ULTRASTRUCTURE OF SPERMS IN BIVALVE MOLLUSKS OF THE MYTILIDAE FAMILY

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.