The ultrastructural investigation of the spermatozoal morphology of the
                    hydrothermal vent crabs, Bythograea thermydron, Austinograea alayseae and
                    Segonzacia mesatlantica (family Bythograeidae) reveals a consistent
                    familial sperm type and a close similarity between the three genera and
                    the deep water trapeziid, Calocarcinus africanus. Association of the
                    bythograeids with Calocarcinus is supported (apart from general
                    similarity) 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 acrosome zone.
                    Calocarcinus is distinguished by shortening of the posterior dense zone
                    into a true xanthid ring and, less so, by a flatter 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 perforation in the
                    operculum). The similarity between the spermatozoa of Calocarcinus and the
                    investigated bythograeids and the dissimilarity between Calocarcinus and
                    other trapeziids is evidence for possible inclusion of Calocarcinus in the
                    Bythograeidae. This hypothesized relationship of Calocarcinus within the
                    Bythograeidae sensu lato and its deep water distribution suggests origin
                    of hydrothermal bythograeids from Calocarcinus-like deep water xanthoids
                    which may have entered the hydrothermal system in or after the Eocene.
                    Further investigation of somatic morphology is required to test this
                    putative relationship.