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.