The
spermatozoon of the freshwater crab Australocarcinus riparius is
heterotreme. However,
absence
of a recognizable acrosome ray zone is not a
general heterotreme
feature,
though constant for thoracotremes. This zone
is also unrecognizable
in potamoids
and in corystoids. The convex ring at
the posterior end of the
inner
acrosome zone is similar to the xanthid
ring, characteristic of
the Xanthidae
and Panopeidae, but homology is
doubtful. Other xanthid
features
such as the accessory opercular ring and
opercular overhang are
absent
from the sperm of A. riparius and
relationship with
xanthoids is
not supported spermatologically. In its
pointed shape, the
perforatorium
resembles that of corystoids but there
the operculum is
perforate. No
special similarities to sperm of potamoids,
which similarly have an
obligatory
freshwater existence at all stages of
the life cycle, are
apparent.
The sperm of A. riparius shows no clear
affinity with those of
any other
Heterotremata although confirming its
inclusion in that group.
In producing
more than one spermatozoon per
spermatophore, A.
riparius,
having marine relatives, may be less evolved
along the path of
lecithotrophy
than are the potamoids, lacking marine
cofamilials.