Hemidonax, Morch, 1778

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 : 335

COMPARISON OF HEMIDONAX SPERM MORPHOLOGY

WITH OTHER HETERODONT BIVALVES

Spermatozoa of Hemidonax pictus are of the simple, aquasperm type characteristic of molluscs showing aquatic fertilization, especially the Bivalvia ( Franzén, 1955, 1983; Popham, 1979; Hodgson et al., 1990; Healy, 1996a), and also seen in a number of other invertebrate groups (e.g. cnidarians, polychaete annelids, brachiopods, echiurids, sipunculans – Franzén, 1956; Baccetti & Afzelius, 1976; Jamieson & Rouse, 1989). Rouse & Jamieson (1987) differentiated aquasperm into those that fertilize eggs in the ambient water (ect-aquasperm) and those that fertilize aquatically but within a protected space such as a worm tube or a molluscan mantle cavity (ent-aquasperm). In the absence of any information on the fertilization biology of H. pictus , it is impossible to characterize the type of aquasperm in this species. However it is worth noting that in shipworms ( Teredinidae, Pholadoidea ) Popham (1974) found that species fertilizing within the mantle cavity had smaller acrosomes than those that fertilized externally (i.e. in the ambient seawater). If such a correlation holds among the Bivalvia in general, then the small size of the acrosome of H. pictus compared with most investigated heterodonts (see Figs 8, 9) would suggest the likelihood of ent-aquatic (mantle cavity) fertilization in this species and probably in other species of Hemidonax .

Whereas it is true to state that spermatozoa of Hemidonax pictus do not exhibit any unique or new features, the precise combination of features is distinctive and presumably characteristic of the genus as a whole. Like other heterodonts, H. pictus shows a well-developed basal ring component of the acrosomal vesicle (see Fig. 8, also for comparative figures and extensive literature see Healy, 1995b, 1996a). Substantial diversity exists among heterodont taxa in the shape of the acrosomal vesicle (and the shape and internal structure of the basal ring), as well as the size and length of the nucleus (short or long, straight, curved or helical) and the number of midpiece mitochondria (four or five, sometimes more, but always with a predominating number). As there are a number of differing opinions relating to the placement and affinities of Hemidonax among the Heterodonta, the following part of the discussion will deal with each of these, including a brief resumé of group features then progressing to a comparison with Hemidonax results.