Limnol. Oceanogr., 44(6), 1999, 1381–1387

The species overlap between European and North American cladoceran faunas has been questioned from the very beginning of faunistic studies on Nearctic zooplankton and still remains unsettled. However, more recently, genetic techniques seem to overcome the limits of morphology as the basis for taxonomy. We analyzed 8–11 allozyme loci to assess the genetic differentiation between intercontinental counterparts of four daphniids: Daphnia curvirostris, Daphnia obtusa, Daphnia pulex, and Daphnia pulicaria. Despite their morphological likeness, all equivalents showed a large amount of genetic divergence between continents due to complete allelic substitutions and major gene frequency shifts. From an evolutionary perspective, this study shows that as in other organisms, geographic isolation seems to be sufficient to minimize gene flow to levels at which the origin of new daphniid species can be considered. From a practical point of view, a revision of Holarctic daphniid taxonomy should be considered. The use of allozyme analysis in taxonomic studies on zooplankton has brought a new dimension to the traditional phenotypic approach by allowing the study of underlying patterns of genetic variability. Allozyme studies have proven invaluable in clarifying species boundaries in a local context, allowing the identification of sibling species and enabling the recognition of interspecific hybrids. Coupled with DNA sequencing studies, these investigations are now providing insights concerning both speciation processes and patterns of phylogenetic diversification. The most intensive genetic studies on the freshwater zooplankton have examined cladoceran crustaceans, especially members of the genus Daphnia. Much taxonomic work has focused on the North American fauna, although detailed studies have also been completed in Australia (Hebert and Wilson 1994). These analyses have shown that there is no species overlap between these two continents, reinforcing the conclusions of earlier morphological work. Interestingly, there has been little effort directed toward examination of the genetic similarity between the European and North American faunas, an issue of more importance, because these faunas show an uncertain amount of overlap in species composition. Birge (1918) concluded that all species of Daphnia in North America were conspecific with European forms. Brooks (1957) reduced the extent of overlap, recognizing six North American endemics, but concluded that the remaining nine species had Holarctic distributions. A recent taxonomic revision of the North American fauna, driven by genetic analyses, raised the number of recognized taxa to 34 and increased the level of endemism to 75%, due largely to the discovery of new species apparently restricted to this continent (Hebert 1995). However, in the sole case where