SPERMATOZOAL ULTRASTRUCTURE IN 3 SPECIES OF HYDROTHERMAL VENT CRAB, IN THE GENERA BYTHOGRAEA, AUSTINOGRAEA AND SEGONZACIA (DECAPODA, BRACHYURA, BYTHOGRAEIDAE)

The ultrastructural investigation of the spermatozoal morphology of th e hydrothermal vent crabs, Bythograea thermydron, Austinograea alaysea e and Segonzacia mesatlantica (family Bythograeidae) reveals a consist ent familial sperm type and a close similarity between the three gener a and the deep water trapeziid, Calocarcinus africanus. Association of the bythograeids with Calocarcinus is supported (apart from general s imilarity) by three synapomorphies: modification of the xanthid ring, development of a well developed periopercular rim (weakly shared with some xanthid and potamid members) and, as a particularly striking link , the unique spiral configuration of the contents of the outer acrosom e zone. Calocarcinus is distinguished by shortening of the posterior d ense zone into a true xanthid ring and, less so, by a natter operculum . Bythograea and Segonzacia are apomorphic in the independent loss of the ragged outer acrosome profile. Their sister genus Austinograea has no distinct apomorphies (with the possible exception of an apical per foration in the operculum). The similarity between the spermatozoa of Calocarcinus and the investigated bythograeids and the dissimilarity b etween Calocarcinus and other trapeziids is evidence for possible incl usion of Calocarcinus in the Bythograeidae. This hypothesized relation ship of Calocarcinus within the Bythograeidae sensu late and its deep water distribution suggests origin of hydrothermal bythograeids from C alocarcinus-like deep water xanthoids which may have entered the hydro thermal system in or after the Eocene. Further investigation of somati c morphology is required to test this putative relationship.