Further expansion of the genus Cercopagis (Crustacea, Branchiopoda, Onychopoda) in the Baltic Sea, with notes on the taxa present and their ecology

The onychopod cladoceran Cercopagis that recently invaded the Baltic Sea is reported from new zones of the northern Baltic proper. Because of successf ul survival and an expanding distribution range, the addition of Cercopagis to the Baltic fauna is considered to be permanent. What has previously bee n cited as Cercopagis pengoi encompasses the morphology of several other sp ecies, subspecies and forms. Either a number of morphologically similar spe cies is present, or there is a number of spurious species in Cercopagis. Th e last hypothesis is favoured. The spatial distribution pattern of Cercopag is, as well as that of total zooplankton, was correlated with depth. Deep ( > 100 m) and shallow (< 10 m) stations had significantly lower abundance th an stations of intermediate depth (< 100 m). An overview of the distributio n of C. pengoi group in fresh and brackish waters suggests a high tolerance to environmental factors, but with differences among taxa. Due to this eco logical flexibility, the colonization of the Baltic is not unexpected. Incr easing salinity may restrict dispersal of cercopagids to the southern areas of the Baltic and to the North Sea, but inland lakes (e.g. in Sweden) pres ent an ecological profile suitable for colonization.