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Summary
Numerous investigations have demonstrated the usefulness of sperm morphology in evaluating molluscan phylogeny. In this work we used transmission and scanning electron microscopy to study the structure of mature spermatozoa from two bivalves, Brachidontes darwinianus and Brachidontes solisianus, and compared them with those of other bivalves, particularly other mytilids. These two species have a wide geographic distribution and are particularly abundant in the intertidal zone of many rocky shores along the Brazilian coast, often in areas with strong water currents. B. darwinianus occurs from the state of Rio de Janeiro, Brazil, to Patagonia, in Argentina, whereas B. solisianus is distributed from Mexico to Uruguay. The spermatozoa of both species were of the primitive or ect-aquasperm form. In both species the spermatozoan head contained an spheroidal nucleus capped by a conical acrosome with an anterior extension. No actin was detected in the subacrosomal region. However, immunocytochemical staining identified actin throughout the nucleus of the sperm of both species. The chromatin was strongly electron-dense, homogenous and compact. The nuclei contained randomly distributed, electron-lucent regions formed by invaginations of the nuclear envelope. These invaginations were detected by E-PTA staining for glycoproteins at low pH. The mid-piece region consisted of five spherical mitochondria grouped in a ring around a pair of short cylindrical centrioles. The flagellum exhibited the typical 9+2 microtubule structure (9 double outer tubules +2 single central tubules). These findings, together with conchological characteristics, can be used to distinguish between B. darwinianus and B. solisianus. The only marked difference in the morphology of spermatozoa from these two species was the longer anterior extension of the acrosomal vesicle in B. solisianus. This elongated acrosome may facilitate penetration of the jelly coat and cytoplasm of large oocytes and could increase the efficiency of fertilization. The resulting enhanced reproductive success could account for the wider geographic distribution of B. solisianus.