Hydrobiologia 429: 207–218, 2000. 207 © 2000 Kluwer Academic Publishers. Printed in the Netherlands. Further expansion of the genus Cercopagis (Crustacea, Branchiopoda, Onychopoda) in the Baltic Sea, with notes on the taxa present and their ecology Elena Gorokhova1;∗, Nikolai Aladin2 & Henri J. Dumont3 1Department of Systems Ecology, Stockholm University, 109 61 Stockholm, Sweden Present address: Department of Biology, Arizona State University, Tempe, U.S.A. Fax: [+1] 480-965-2519. E-mail:
[email protected] 2Zoological Institute, Russian Academy of Science, 199034 St. Petersburg, Russia 3Laboratory of Animal Ecology, University of Ghent, B-9000 Ghent, Belgium Received 5 May 1999; in revised form 10 September 1999; accepted 20 September 1999 Key words: Cercopagis, invasion, Baltic Sea, depth, temperature, salinity, morphological variations Abstract The onychopod cladoceran Cercopagis that recently invaded the Baltic Sea is reported from new zones of the north- ern Baltic proper. Because of successful survival and an expanding distribution range, the addition of Cercopagis to the Baltic fauna is considered to be permanent. What has previously been cited as Cercopagis pengoi encompasses the morphology of several other species, subspecies and forms. Either a number of morphologically similar species is present, or there is a number of spurious species in Cercopagis. The last hypothesis is favoured. The spatial distribution pattern of Cercopagis, as well as that of total zooplankton, was correlated with depth. Deep (>100 m) and shallow (<10 m) stations had significantly lower abundance than stations of intermediate depth (<100 m). An overview of the distribution of C. pengoi group in fresh and brackish waters suggests a high tolerance to environmental factors, but with differences among taxa.