Jamieson, B.G.M. 1993c. Ultrastructure of the
spermatozoon of Potamonautes perlatus sidneyii (Heterotremata,
Brachyura, Crustacea). South African Journal of Zoology 28,
40-45.
The spermatozoa of Potamon
fluviatile and P.
ibericum are virtually indistinguishable and do not support
separate
subgeneric
rank (Potamon and Pontipotamon, respectively).
Synapomorphic with the
spermatozoa of the South African freshwater crab Potamonautes perlatus
sidneyi
are the elongation of the two centrioles and disposition of the
centrioles
almost parallel to each other, unknown elsewhere in the Brachyura, and
reduction of the thickened ring (homoplasic with grapsids and
gecarcinids).
Other, probably synapomorphic, similarities of Potamon and Potamonautes
include
the wide inner acrosome zone, absence of a definite acrosome ray zone
(homoplasic in other families) and the cleistospermial spermatophores.
Further
similarities, of questionable polarity, are the simple, not
multilaminar,
nuclear membrane and the tendency of the nuclear arms to wrap around
the
nucleus. Differences of Potamon from
Potamonautes, which possibly
support their
present generic separation and give weak support to their former
separate
familial placement in the Potamidae and Potamonautidae respectively,
are
perforation of the operculum and the weak, rather than strong,
development of a
periopercular rim. Absence in Potamon
and Potamonautes of an
accessory
opercular ring and a xanthid ring separates them from xanthids. No
close affinities
with other heterotreme families are seen but their assignment to the
Heterotremata is not in doubt. Their spermatozoa lack two of the
distinctive
features of thoracotreme spermatozoa (apical button on the operculum
and
concentric lamellation of the outer acrosome zone). No clear correlates
of
spermatozoon structure with a freshwater existence are recognizable but
reduction of the thickened ring possibly relates to peculiarities of
the
acrosome reaction. However, the production of spermatophores with
single
spermatozoa (cleistospermia) is possibly a device to prevent polyspermy
and
wastage of the small number of lecithotrophic eggs produced in potamids.