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.