Phylogenetic relationships and evolution in the genus Dissodactylus Smith, 1870 (Crustacea: Brachyura: Pinnotheridae)

Abstract

A cladogram indicating sister-group relationships within the pinnotherid crab genus Dissodactylus is constructed from analysis of 28 morphological characters. The outgroup method was used to determine character polarities. Parapinnixa and Sakaina are shown to be a monophyletic group, and are suggested to be the sister group of Dissodactylus. Within Dissodactylus, two mutually exclusive, monophyletic subgroups are found, one containing nine, the other four species. Both of these groups include Atlantic and Pacific representatives. Two Atlantic–Pacific species pairs are suggested to be sister species (D. mellitae–D. glasselli and D. primitivus–D. schmitti). Members of the group of nine species are inhabitants primarily of mellitid sand dollars, although one species, D. primitivus, is a symbiont of heart urchins (Spatangoida). Members of the group of four species are inhabitants of sea biscuits of the genus Clypeaster (Clypeasteridae). An evolutionary scenario is presented, which suggests that a free-living ancestor colonized mellitid sand dollars, and subsequent specializations led to the colonizations of the genus Clypeaster and genera of Spatangoida.