T h e C a s p i a n p r e d a t o r y c l a d o c e r a n C e r c o p a g i s p e n g o i i n v a d e s the Gulf of Finland

Vadim E. Panov, Piotr I. Krylov and Irena V. Telesh Zoological Institute of the Russian Academy of Sciences

During recent decades the Baltic Sea has experienced invasions by dozens of exotic species and most of these invasions were accidental (Leppakoski, 1984; Jansson, 1994). Because of the negative consequences at community and ecosystem level, the introduction of alien species can be considered as biological pollution. The adverse effects of biological pollution can be similar to, and sometimes exceed, the effects of other forms of anthropogenic impact, such as eutrophication and toxic pollution. In becoming established in new environments, the density of alien species often explode because of the absence of the specific parasites, predators and competitors that would normally control the population of the species in their native habitats. Until recently few invaders of the Baltic Sea had been observed to cause clear, adverse effects on Baltic ecosystems (Leppakoski, 1993), but the recent arrival of the Caspian 'zooplanktoner' Cercopagis pengoi begins to represent an example of an accidental introduction of an alien species which could have a profound effect on the whole Baltic Sea ecosystem.

In the Baltic, Cercopagis was first discovered ?n the Gulf of Riga in 1992, and probably entered this area in the ballast water of ships (Ojaveer, Lumberg, 1995). In the autumn of 1995 Cercopagis was registered, for the first time, as abundant zooplankton in the eastern Gulf of Finland (Bychenkov, Rodionova, 1996, Avinsky, 1997) and also as a biofouler of fishing nets (Kivi, 1995). Cercopagis pengoi belongs to the class Crustacea, order Cladocera, family Cercopagidae and the genus Cercopagis, which has 13 species most of which are restricted to the Caspian Sea. Unlike most Cercopagis species, Cercopagis pengoi is known to be able to spread out from its native habitats in the Caspian. It has been found in the Aral Sea and the Azov Sea, and in the brackish lagoons and freshwater reservoirs of the Volga and Don basins at latitudes lower than 50°N. But, until the recent invasion of the Baltic Sea Cercopagis had not been registered in areas at higher latitudes. Cercopagis pengoi is easily recognized by its unique, long, tail spine, which accounts for more than 80 percent of the animal's total length, which in adults is about 1 cm (Figure 1). The spine usually contains from one to three pairs of thorn-like barbs and is, most likely, a defensive structure against fish predation. It has been shown that juvenile fish in the Gulf of Riga are not able to consume Cercopagis (Ojaveer, Lumberg, 1995). The head consists primarily of a single, large eye filled with orange-brown pigment. Just behind the head is a pair of swimming antennae. On their backs the reproductive females carry a forward-directed, sharply-formed, brood pouch filled with eggs or developing offspring. The biology of Cercopagis in the Caspian Sea basin is poorly known. As with most cladocerans, Cercopagis exibits parthenogenesis (asexual reproduction) during most of the season, with males appearing under unfavourable conditions e.g. low food or decreasing temperature in autumn. These males are able to mate with females, who produce resting eggs. The resting eggs are first carried as brown spheres in the female brood pouch. They are later released and fall to the bottom, where they are able to survive the cold winter. In spring or early summer these eggs hatch into juvenile females that begin the parthenogenic reproduction cycle once again. Resting eggs can remain dormant for long periods of time and their transportation in the ballast water of ships is most likely the main mechanism of their dispersal and the means by which Cercopagis arrived in the Baltic Sea. The Cercopagis populations that have become established in the eastern Gulf of Finland exibit a remarkably different pattern of reproduction to those in the Caspian. Females with resting eggs (gamogenetic females) were first found in early August, when water temperatures are highest and the time when the Cercopagis population is on the increase. Infact, gamogenetic females comprised over 60% of the total Cercopagis population.This reproductive strategy leads to the development of a large store of resting eggs in the eastern Gulf environment. Because these eggs are able to survive unfavourable conditions, this large pool of resting eggs has enabled Cercopagis to survive in this new environment and achieve fast population growth under favourable conditions. In some locations in the Baltic Sea, population densities of Cercopagis may exceed 300 individuals per cubic meter, which is a very high number for a relatively large zooplankton predator. The ability of the Gulf of Finland Cercopagis to produce a large number of resting eggs throughout most of the season increases the risk that this predator will be dispersed, in the ballast waters of ships, from the Gulf of Finland to other aquatic ecosystems around the world.

The possible adverse consequences of the Cercopagis invasion for Baltic Sea ecosystems may be summarised as follows: 1) alterations to the structure of the zooplankton community, including possible elimination of some species; 2) strong competition for food with planktivorous fish, because small grazing cladocerans are also the main food source for Cercopagis. The lengthening of food chains can also have a negative effect on fish yield by reducing the amount of available energy;

3) increasing total predation on grazing cladocerans can enhance the growth of undesirable algae, because grazing cladocerans, to some extent, control the growth of planktonic algae; 4) the long, sharp, tail spine of Cercopagis can cause injury in fish, as has been shown for species of Bythotrephes (Barnhisel, 1991); 5) increase in the levels of toxic substances in piscivorous fish, because of the lengthening of their supporting food chain; 6) biofouling of fishing equipment may also be seen as a problem. There is an urgent need for more studies on the effects of Cercopagis and other invading species on the food webs and fish communities of the Baltic Sea. These studies should be made in combination with special efforts to assess the risk of new invasions by alien species, especially with regard to the on-going development of new ports in the eastern Baltic. Studies on the biology of invading species and proper attention to risk assessment of exotic introductions would provide the basis for effective management decisions to deal with the increasing risk of further biological pollution of Baltic ecosystems.

Acknowledgements The present study is being supported by the Russian Fund for Basic Research (N 96-04-48109). We thank Mr. Richard Thompson Coon for correcting the English and valuable discussion.

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Vadim E. Panov, Piotr I. Krylov and Irena V. Telesh Zoological Institute of the Russian Academy of Sciences 199034 St. Petersburg, Russia E-mail:[email protected]