zoopsmf.htm)

2. Identification
The body is elongated and slender.
Female. The abdomen of the female is very short, somewhat shorter than ¼ of cephalothorax that has rounded posterior margins. The abdomen is armed with thin bundles of hairs on the sides of the anal segment. The first antennae reach the middle of the genital segment.The distal segment of the fifth pair of female's podites has a rounded projection on the inner margin. The apical seta is coarsely serrated at the distal end, it is almost equal in length to the lateral seta.

Male. The lateral part of the genital segment has bundles of short setae. The caudal branches are asymmetrical, with bundles of setae on the inner margin. The fifth pair of the male's podites is markedly asymmetrical, uniramous. The third segment of the right podite has a large inner projection. The forth segment is elongated, curved, armed with thin long setae. Basipodite (left) has a large projection. (Kurashova, E.K. )

3. Natural distribution
Acartia tonsa is a widely spread species: Atlantic and Pacific coasts of the North and South America, Indian Ocean.

4. First finding in Estonia
First known records in the following sub-basins of the Baltic Sea that surround Estonia: Gulf of Riga in 1924 (Berzins, 1940), Gulf of Finland in 1934 (Smirnov, 1935).

5. Invasion history in the Baltic Sea
The species has probably dispersed by human activities - aquaculture (Belmonte, Potenza, 2001), shipping - ballast water (Brylinski, 1981; Leppäkoski, 1984; Jansson, 1994).
Difficulties in taxonomic identification of the species, especially distinguishing from Acartia bifilosa make the results in presence/absence and distribution area of the species questionable.
In 1916, the species was identified in the North Sea (Redeke, 1934 cit. Berzins, 1940). In the Baltic Sea, the first record date back to 1924 when the species was identified in the Gulf of Riga. In 1925, presence of the species was recorded in the Gulf of Gdansk. Acartia tonsa spread afterwards to north - Gulf of Finland in 1934 in Luga Bay and in Espoo in 1939 (Vorstmann, 1946).
6. Distribution and population dynamics in Estonia and adjacent waters
Brackishwater euryhaline and eurythermic species (Lumberg, 1976; Lumberg, Ojaveer, 1991, Silina, 1989). The typical warm-water species. Showed mass occurrence in very warm summers (Behrends, 1999).
In general, Acartia spp. (A. bifilosa and A. tonsa) are not identified to the species level in the Baltic Sea zooplankton studies. Therefore, the current distribution area and population parameters are largely unknown.
A few data for the species is available for the Gulf of Finland in 1963 - 1973 (Lumberg, 1976) and in 1983 - 1988 (Silina, 1989). Comparative data are also available in the southern Baltic. These suggest that Acartia tonsa prefers warmer water than Acartia bifilosa. In the Gulf of Finland, Acartia tonsa is present in the zooplankton community from August to the beginning of December by reaching the abundance peak in August (Lumberg, 1976; Silina, 1989). In the Gulf of Riga, A. tonsa has been found from July to September (Berzins, 1940; Nikolaev, 1961). In the southern Baltic, Kieli ja Mecklenburg bights, mass development of the species occurs in summer-autumn period(Rzoska, 1938; Siudzinski, 1968; Hirche, 1974; Arndt, Heidecke, 1973; Arndt, Schnese, 1986; Behrends, 1999).

7. Ecological and economical impact
Poorly known. Dietary overlap between A. tonsa and A. bifilosa is possible. In case of food shortage, A. tonsa may act as food competitor for A. bifilosa. A. tonsa is preyed by several invertebrates (e.g., mysids) and planktivorous fish stickleback, herring, perch (Arndt and Schnese, 1986).

