Sperm ultrastructure in Hemidonax pictus (Hemidonacidae, Bivalvia, Mollusca):: comparison with other heterodonts, especially Cardiidae, Donacidae and Crassatelloidea

The systematic position and affinities of the marine bivalve genus Hemidonax (Heterodonta, Veneroida) are investigated using comparative sperm ultrastructure, with particular focus on the various groups to which this genus has been assigned [Donacidae (Tellinoidea), Cardiidae (Cardioidea) and Crassatellidae (Crassatelloidea)]. Ultrastructural examination (using transmission electron microscopy) reveals that Hemidonax pictus produces sperm of the aquasperm type, with a short, rounded-conical acrosomal vesicle, a short, barrel-shaped nucleus, a short midpiece (composed of two centrioles and four surrounding mitochondria) and a flagellum containing a conventional 9 + 2 pattern axoneme. The acrosomal vesicle exhibits a wedge-shaped, highly electron-dense, basal ring component, and less dense anterior component (including a thin, electron-lucent layer apically, which may prove to be a useful apomorphy for Hemidonax). A loose, granular deposit of subacrosomal material is located within a narrow invagination traversing most of the length of the vesicle. Comparison with sperm of other heterodont bivalves shows no close connection between Hemidonax and the Donacidae (Tellinoidea) or with the Crassatellidae (or other crassatelloideans). Although certain Veneridae (Veneroidea) and Cardiidae (Cardioidea, especially Fragum) show much better conformity in sperm morphology to that observed in Hemidonax, no complete match could be found among studied taxa. We conclude that Hemidonax should be retained in its own, previously introduced family Hemidonacidae, and the latter be placed incertae sedis within the Euheterodonta.(c) 2008 The Linnean Society of London.