CONTRASTING SPERMATOZOAL ULTRASTRUCTURE IN 2 THORACOTREME CRABS, CARDISOMA-CARNIFEX (GECARCINIDAE) AND VARUNA-LITTERATA (GRAPSIDAE) (CRUSTACEA, BRACHYURA)

Despite its terrestrial mode of life, Cardisoma carnifex (Gecarcinidae ) returns to the sea to breed, and its spermatozoon displays all of th e diagnostic features of thoracotreme sperm: absence (clearly loss) of the acrosome ray zone; presence of an apical button filling an opercu lar perforation; concentric lamellation of the outer acrosome zone; an d the near-horizontal accessory opercular ring. In contrast, Varuna li tterata (Grapsidae, subfamily Varuninae), a freshwater species, though again returning to marine (or estuarine) conditions to breed, shows o nly a negative state, absence of acrosome rays (other than a questiona ble presence of concentric lamellae) which would place it in the Thora cotremata. It does not display the grapsid synapomorphy, loss of the t hickened ring. The distinctness spermatologically of Varuna from other grapsids is susceptible to a number of explanations among which are: (1) it is not a grapsid but a thoracotreme descended from a stock whic h had not yet developed typical thoracotreme spermatozoal ultrastructu re; (2) it is a grapsid which has lost typical thoracotreme spermatozo al ultrastructure; (3) it is a heterotreme which has developed a thora cotreme configuration of genital pores independently of true thoracotr emes. Of these alternatives, (3) is considered unacceptable, but the c hoice between (1) and (2) is difficult, although classical taxonomy wo uld favour alternative (2). Absence of a recognizable acrosome ray zon e, in Varuna sperm, presence of which is a typical heterotreme feature is also noted for the heterotremes Potamonautes, Potamon (Potamidae), and Australocarcinus (Goneplacidae), all of which are freshwater gene ra.