The evolutionary significance of heterochrony in the abbreviated zoeal development of pilumnine crabs (Crustacea: Brachyura: Xanthoidea)

The zoeal development of Pilumnus hirtellus (Linnaeus, 1761) is redescribed and the four stages are compared with the abbreviated development of Actumnus setifer (de Haan, 1835) with three stages, and Pilumnus sluiteri De Man, 1892 with two stages. A number of characters are not affected by abbreviated zoeal development and do not change during successive stage moults. Of these, some traits remain conservative at higher taxonomic level, whereas others varied between closely related pilumnid taxa, but neither provided phylogenetic information within the three pilumnines studied. However, abbreviated zoeal development affected 23 pilumnine characters that change with successive stage moults. Their timing of appearance and rate of development occur at different stages relative to the homologous process in an ancestral sequence with more zoeas, and can be attributed to three heterochronic mechanisms; postdisplacement, predisplacement and acceleration. These processes collectively appear to provide the predominant mechanism underlying the evolution of oligomerization within pilumnine zoeas.