8. More information
Mart Simm, e-mail: mart@klab.envir.ee

9. References
Arndt, H. and Schnese, W. 1986. Population dynamics and production of Acartia tonsa (Copepoda; Calanoida) in the Darss-Zingst estuary, southern Baltic. Ophelia (Suppl.) 4: 329-344.
Behrends, G. 1999. Seasonal and interannual variability of abundance of the four Acartia species occurring in the Kiel Bight, western Baltic Sea, 1985 - 1998. 16th BMB Symposium, Klaipeda, Lithuania. Abstract, p. 14.
Belmonte, G., Mazzocchi, M. G., Prusova, I. Y. and Shadrin, N. V. 1994. Acartia tonsa: a species new for the Black Sea fauna. Hydrobiologia 292/293: 9 - 15.
Belmonte, G. and Potenza, D. 2001. Biogeography of the family Acartiidae (Calanoida) in the Ponto-Mediterranean Province. Hydrobiologia 453/454: 171-176.
Berzins, B. 1940. Acartia tonsa Dana in the Gulf of Riga. Folia Zool. Hydrobiol. 10: 484-487.
Brylinski, J. M. 1981. Report on the presence of Acartia tonsa Dana (Copepoda) in the harbour of Dunkirk (France) and its geographical distribution in Europe. J. Plankton Res. 3: 255 - 260.
Gaudy, R. and Vi?as, M. D. 1985. Premiere signalisation en Mediterranee du Copepode pelagique Acartia tonsa. Rapp. Comm. Int. Mer Medit. 29: 227 - 229.
Hirche, H.-J. 1974. Die Copepoden Eurytemora affinis Poppe und Acartia tonsa Dana und ihre Besiedlung durch Myoschiston centropagidarum Precht (Peritricha) in der Schlei. Kieler Meeresf. 30: 43-64.
Jansson, K. 1994. Alien species in the marine environment. Introductions to the Baltic Sea and the Swedish West Coast. - Swedish Environmental Protection Agency: 1 - 68.
Kurashova, E.K. and N.M. Abdullayeva, 1984. Acartia tonsa (Calanoida, Acartiidae) in the Caspian Sea. Zool.J. 63: 6 (in Russian ).
Leppäkoski, E. 1984. Introduced species in the Baltic Sea and its coastal ecosystems. - Ophelia Suppl. 3: 123 - 135.
Lumberg, A. J. 1976. On zooplankton in the Gulf of Finland. Rybokoz. issled. bass. Balt. Morya 12: 10 -28 (In Russian).
Nikolaev, I. I. 1961. New immigrants of fauna and flora from the North Sea to the Baltic Sea. Zool. Zh. 42: 20 - 27 (In Russian).
Rzoska, J. 1938. Acartia (Acanthacartia) tonsa Dana, new component of the Baltic fauna. (In Polish). - Archiwum Hydrobiologii i Rybactwa., XI.
Silina, N.I. 1989. Species of the genus Acartia in the plankton of eastern part of the Gulf Finland, Baltic Sea. - Proc. Zool. Inst. Leningrad 205: 108 - 118 (In Russsian).
Siudzinski, K. 1968. Ecology of species of the genus Acartia in the Baltic. ICES C.M./L:11.
Smirnov, S. S. 1935. About appearance of Acartia tonsa Dana (Copepoda) in the Gulf of Finland. Dokl. ANSSSR 3: 237 - 240 (In Russian).
Vorstmann, A.G. 1946. A. tonsa on the south coast of Finland. - 13th Biologisch Jahrbock Dodonea, Brüssel: 184 - 188.

1. Scientific name: Cercopagis (Cercopagis) pengoi (Ostroumov 1891) (Cladocera, Crustacea)
Common name: fish-hook waterflea

Photo by Mart Simm.

2. Identification
The following most pronounced parts of the body can be identified: the head, the second pair of antenna, four pairs of thoracic legs (the first leg is 3-4 times longer than other legs), abdomen, caudal process and a brood pouch in females. Body length of females varies from 1.2 to 2.0 mm and that of males from 1.1-1.4 whereas the caudal process exceeds the main body 5-7 times by length (Mordukhai - Boltovskoi and Rivier, 1987). The most distinctive feature is the long caudal process which has a loop-like curvature at the end. However, individuals with a straight and relatively short caudal process without characteristic S-bend dominate amongst Cercopagis community in spring (see photo below). In addition to differing caudal process, the 'spring individuals' bear backwardly bent or straight tips of barbs. In adults, the caudal process consists generally of three articles, each of which bear a pair of ventrally situated stout spines. The head is essentially composed of a large single eye, where the amount of black pigment makes less than one half of the diameter of the eye. Characteristically for C. pengoi, length of the abdomen is about equal to that of the remaining body (the caudal process excluded). The second antenna is a large appendage containing of two branches - the endopod and exopod. The number of setae on each branch has been used as one of the distinguishing characteristics to separate two very similar genera: Cercopagis and Bythotrephes. There are 7 setae on each ramus in Cercopagis but 7 and 8 on the inner (endopod) and outer ramus (exopod) of all species of Bythotrephes, respectively. Mordukhai - Boltovskoi, 1967, 1968; Mordukhai - Boltovskoi and Rivier, 1987; Martin and Cash-Clark, 1995; Rivier 1998).

