Contrasting spermatozoal ultrastructure in two thoracotreme crabs, cardisoma carnifex (gecarcinidae) and varunu 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 the diagnostic features of thoracotreme sperm: absence (clearly loss) of the acrosome ray zone; presence of an apical button filling an opercular perforation; concentric lamellation of the outer acrosome zone; and the near-horizontal accessory opercular ring. In contrast, Varuna litterata (Grapsidae, subfamily Varuninae), a freshwater species, though again returning to marine (or estuarine) conditions to breed, shows only a negative state, absence of acrosome rays (other than a questionable presence of concentric lamellae) which would place it in the Thoracotremata. It does not display the grapsid synapomorphy, loss of the thickened 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 which had not yet developed typical thoracotreme spermatozoal ultrastructure; (2) it is a grapsid which has lost typical thoracotreme spermatozoal ultrastructure; (3) it is a heterotreme which has developed a thoracotreme configuration of genital pores independently of true thoracotremes. Of these alternatives, (3) is considered unacceptable, but the choice between (1) and (2) is difficult, although classical taxonomy would favour alternative (2). Absence of a recognizable acrosome ray zone, 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 genera