Evolution of sperm morphology in potamid freshwater crabs (Crustacea: Brachyura: Potamoidea)

We investigated sperm cells and spermatophores of four species of Old World freshwater crabs belonging to three different genera of the subfamily Potaminae (family Potamidae). Characters previously believed to be apomorphic for the potamid subfamily Potamiscinae were also found to occur in the Potaminae. To infer the morphological ancestral character state combination of the Potamidae, ancestral character state analysis of four different sperm traits was performed, based on a 16S rDNA phylogeny of the investigated species. Comparing molecular phylogeny and character state distribution, several cases of convergent evolution could be identified. The densely packed, coenospermic spermatophores and the occurrence of a ‘tongue‐and‐groove’ connection between operculum and acrosomal zones are probably apomorphies for the whole Potamidae. The spermatozoa of Socotrapotamon socotrense show several unique characters. We also analysed the evolution of acrosome size. The sperm cells of the Potamidae and their sister‐group Gecarcinucidae only slightly overlap in acrosome size. Within the investigated species, the ‘East Asia’ subclade (subfamily Potamiscinae) developed significantly larger acrosomes than the subfamily Potaminae. Our results suggest that the use of brachyuran acrosome morphology for phylogenetic inference at the family level is strongly affected by small sample size, and by convergent character evolution. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010.