The 'spring form' of Cercopagis pengoi. Photo by Henn Ojaveer

3. Natural distribution
Cercopagis pengoi prefers, in general, the brackish-water environment, but the waterflea has also been found in pure freshwater conditions. Until the invasion into the Baltic Sea, the distribution area of C. pengoi has been mainly restricted to the Ponto-Caspian region: the Caspian, Azov and Aral seas together with lower reaches of the rivers entering to these waterbodies - Danube, Dniester, Bug, Dnieper, Don and Volga. The animal has also been identified in the coastal lakes in Bulgaria, and in the Tsimlyansk and Kahovka reservoirs at Dniepr and Don, respectively (Mordukhai-Boltovskoi and Rivier, 1987; Rivier, 1998).

4. First finding in Estonia
Southern coast of the Gulf of Finland (Muuga Bay) and the northeastern Gulf of Riga (Pärnu Bay), 1992.

5. Invasion history in the Baltic Sea
In difference from Gulf of Riga where the species continued its presence in 1993-1994, Cercopagis was not encountered in zooplankton samples in the Gulf of Finland in these years. Since 1995, the species was continuously recorded in several localities in the southern and eastern Gulf of Finland (e.g., Kivi 1995, Avinski 1997). During next years, the distribution area of Cercopagis substantially expanded: 1) to open and western Baltic (Gotland basin, west coast of the Baltic Proper, 1997), 2) to north (Gulf of Bothnia, 1999) and 3) to south (Gulf of Gdansk, Vistula lagoon, 1999) (Gorokhova et al. 2000, Naumenko and Polunina 2000, Zmudzinski 1999, Bielecka et al, 2000). After invasion to Lake Ontario (North America) in 1998, the species was observed in the following year in Lake Michigan and five New York Fingerling lakes (MacIsaac et al. 1999, Makarewicz et al. 2001).

6. Distribution and population dynamics in Estonia and adjacent waters
The wide range of optimal salinity (from 2 to 10 PSU) does not restrict spreading of Cercopagis throughout most of the Baltic Sea. The basic factors limiting the distribution of the species are water temperature and food availability (Avinski 1997). Usually, the species appears in pelagic mesozooplankton community at water temperatures over 15 ?C and starts to disappear when temperature falls below 8 ?C. In some years, however, the species was unexpectedly encountered in zooplankton samples at temperatures around and below 10 ?C. Additionally, stability of the water column may be important: higher abundances have been recorded in sheltered locations (less affected by wind-induced water movements) and in the areas of lack of currents (e.g., Avinski 1997).
Cercopagis prefers warm water environment: its distribution is mostly confined to upper layers during the day and night. Only a few individuals have been recorded in colder waters beneath the seasonal thermocline. The species does not perform diurnal vertical migrations (Avinski 1997, Krylov et al. 1999).
The species is widely distributed in all parts of Estonian marine waters: Gulf of Finland, Gulf of Riga, Baltic Proper and Väinameri Archipelago. Long-term studies are being carried out in the Gulf of Riga. Presence of the cladoceran in the pelagic plankton community varied during the first ten years of invasion (1992-2001) from 7 to 24 weeks (the long-term mean 14.6) whereas the duration of presence of Cercopagis has clearly increased over the time. This pattern was accompanied by another long-term shift: appearance of the species into the plankton community tended to occur earlier over the years studied. The last event seems not to be regulated directly by the water temperature regime alone, but perhaps mediated via production of potential prey in spring (Eurytemora).
The overall mean long-term population abundance of Cercopagis was relatively low: 126±35 (s.e.) ind. m-3. Cercopagis density has increased exponentially during the first ten years of invasion. The exponential growth is mainly due to high abundance in 2001 (mean 420±109, max 1,284 ind. m-3).

