ORIGIN AND RECENT ENDEMIC DIVERGENCE OF A CASPIAN MYSIS SPECIES FLOCKWITH AFFINITIES TO THE GLACIAL RELICT CRUSTACEANS IN BOREAL LAKES

Aspects of the evolution of intralacustrine species flocks and of the origin of the Arctic or ''glacial-relict'' zoogeographical element in Eurasian inland waters were elucidated in an allozyme study of the cru stacean genus Mysis. This element, of supposedly northern marine ances try, is represented by vicarious taxa in the deeper parts of the Caspi an Sea (an enclosed ancient basin) and in young boreal lakes. The thre e endemic Caspian Mysis species studied are very close genetically (Ne i's D = 0.06), which suggests a recent intrabasin radiation and rapid morphological divergence. This is in contrast to the pattern in postgl acial Holarctic boreal lakes, where the Mysis relicta group is represe nted by a set of morphologically uniform but probably much older sibli ng species (D = 0.3-0.6). The results provide a parallel to those on t he recent diversification of some fish species flocks in ancient fresh water lakes. The situation is, however, unusual in that the Caspian sy mpatric Mysis flock is pelagic, and conditions promoting speciation th rough allopatric isolation or spatial segregation by trophic substrate specialization seem implausible. The monophyletic Caspian Mysis clade shows a relatively strong divergence from both the northern lacustrin e and the Arctic marine congeners (D = 0.6-1.0); the phylogenetic bran ching order of these three zoogeographical groups is not conclusively resolved. The results contradict the prevailing hypothesis of a recent Pleistocene origin of the Caspian Arctic element by invasion from Eas tern European continental proglacial lakes that drained south to the C aspian basin during the glacial maxima and served as refugia for the b oreal lacustrine taxa.