An increase of xenodiversity in plankton and benthos in the eastern Gulf of Finland was observed from 1998 to 2004. Nonindigenous species account for 4.8% of all species found and up to 96% of total biomass. Invasive benthic omnivores, the alien amphipods Gmelinoides fasciatus and Pontogammarus robustoides and the predaceous fish Perccottus glenii with their versatile diets strongly affect the community structure. Invasive sessile seston-feeders that directly (through grazing and water clearance) and indirectly (through recycling of nutrients) interact with other ecosystem components, are mainly represented by the zebra mussel Dreissena polymorpha, which affect the structure of benthic and planktonic communities as well as benthicpelagic coupling. The invasive predatory cladocerans Cercopagis pengoi and Evadne anonyx and larvae of D. polymorpha are only temporary components in the zooplankton, which is limiting their overall effect. Alien benthic bioturbators, the polychaetes Marenzelleria neglecta and the oligochaete Tubificoides pseudogaster account for a high proportion of total abundance and biomass but their effects on native species need further research.

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Morphological variation in the genus Bythotrephes was studied. We analyzed samples from 29 locations through the generic range. Based on the core body length and the total length, as well as several other morphological characters, differences were found that are consistent across five groups of populations previously described as a separate species (B. cederstroemii, B. crassicaudus, B. transcaucasicus, B. brevimanus, and B. longimanus). Individuals with intermediate morphological characteristics were detected where B. cederstroemii, B. brevimanus and B. crassicaudus have geographic overlap. Based on allozyme analysis, hybrids of B. cederstroemii and B. brevimanus were revealed in the zone of contact of these two species in the Volga River drainage and northwestern Russia.

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