Abundance dynamics (mean±S.E.) of Cercopagis in Pärnu Bay (NE Gulf of Riga) in 1992-2001.

Although the mean temperature for June-September months tended to increase over the years studied, correlation with the mean annual abundance of Cercopagis was insignificant (p>0.05) suggesting that water temperature is not an important factor regulating multi-annual development of Cercopagis population. As a long-term mean, Cercopagis abundance reached the peak during the first week of August (mean 269 ind. m-3) and development of the population density at the seasonal scale closely followed course of the annual water temperature.

7. Ecological and economical impact
Changes in food-web and energy transfer in lower trophic levels, due to the invasion of Cercopagis will likely impact the structure of fish stocks. Introduction of the cladoceran may be beneficial to commercial fish production (mainly gulf herring) if it enhances transfer of less favourable mesozooplankton production to higher trophic levels (e.g., Bosmina à Cercopagis à planktivorous fish). This is probably most important during the periods of shortage of energetically suitable food that have resulted in decreased individual body weight of herring in the northern Baltic. However, possible dietary overlap between Cercopagis and planktivorous fish for copepods and Bosmina and may result in reduced prey resource and reduced fish recruitment (see below).
Invasion of the predatory cladoceran obviously increases overall stability and functional diversity of the Baltic Sea by incorporating additional trophic link in the food-web of the invaded sub-systems. Presently, the invasion has additionally resulted in elevated relative importance of the warm-water planktonic invertebrates in the energy flow to cold-water bentho-pelagic fish (through direct predation). In some sheltered coastal shallow areas characterised by high Cercopagis but low predator abundance in the warm season, part of the Cercopagis production may die and sink to the bottom, and undergo there heterotrophic decomposition processes. This obviously complicates energy transfer to higher trophic levels in these areas. However, studies to date have shown that sinking of dead animals to bottom is probably not intense in deeper areas. As this euryhaline species originates from warmer climate conditions, the global warming should favour the extension of its area in the Baltic Sea region and support further increase in abundance.
Density of the most abundant small-sized cladoceran in this area - Bosmina coregoni maritima - has significantly decreased after the invasion of the predatory Cercopagis. Although the abundance of Bosmina. was already in the declining phase before Cercopagis invasion there were no such a long-lasting low-abundance values recorded since 1970.

Abundance dynamics of Bosmina coregoni maritima in Pärnu Bay (NE Gulf of Riga) in 1970-2001.

Significant changes have also taken place at the seasonal scale: after the invasion, Bosmina disappeared from the zooplankton community several weeks earlier compared to the pre-invasion time. The field observations suggest that, at the seasonal scale, the decline of copepods (e.g., nauplii, Acartia spp, Eurytemora affinis) was recorded in the time when abundance of Bosmina was not sufficient to facilitate energetic needs of Cercopagis in spring- early summer (copepod nauplii) and when it was nearly depleted in late summer (copepod nauplii, Acartia spp).
Cercopagis is generally not a favoured prey for planktivorous fish and 0-group individuals totally avoid the cladoceran. However, being energetically profitable prey (probably due to relatively large dimensions) the cladoceran can periodically make a substantial portion (up to 100%) of fish diet. Consumption of Cercopagis by some fish with different abiotic preferences (e.g., cold-water preferring and therefore deeper water column inhabiting smelt and older herring) may reflect forced dietary selection of these fish due to shortage or lack of suitable prey in deeper water layers.
The direct impact of this invasion to fisheries is through chocking of fishing equipment. This may cause substantial harm to fishers - the estimated economic loss in one fish farm in the eastern Gulf of Finland averaged for 1996 -1998 at minimum 50,000 USD (Panov et al. 1999).

