GENETIC ZOOGEOGRAPHY OF THE MYSIS-RELICTA SPECIES GROUP (CRUSTACEA, MYSIDACEA) IN NORTHERN EUROPE AND NORTH-AMERICA

The zoogeography and systematics of the Mysis relicta species group we re elucidated in an allozyme survey of populations across northern Eur ope and North America. The North American populations are here identif ied as an independent species (sp. IV), distinct from the three previo usly recognized European M. relicta group taxa (spp. I-III). The geogr aphical pattern of gene frequency variation in North America supports a late-glacial colonization by separate eastern and western refugial s tocks of sp. IV. In Europe, sp. III is known from a single subarctic l ake, while both spp. I and II are widespread. They coexist in the Balt ic Sea, but their lacustrine distributions are largely different. Spec ies I accounts for most Fennoscandian populations and those in Poland and Germany whereas sp. II lives in Ireland, parts of southwestern Sca ndinavia, and Karelia. With the paleohydrographical reference, the dis tributions suggest that both species survived the last glaciation in p roglacial lakes east of the Scandinavian Ice. Subsequent distributiona l differentiation was influenced by environmental variations; the disp ersal of sp. II in southwestern Scandinavia was facilitated by a broad er euryhalinity than that in sp. I and other stenohaline ''glacial rel ict'' crustaceans. The Irish populations may represent a distinct refu gial stock within sp. II.