Healy, J.M., Buckland-Nicks, J.A.
and Jamieson, B.G.M.
2001e. Spermatozoal ultrastructure of spiny oysters (Spondylidae,
Bivalvia) including a comparison with other bivalves. Invertebrate
Reproduction and Development 40 (1): 27-37.
Sperm ultrastructure in three representative
species of the marine bivalve family Spondylidae ("spiny or thorny
oysters") is examined and compared with available data on other
bivalves,
especially other families of the subclass Pteriomorphia. Spondylid
spermatozoa
are of the externally fertilizing aquasperm type (ect-aquasperm). The
acrosomal
vesicle is conical with a deep basal invagination extending almost the
full
length of the vesicle. Vesicle contents are divisible into an inner,
highly
electron-dense anterior layer and a less dense posterior layer. The
anterior
layer is folded back on itself posteriorly and exhibits radiating
plates (best
developed peripherally). The vesicle rests on, and is partially
embedded in, an
extensive granular deposit of subacrosomal material at the nuclear
apex. This
deposit extends partly into acrosomal vesicle invagination and also
fills a
broad depression in the anterior of the nucleus. No pre-formed axial
rod
(perforatorium) is present. The nucleus is round-pyriform and its
contents
coarsely fibro-granular. At the base of the nucleus, four broad
depressions
partially accommodate the midpiece mitochondria. The midpiece consists
the four
spherical mitochondria and the proximal and distal centrioles. The
centrioles
are arranged at approximately 90degree to each other, and each consists
of
nine, angularly-oriented, microtubular triplets embedded in a granular
matrix.
A short, periodically banded rootlet connects the proximal centriole to
the
nuclear fossa, whereas the distal centriole, which forms the basal body
to the
flagellar axoneme, is anchored to the plasma membrane by nine
terminally forked
satellite fibres. Extensive deposits of putative glycogen rosettes
surround the
centrioles and mitochondria. The flagellum consists of a 9+2 axoneme
sheathed
by the plasma membrane. Spondylid spermatozoa strongly resemble those
of the
Pectinidae, further confirming the traditional view (based on
comparative
anatomy and shell morphology) of a close relationship between the
Spondylidae
and the Pectinidae. Differences in acrosomal shape and dimensions were
noted
between the three species examined, indicating potential taxonomic
utility for
comparative sperm ultrastructure within the Spondylidae.