8. More information
Henn Ojaveer, e-mail: henn@sea.ee
Mart Simm, e-mail: mart@klab.envir.ee

9. References and other related literature
Antsulevich, A., Välipakka, P. 2000. Cercopagis pengoi - New Important Food Object of the Baltic Herring in the Gulf of Finland. - Internat. Rev. Hydrobiol. 85: 609-619.
Avinski, V. 1997. Cercopagis pengoi - a new species in the eastern Gulf of Finland ecosystem. In Proceedings of the final seminar of the Gulf of Finland year 1996. March 17-18, Helsinki, Finland. Edited by J. Sarkkula. Suomen Ympäristokeskus, Helsinki, pp. 247-256.
Bielecka, L., Zmijewska, M.I. and Szymborska, A. 2000. A new predatory cladoceran Cercopagis (Cercopagis) pengoi (Ostroumov 1891) in the Gulf of Gdansk. Oceanologia 42: 371 - 374.
Cristescu M.E.A., Hebert P.D.N., Witt J.D.S., MacIsaac H.J. and Grigorovich I.A. 2001. An invasion history for Cercopagis pengoi based on mitochondrial gene sequences. Limnol. Oceanogr. 46: 224 - 229.
Gorokhova, E., Aladin, N. and Dumont, H. 2000. Further expansion of the genus Cercopagis (Crustacea, Branchiopoda, Onychopoda) in te Baltic Sea, with notes on the taxa present and their ecology. Hydrobiologia 429: 207-218.
Grigorovich, I.A., MacIsaac, H.J., Rivier, I.K., Aladin, N.V. and Panov, V.E. 2000. Comparative biology of the predatory cladoceran Cercopagis pengoi from Lake Ontario, Baltic Sea and Caspian Sea. - Arch. Hydrobiol. 149: 23-50.
Hebert, P.D.N. and Cristescu M.E.A. 2002. Genetic perspectives on invasions: the case of the Cladocera. Can. J. Fish. Aquat. Sci. 59: 1229 - 1234.
Kivi, K. 1995. Petomainen vesikirppu saattaa kotiutua Suomeen. Helsingin Sanomat 23-9-1995.
Krylov, P.I., Bychenkov, D.D., Panov, V.E., Rodionova, N.V. and Telesh, I.V. 1999. Distribution and seasonal dynamics of the Ponto-Caspian invader Cercopagis pengoi (Crustacea, Cladocera) in the Neva Estuary (Gulf of Finland). Hydrobiologia 393: 227-232.
Krylov, P.I. and Panov, V.E. 1998. Resting eggs in the life cycle of Cercopagis pengoi, a recent invader of the Baltic Sea. - Arch. Hydrobiol. Spec. Issues Advanc. Limnol. 52: 383 - 392.
Leppäkoski, E., Gollasch, S., Gruszka P., Ojaveer H., Olenin S. and Panov V. 2002. The Baltic - a sea of invaders. Can. J. Fish. Aquat. Sci. 59: 1175 - 1188.
MacIsaac, H.J., Grigorovich, I.A., Hoyle, J.A., Yan, N.D., and Panov, V. 1999. Invasion of Lake Ontario by the Ponto-Caspian predatory cladoceran Cercopagis pengoi. Can. J. Fish. Aquat. Sci. 56: 1-5.
Makarewicz, J.C., Grigirovich. I.A., Mills, E., Damaske, E., Cristescu, M.E., Pearsall, W., LaVoie, M.J., Keats, R., Rudstam, L., Hebert, P., Halbritter, H., Kelly, T., Matkovich, C. and MacIsaac, H.J. Distribution, fecundity, genetics and dispersal of Cercopagis pengoi (Crustacea, Cladocera) - a new exotic zooplankter in the Great Lakes basin. Journal of Great Lakes Research 27: 19-32.
Martin, J.W. and Cash-Clark, C.E. 1995. The external morphology of the onychopod 'cladoceran' genus Bytotrephes (Crustacea, Branchiopoda, Onychopoda, Cercopagididae), with notes on the morphology and phylogeny of the order Onychopoda. Zoologica Scripta, 24: 61-90.
Mordukhai-Boltovskoi, F.D. 1967. On the males and gamogenetic females of the Caspian Polyphemidae (Cladocera). Crustaceana 12: 113-123.
Mordukhai-Boltovskoi, F.D. 1968. On the taxonomy of the Polyphemidae. Crustaceana 14: 197-209.
Mordukhai-Boltovskoi, F. D. and Rivier, I. K. 1987. Predatory cladocerans (Podonidae, Polyphemidae, Cercopagidae and Leptodoridae) in the world's fauna. Leningrad, "Nauka" 1 - 182 (in Russian).
