Phylogeography of the whelk genus Cominella (Gastropoda: Buccinidae) suggests long-distance counter-current dispersal of a direct developer

Abstract

The gastropod genus CominellaGray, 1850 consists of approximately 20 species that inhabit a wide range of marine environments in New Zealand and Australia, including its external territory, the geographically isolated Norfolk Island. This distribution is puzzling, however, with apparently closely-related species occurring either side of the Tasman Sea, even though all species are considered to have limited dispersal abilities. To determine how Cominella attained its current distribution, we derived a dated molecular phylogeny, which revealed a clade comprising all the Australian and Norfolk Island species nested within four clades of solely New Zealand species. This Australian clade diverged well after the vicariant separation of New Zealand from Australia, and implies two long-distance dispersal events: a counter-current movement across the Tasman Sea from New Zealand to Australia, occurring at the origination of the clade, followed by the colonization of Norfolk Island. The biology of Cominella suggests that the most likely method of long-distance dispersal is rafting as egg capsules. Our robust phylogeny also means that the current Cominella classification requires revision. We propose that our clades be recognized as subgenera: Cominella (s.s.), Cominista, Josepha, Cominula, and Eucominia, with each subgenus comprising only of New Zealand or Australian species.