ULTRASTRUCTURE AND RELATIONSHIPS OF SPERMATOZOA OF THE FRESH-WATER CRABS POTAMON FLUVIATILE AND POTAMON IBERICUM (CRUSTACEA, BRACHYURA, POTAMIDAE)

The spermatozoa of Potamon fluviatile and P. ibericum are virtually in distinguishable and do not support separate subgeneric rank (Potamon a nd Pontipotamon, respectively). Synapomorphic with the spermatozoa of the South African freshwater crab Potamonautes perlatus sidneyi are th e elongation of the two centrioles and disposition of the centrioles a lmost parallel to each other, unknown elsewhere in the Brachyura, and reduction of the thickened ring (homoplasic with grapsids and gecarcin ids). Other, probably synapomorphic, similarities of Potamon and Potam onautes include the wide inner acrosome zone, absence of a definite ac rosome ray zone (homoplasic in other families) and the cleistospermial spermatophores. Further similarities, of questionable polarity, are t he simple, not multilaminar, nuclear membrane and the tendency of the nuclear arms to wrap around the nucleus. Differences of Potamon From P otamonautes, which possibly support their present generic separation a nd give weak support to their former separate familial placement in th e Potamidae and Potamonautidae respectively, are perforation of the op erculum and the weak, rather than strong, development of a periopercul ar rim. Absence in Potamon and Potamonautes of an accessory opercular ring and a xanthid ring separates them from xanthids. No close affinit ies with other heterotreme families are seen but their assignment to t he Heterotremata is not in doubt. Their spermatozoa lack two of the di stinctive features of thoracotreme spermatozoa (apical button on the o perculum and concentric lamellation of the outer acrosome zone). No cl ear correlates of spermatozoon structure with a freshwater existence a re recognizable but reduction of the thickened ring possibly relates t o peculiarities of the acrosome reaction. However, the production of s permatophores with single spermatozoa (cleistospermia) is possibly a d evice to prevent polyspermy and wastage of the small number of lecitho trophic eggs produced in potamids.