Naumenko, E. and Polunina, Yu.Yu. 2000. New cladocera species - Cercopagis pengoi (Ostroumov, 1891) (Crustacea) in the Vistula Lagoon of the Baltic Sea. ICES C.M./U:14.
Ojaveer, E., Lumberg, A. and Ojaveer, H. 1998. Highlights of zooplankton dynamics in Estonian waters (Baltic Sea). ICES Journal of Marine Science 55: 748 - 755.
Ojaveer, H., Kuhns, L.A., Barbiero, R.P. and Tuchman, M.L. 2001. Distribution and Population Characteristics of Cercopagis pengoi in Lake Ontario. J. Great Lakes Res. 27: 10 - 18.
Ojaveer, H., Leppäkoski, E., Olenin, S., and Ricciardi, A. 2002. Ecological impacts of Ponto-Caspian invaders in the Baltic Sea, European inland waters and the Great Lakes: an inter-ecosystem comparison. In (eds. E. Leppäkoski, S. Gollasch and S. Olenin) Invasive Aquatic Species of Europe: Distribution, Impacts and Management. Kluwer Scientific Publishers, Dorthrecht, The Netherlands.
Ojaveer, H. and Lumberg, A. 1995. On the role of Cercopagis (Cercopagis) pengoi (Ostroumov) in Pärnu Bay and the NE part of the Gulf of Riga ecosystem. - Proceedings of the Estonian Academy of Sciences, Ecology 5: 20 - 25.
Ojaveer, H., Simm, M. Lankov, A. and Lumberg, A. 2000. Consequences of invasion of a predatory cladoceran. ICES C.M. 2000/U:16.
Panov, V.E., Krylov, P.I. and Telesh, I.V. 1999. The St. Petersburg harbour profile. In Initial risk assessment of alien species in Nordic coastal waters. Edited by S. Gollasch and E. Leppäkoski. Nordic Council of Ministers, Copenhagen, pp. 225-244.
Rivier, I. 1998. The predatory Cladocera (Onychopoda: Podonidae, Polyphemidae, Cercopagidae) and Leptodoridae of the world. Backhuys Publishing, Leiden, The Netherlands.
Rivier, I. 2000. On the species diversity of subgenus Cercopagis (Cercopagidae, Polyphemoidea) in the Baltic Sea. Abstracts of Aquatic Sciences Meeting Research Across Boundaries. June 5 - 9, Copenhagen, Denmark. American Society of Limnology and Oceanography (ASLO), 2000, SS21- p06.
Simm, M. and Ojaveer, H. 1999. Occurrence of different morphological forms of Cercopagis in the Baltic Sea. Proceedings of the Estonian Academy of Sciences, Biology, Ecology 48: 169 - 172.
Strake S. 2000. Variations of mesozooplankton species composition, abundance, and biomass in the littoral zone of the Gulf of Riga, Latvia. Proceedings of the Latvian Academy of Sciences. Section B 54: 155 - 159.
Telesh, I.V., Alimov A.F., Golubkov, S.M., Nikulina V.N. and Panov V.E. 1999. Reponse of aquatic communities to anthropogenic stress: a comparative study of Neva Bay and the eastern Gulf of Finland.. Hydrobiologia 393: 95 - 105.
Telesh, I. and Ojaveer, H. 2002. Cercopagis pengoi (Ostroumov, 1891) in the Baltic Sea: invasion history, distribution and implications to ecosystem dynamics. In (eds. E. Leppäkoski, S. Gollasch and S. Olenin) Invasive Aquatic Species of Europe: Distribution, Impacts and Management. Kluwer Scientific Publishers, Dorthrecht, The Netherlands.
Uitto, A., Gorokhova, E., Välipakka, P. 1999. Distribution of the non-indigenous Cercopagis pengoi in the coastal waters of the eastern Gulf of Finland. ICES Journal of Marine Science 56 Supplement: 49 - 57.
Vanderploeg, H.A., Nalepa T.F., Jude D.J., Mills E.L., Holeck K.T., Liebig J.R., Grigorovich I.A., Ojaveer H. 2002. Dispersal and emerging ecological impacts of Ponto-Caspian species in the Laurentian Great Lakes. Canadian Journal of Fisheries and Aquatic Sciences 59: 1209 - 1228.
Zmudzinski, L. 1999. Cercopagis pengoi (CLADOCERA) conquered the southern Baltic Sea. Baltic Coastal Zone 2: 95-96.

