Mitochondrial sequence variation suggests extensive cryptic diversity within the Western Palearctic Daphnia longispina complex

We ; report on a discovery of six divergent lineages within the European Daphnia longispinacomplex from various localities in central to northeast Europe. The levels of divergence from well-known species of the complex suggest that they represent as yet unrecognized distinct taxa. These newly recognized lineages always coexisted in syntopy with widespread species of the complex. Particularly rich in cryptic diversity (with four of the six lineages present) was the delta of the River Pechora in northern European Russia, a region not covered by an ice sheet during most of the last glacial period. We suggest that ice-free regions of northeastern Europe may have been important refugia for planktonicspecies, and still are overlooked hot spots of diversity. Our findings confirm that the real diversity within widespread crustacean planktonictaxa is much higher than presently recognized. The potential presence of cryptic species should be considered in ecological studies. Cryptic species, i.e., phenotypically similar but evolutionarily distinct (and often genetically highly divergent) species, are common throughout the animal kingdom (Pfenninger and Schwenk 2007), and cladocerans are not