THE SPERMATOZOON OF CALOCARCINUS-AFRICANUS (HETEROTREMATA, BRACHYURA,CRUSTACEA) - ULTRASTRUCTURAL SYNAPOMORPHIES WITH XANTHID SPERM

Features of the spermatozoon of Calocarcinus africanus which are gener al for heterotremes and endorse its inclusion in the Heterotremata are : extension of the subacrosomal chamber almost to the anterior apex of the sperm, presence of an acrosome my zone, and presence of a thicken ed ring where the capsule surrounds the base of the subacrosomal chamb er. A feature shared with ''higher'' heterotremes is the restriction o f cytoplasm to the periacrosomal region, the arms being nuclear only, in contrast with invasion of their chromatin with cytoplasm and microt ubules in majids; and loss of a posterior median process, containing c hromatin, which is present in majids as in raninids and homolids. The relationship of Calocarcinus with xanthids is unequivocally supported by (1) presence of a posterior circumperforatorial zone, the xanthid r ing; (2) the precise form of the acrosome ray zone which is wide anter iorly and sends a long slender extension posteriorly to meet the ring; (3) the irregular external margin of the outer acrosome zone; (4) div ision of the operculum complex into a distinct upper zone and a lower, subopercular zone of lesser diameter; and (5) presence of an accessor y ring around the operculum. Modification of the xanthid ring is typic al of thoracotremes and partial modification in Calocarcinus may indic ate origin of thoracotremes from a related xanthoid stock. A differenc e of Calocarcinus sperm from those of xanthids is the (plesiomorphic) presence of centrioles, also seen in some heterotremes and thoracotrem es. No synapomorphies which are not common to other heterotremes are s hared between Calocarcinus and trapeziid sperm. Trapeziid sperm (plesi omorphically?) lack the xanthid ring, the posterior extension of the a crosome ray zone and the irregular margin of the outer acrosome zone o f Calocarcinus and xanthids. Apomorphic features of the Calocarcinus a fricanus sperm include a spiral configuration of the contents of the o uter acrosome zone (autapomorphy?), as seen in cross-section, and pres ence of a periopercular rim. A well developed periopercular rim is kno wn elsewhere only in Potamonautes (family Potamidae), but a rudiment o ccurs in some xanthids (e.g., Etisus). The periopercular rim is probab ly a true synapomorphy indicative of relationship of potamids to xanth oids (represented by Calocarcinus) which has been postulated elsewhere on morphological grounds.