ULTRASTRUCTURE OF THE SPERMATOZOA OF CORYSTES-CASSIVELAUNUS (CORYSTIDAE), PLATEPISTOMA-NANUM (CANCRIDAE) AND CANCER-PAGURUS (CANCRIDAE) SUPPORTS RECOGNITION OF THE CORYSTOIDEA (CRUSTACEA, BRACHYURA, HETEROTREMATA)

A combination of characters, not individually unique, possessed by the corystid, Corystes cassivelaunus, and the two cancrids, Platepistoma nanum and Cancer pagurus, defines a corystoid-type of spermatozoon: th e basally bulbous, anteriorly narrowing perforatorium, the extent of t his almost to the plasma membrane through a widely perforate operculum , and the simple inner acrosome zone, lacking an acrosome ray zone. Th e sperm of the two cancrids are closely similar, that of the corystid differing, for instance, in the less pointed, and less tapered, form o f the perforatorium. This relative uniformity of spermatozoal ultrastr ucture in the cancrid+corystid assemblage so far investigated supports inclusion of the two families in the superfamily Corystoidea by Guino t (1978). The combination of perforation of the operculum and absence of an acrosome ray zone (at least in a clearly recognizable form) are features of the Potamidae which possibly indicate that the latter fami ly, modified for a freshwater existence, is related to the cancrid+cor ystid assemblage. Some elongation of the centrioles, apparent at least in Corystes, may be a further link with potamids in which they are gr eatly elongated. The coenospermial spermatophores of cancridoids are a notable difference from the cleistospermia of potamids; but the latte r is probably an apomorphic modification for fertilization biology.