151. Jamieson, B.G.M., Guinot, D. and Richer de Forges, B. 1996d. Contrasting spermatozoal ultrastructure in two thoracotreme crabs, Cardisoma carnifex (Gecarcinidae) and Varuna litterata (Grapsidae) (Crustacea: Brachyura). Invertebrate Reproduction and Development 29, 111-126.
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.