Hemidonax, AND THE

Healy, John M., Mikkelsen, Paula M. & Bieler, Rüdiger, 2008, Sperm ultrastructure in Hemidonax pictus (Hemidonacidae, Bivalvia, Mollusca): comparison with other heterodonts, especially Cardiidae, Donacidae and Crassatelloidea, Zoological Journal of the Linnean Society 153 (2), pp. 325-347 : 337

HEMIDONAX AND THE CARDIIDAE (CARDIOIDEA)

A number of species of Cardiidae (sensu Schneider, 1992, 1995; now inclusive of Tridacnidae as subfamily Tridacninae ) have been examined for sperm ultrastructure ( Popham, 1979; Sousa & Azevedo, 1988; Healy, 1995b; Sousa et al., 1998; Keys & Healy, 1999, 2000; Drozdov et al., 2001; present study) and even though many genera and even some subfamilies await investigation, enough is known to provide a meaningful comparison with Hemidonax pictus (see Figs 8, 9). Sperm morphology in the Cardiidae varies widely between taxa, to such an extent in fact that no distinctive, family-defining characters are yet apparent. However, it can be stated that in all investigated species: (1) the acrosomal vesicle is never elongate (usually with a rounded or truncated apex); (2) the acrosomal complex is never seated in a depression of the nuclear apex; (3) the subacrosomal material is never formed into a well-defined rod (herein ‘axial rod’ or ‘perforatorium’; although this does not preclude the generation of such a structure via polymerization of the subacrosomal material during the acrosome reaction); and (4) the midpiece almost always features four mitochondria (five occurring occasionally as a variant, but not predominating). Comparison of H. pictus with the range of sperm morphologies encountered in the Cardiidae to date (see Figs 8, 9) reveals that only Fragum unedo (Fragiinae) shows any degree of similarity to H. pictus , although the acrosomal vesicle is larger and more compressed and the nuclear profile is curved in longitudinal section (see Fig. 3). The basal invagination of the acrosomal vesicle of H. pictus and F. unedo is narrow within the anterior half of the vesicle, and the wedge-shaped basal ring profile is comparable between the two taxa. In almost all other investigated species of Cardiidae (including Lunulicardia hemicardium of the Fragiinae) the anterior region of the invagination is either as wide or wider than the basal region. A narrow invagination also appears present in Cerastoderma spp. (published micrographs are, unfortunately, few in number and lacking in much detail – see Sousa & Azevedo, 1988; Sousa et al., 1998; Drozdov et al., 2001), but, unlike that in H. pictus or F. unedo , and like certain species of Tridacninae ( Keys & Healy, 1999, 2000), the nuclear apex projects deeply into this invagination (see Fig. 8). Cerastoderma spp. also exhibit marked helical coiling of the nucleus, a feature long ago recorded by Retzius (1905) and one also observed in the enigmatic cardioidean (?cardiid) genera Monodacna , Didacna and Adacna (see comparative light microscopic account by Karpevich, 1961). An apparent difference between H. pictus and the Cardiidae (based on available data) is the electron-lucent layer underlying the curved apex observed in the former. A similar layer has been seen by us in at least one venerid species ( Lioconcha annettae Lamprell & Whitehead, 1990 ) ( R. Bieler, P. M. Mikkelsen & J.M. Healy, unpubl. observ.) but we are hesitant to attach undue significance to it, as the apical region of the acrosomal vesicle is the most structurally unstable component of the acrosomal complex. If the electron-lucent layer is not an artefact of fixation, and occurs in other Hemidonax species , it may prove a useful genus-defining or even familydefining feature.