A taxonomic reevaluation of North American Daphnia (Crustacea: Cladocera). II. New species in the Daphnia pulex group from the south-central United States and Mexico

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Abstract
Although the establishment of species boundaries in the genus Daphnia is complicated by the prevalence of interspecific hybrids and by phenotypic plasticity, genetic studies can resolve these complexities. This investigation employed allozyme analyses to critically assess species boundaries in members of the Daphnia pulex group from the south-central United States and Mexico. These studies demonstrated the occurrence of three common Nearctic species (obtusa, pulex, pulicaria), but also revealed the occurrence of three previously unrecognized taxa (cheraphila, pileata, prolata). All of these newly described species have their distributional centroid in this region of North America and are restricted to clay-water habitats. F1 hybrids were detected between three pairs of species (cheraphila × prolata, obtusa × pileata, pulex × pulicaria), but only the latter hybrids were common. The discovery of daphniid taxa endemic to this region of North America contrasts with the results of a broader survey of sites in Canada, and suggests that additional species await description from other unglaciated regions of North America.