1. Scientific name: Maeotias marginata (Modeer; 1791) (= M. inexspectata Ostroumoff, 1896) (Limnomedusae, Hydrozoa)
Common name: None

Adult medusa: Photo by David Wrobel (http://jellieszone.com/maeotias.htm)

Young medusa: Photo by Arno Põllumäe

2. Identification
The umbrella of M. marginata is roughly hemispherical. The bell diameter of mature medusa is 30-50 mm. Gonads hang down from each of the four radial canals in folded sheets. The mesoglea of the bell is clear in young specimens becoming more opaque as the medusa develops. The rim of the bell has red shadow due to discrete areas of pigmentation at the bases of tentacles. Mature medusa has about 450-600 tentacles, all of identical structure and up to 0,5 mm thick. Characteristic for Maeotias are also centripetal canals visible between four radial canals. Each centripetal canal is beginning at the ring canal and extending towards but not actually reaching the stomach (Väinölä and Oulasvirta, 2001, Rees and Gershwin, 2000).

3. Natural distribution
Although the first description of Maeotias marginata was made on the basis of specimens collected in the Netherlands in 1762, this species is considered as native to Azov and Black Sea, the mouths of rivers Don and Kuban and Danube estuary. In the region of origin it has described as very rare species, the polyp stage of Maeotias is never found in nature there. All locations where the species is found are characterized by waters of low salinity (from 4.2 to 10.7 PSU) (Mills & Rees, 2000).

4. First finding in Estonia
Väinameri Archipelago, western Estonia in 1999.

5. Invasion history in the Baltic Sea
First found in Väinameri Archipelago in late summer 1999, where it was observed in four locations (Väinölä R and Oulasvirta P 2001). Presence of the species was confirmed in the same region in 2002. No other findings of Maeotias are known from the Baltic Sea so far.

6. Distribution and population dynamics in Estonian waters
Density of Maeotias in Väinameri is very low. In 1999 about dozen specimens were found in four locations (Väinölä R and Oulasvirta P 2001) and in 2002 only two specimens were caught in one sampling site.

Finding locations of Maeotias in Estonian waters

7. Ecological and economical impact
There is no published information about the effect of Maeotias marginata on surrounding benthic or planktonic communities. The medusa is feeding nearly exclusively on small crustacean planktonic and infaunal organisms. The feeding rates of M. marginata have found high. Nearly all individuals examined had full guts with many food items (Mills and Sommer, 1995). Since there is no information about species reaching the bloom status, the drastic ecological or economical impacts are not expected. With reference to spreading ecology of Maeotias it must be consider that hydrozoan polyps also propagate asexually and produce resting stages, pedal cysts. The importance and efficiency of the asexual mode for colonizing hydrozoans, and the inefficiency of sexual medusa for established stocks, is evident from the fact that introduced medusa populations are often unisexual, i.e., probably derived from and propagated through one or a few colonizing genets of single sex (Dumont, 1994).

8. More information
Arno Põllumäe, e-mail: arno@sea.ee

9. References
Dumont, H J. 1994. The distribution and ecology of the fresh- and brackish-water medusae of the world. Hydrobiologia 272: 1-12.
Mills, C.E. and Rees, J.T. 2000. New observations and corrections concerning the trio of invasive hydromedusae Maeotias marginata ( =M. inexpectata), Blackfordia virginica, and Moerisia sp. in the San Francisco Estuary. Scientia Marina 64 Suppl. 1: 151-155.
Mills, C.E. and Sommer, F. 1995. Invertebrate introductions in marine habitats: two species of hydromedusae (Cnidaria) native to the Black Sea, Maeotias inexspectata and Bkackfordia virginica, invade San Francisco Bay. Marine Biology 122: 279-288.
Rees, J.T. and Gershwin, L.-A. 2000. Non-indigenous hydromedusae in California's upper San Francisco Estuary: life cycles, distribution, and potential environmental impacts. Sci. Mar. 64 Suppl. 1: 73-86.
Väinölä, R. and Oulasvirta, P. 2001. The first record of Maeotias marginata (Cnidaria, Hydrozoa) from the baltic Sea: a Pontocaspian invader. Sarsia 86: 401-404