O. Garrido et Cs. Gallardo, ULTRASTRUCTURE OF SPERMS IN BIVALVE MOLLUSKS OF THE MYTILIDAE FAMILY, INVERTEBRATE REPRODUCTION & DEVELOPMENT, 29(2), 1996, pp. 95-102
The sperm structure of five external fertilizing bivalve molluscs pert
aining to the Mytilidae family is compared. Apart from examining if su
ch gamete structure conforms to the primitive sperm form of the type I
predicted by Franzen, a major aim of the study is to compare the deve
lopment and structure of some organelles whose evolution among the biv
alve sperms has also been associated with the style of fertilization (
whether external or within brooding chambers where the retained eggs a
re fertilized). Observations are specially focused on examining the ex
istence of the acrosome, its morphology and the way in which it develo
ps as well as on the pattern of condensation of the nucleus. Finally,
the morphology of the mitochondria, conforming the mid-piece, is also
examined. The sperms studied conform to the primitive general type pro
posed for external fertilizing species as well to the morphological pa
ttern presented by other free-spawning bivalve molluscs. A well develo
ped acrosome is observed, supporting the contention that such organell
es tend to persist among the typical broadcast fertilizer bivalves, pr
obably linked to egg coat structure variations. This acrosome, however
, appears as the most variable organelle, with diversity of shape, siz
e and inner structure, particularly with respect to the number of comp
artments structuring it. Most of the species examined show absence of
an axial rod as part of the apparatus making up the acrosome, a featur
e considered primitive by some authors; the exception is Mytilus chile
nsis where such structure does differentiate. This characteristic - al
so presented in the literature about other Mytilus species - adds to o
ther traits shared by the sperms of this mytilid genus, thus also show
ing the potential value that sperm morphology could have to substantia
te the studies of taxonomic affinities among these bivalve molluscs.