A molecular phylogeny of the patellid limpets (Gastropoda: Patellidae) and its implications for the origins of their antitropical distribution

The geographical distribution of the limpet family Patellidae is essentially antitropical, with 18 species in southern Africa, 10 in the northeastern Atlantic, and only 11 species elsewhere (although 4 of these do occur in the tropics). One possible explanation for this distribution is the suggestion of a recent, perhaps Early Pliocene, migration from southern Africa northward. We tested this hypothesis by constructing a molecular phylogeny, derived from partial sequences of the 12S and 16S mitochondrial genes, obtained from 34 of the 38 patellid species. Five species of Nacellidae and 3 of Lottiidae were included as potential outgroups. Analysis revealed that two patellid clades are represented in the northeastern Atlantic. The typical European patellids (Patella sensu stricto) form a single clade within which there is little molecular divergence, but are distant from all other patellids, thus refuting the idea of recent southern ancestry. From the limited fossil record and estimated rates of molecular divergence, we suggest that Patella s.s. may have originated at least as early as the Upper Cretaceous and that its northern distribution may have been achieved at the same time. The second patellid clade present in the northeastern Atlantic is the genus Cymbula, of which the single species Cymbula safiana extends from West Africa to the Mediterranean. In contrast to Patella s.s., C. safiana is indeed a member of an otherwise southern African clade and may have attained its present distribution more recently, during the Miocene. The geographical origin of the family remains unclear, but a Mesozoic radiation in southern Gondwana is possible. By optimizing morphological characters on our molecular tree, we consider the evolution of shell mineralogy and sperm ultrastructure. We also discuss the phylogenetic classification of the patellids and present some evidence that the family may not be monophyletic.