Trans-Arctic Migration and Speciation Induced by Climatic Change: The Biogeography of Littorina (Mollusca: Gastropoda)

The genus Littorina comprises 18 or 20 species, confined to the northern hemisphere. A cladistic phylogeny and distributions of these taxa form the basis of a discussion of the historical biogeography of the group. Successively more derived taxa are present in the tropical Atlantic, western North America, Southeast Asia, Japan, the northern Pacific and the northern Atlantic, and similar area relationships are suggested by some other taxonomic groups. A dispersalist interpretation suggests a Tethyan origin and westward range expansion, but in the absence of a complete fossil record this cannot be confirmed. It is argued that westward trans-Pacific dispersal is unlikely. Instead, a vicariant model is proposed, involving speciation in response to climatic cooling during the Cenozoic. This explains the progression of more derived species at higher latitudes on the Asian coast, which could be a common pattern in marine shallow-water organisms on continental margins. The cladogram suggests that the northern Atlantic species originated from at least two immigrants from the Pacific. The fossil record indicates that these reached the Atlantic by an Arctic route following the opening of the Bering Strait in the Upper Pliocene. In both oceans, clades with nonplanktotrophic development have shown the greater rates of speciation. Within the Atlantic Ocean there is no clear area cladogram; instead there is an ecological pattern: species with a potential for wider dispersion have wider geographical ranges.