Cryptic species in a "living fossil" lineage: taxonomic and phylogenetic relationships within the genus Lepidurus (Crustacea: Notostraca) in North America.

Lineages which exhibit little morphological change over geologic time are evolutionarily and ecologically interesting, but often taxonomically difficult. For some morphologically conservative groups, not only is it almost impossible to identify significant changes in fossil forms over time, but the relationships among extant populations are often poorly understood due to lack of known characters which clearly delimit species. Notostracan crustaceans are a classic example of such "living fossils." The paradoxical characteristics of long-term stasis in gross morphology and hypervariability of many individual morphological characters make notostracans an especially taxonomically challenging group. We used molecular and biochemical techniques to investigate the taxonomic and phylogenetic relationships of four nominal species within the genus Lepidurus in North America, three of which had been alternately abandoned, resurrected, or synonymized under a single, globally distributed morphospecies since their original descriptions in the 1800s. Data from a 330-bp sequence of the mitochondrial 12S rDNA gene and from nine allozyme loci consistently indicate five highly genetically divergent clades among the populations we used to represent the four nominal species. Diagnostic molecular characters, magnitudes of genetic divergence among clades, and the fact that these genetically distinct clades have broadly overlapping geographical ranges strongly suggest that the five clades are reproductively isolated species. One of the nominal species (L. couesii) is not monophyletic, but rather consists of two species which are not sister taxa. The other three nominal species (L. lemmoni, L. packardi, and L. bilobatus) are supported as valid phylogenetic species. The best current hypothesis for phylogenetic relationships among the five species is provided by a simultaneous analysis of both 12S rDNA and allozyme data, which places L. bilobatus and L. "couesii"-1 as sister taxa and L. "couesii"-2 as the most basal of all the Lepidurus species included in this study. These results point to the existence of cryptic species within the current classification scheme for Lepidurus, the need for further taxonomic work within the Notostraca in general, and the role that genetic techniques can play in clarifying the systematics of morphologically conservative groups.