SYSTEMATIC REVISION OF PATELLOIDA PYGMAEA (DUNKER, 1860) (GASTROPODA: LOTTIIDAE), WITH A DESCRIPTION OF A NEW SPECIES

Abstract

Patelloida pygmaea (Dunker) and its closely allied species, P. heroldi (Dunker) and P. conulus (Dunker) have caused nomenclatural confusion because of their variable shell morphology and distinctive habitats. According to current nomenclature, these species of Patelloida have been synonymized and treated as one species with two ecological forms. Patelloida pygmaea lives on the shell of Crassostrea gigas (Ostreidae), P. pygmaea form heroldi occurs on intertidal rocks on sheltered shores and P. pygmaea form conulus is found on the shell of Batillaria multiformis (Batillariidae). Their taxonomic relationships and possible species status are, however, unclear. Using two mitochondrial genes (fragments of COI and 16S ribosomal RNA; total 1192 sites), we analysed 88 specimens of P. pygmaea, P. pygmaea form heroldi and P. pygmaea form conulus from 37 localities in East Asia. In the resulting molecular phylogenetic trees, the specimens of Patelloida fall into four clades with high bootstrap probabilities; these clades correspond taxonomically to P. pygmaea, P. conulus, P. heroldi and a fourth previously unrecognized taxon, Patelloida ryukyuensis n. sp., described here. A minimum-spanning network for 29 unique mitochondrial COI haplotypes obtained from 45 specimens in the same bay in central Japan form three distinct clusters, consisting of P. pygmaea, P. conulus and P. heroldi, respectively. This suggests that reproductive isolation has been established between each group. A detailed examination of radular and shell morphologies of the four taxa clarifies the morphological distinction between these species. The four species form a rather young clade in the genus Patelloida that diverged during the Pliocene. They provide an example of habitat segregation in a restricted marine environment.