Hidden diversity in eastern North America: The genus Ligidium (Oniscidea, Ligiidae) in the southern Appalachian Mountains

The terrestrial isopod genus Ligidium includes 58 species from Europe, Asia, and North America. In Eastern North America four species are recognized: L. floridanum and L. mucronatum, known just from their type localities in Florida and Louisiana respectively, L. blueridgensis, endemic to the southern Appalachians, and L. elrodii, widespread from Georgia to Ontario. The genus shows a marked morphological conservatism, and species are differentiated mostly by small morphological differences; it is not always easy to determine if such variability represents inter‐ or intraspecific variation. Here, we explore the diversity of Ligidium from the southern Appalachian Mountains, exploring the congruence of morphologically defined groups with multilocus phylogenetic reconstructions and molecular species delimitation methods. We have studied a total of 130 specimens from 37 localities, mostly from the southern Appalachians, and analysed mtDNA (Cox1) and nuclear (28S, NaK) sequences. Morphologically, we recognized eight morphotypes, most of them assignable to current concepts of L. elrodii and L. blueridgensis. Phylogenetic analyses supported the evolutionary independence of all morphotypes, and suggest the existence of 8–9 species, including limited cryptic diversity. Single‐locus delimitation analyses based on mtDNA data suggest the existence of a much higher number of species than the multilocus analyses. The estimated age of the ancestors of sampled lineages indicates a long presence of the genus in eastern North America and old speciation events through the Miocene. Our results indicate a higher diversity than previously thought among the Ligidium populations present in the southern Appalachian Mountains, with several species to be described.