Spermatogenic ultrastructure in the anomalodesmatan bivalve M yochama anomioides ( M ollusca: M yochamidae) – does the nucleus help position the ‘temporary’ acrosome?

Spermatogenic ultrastructure in the marine bivalve mollusc M yochama anomioides ( M yochamidae) is described and contrasted with other bivalves, especially other euheterodonts. Small (0.1 μm diameter), primary proacrosomal vesicles produced in spermatocytes give rise to much larger (0.4 μm diameter) secondary proacrosomal vesicles in early spermatids, which in turn form the dished‐shaped, definitive acrosomal vesicle (diameter 1.0 μm) of later spermatids. The acrosomal vesicle acquires a deposit of subacrosomal material and comes to lie close to or in contact with the plasma membrane. The acrosomal complex (acrosomal vesicle + subacrosomal material) initially positions itself at the apex of the condensing, fibrous nucleus (the so‐called temporary acrosome position), but subsequently begins to move posteriorly. The condensing nucleus becomes markedly folded so that its apex is posteriorly orientated towards the migrating acrosomal complex and the midpiece (mitochondria and centrioles). The close spatial relationship of nuclear apex to acrosomal complex during this folding strongly suggests that acrosomal migration in M . anomioides is assisted, at least in part, by movement of the late spermatid nucleus. Similar nuclear folding has previously been demonstrated in an early stage of fertilization in another anomalodesmatan ( L aternula limicola ) raising the possibility that one event might be a reversal of the other.