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 DONACIDAE (TELLINOIDEA)

To date, five species of Donacidae have been examined in detail for sperm ultrastructure (the South African Donax serra Dillwyn, 1817 , D. madagascariensis Wood, 1828 , D. sordidus Hanley, 1845 – see Hodgson et al., 1990; the eastern Atlantic/Mediterranean Donax trunculus – see Sousa & Oliveira, 1994; the Australian D. deltoides Lamarck, 1818 – this study, see also Healy, 1995b). In all of these species (see Figs 8, 9), with the exception of D. trunculus , the acrosomal vesicle is large, with prominent, parallel layers within the basal ring, the nucleus is barrelshaped and the midpiece contains the centrioles and four mitochondria. In D. trunculus the nucleus is appreciably longer than in other investigated species of Donax , but four midpiece mitochondria are observed (as also in other species). More significantly, no layering within the basal ring of the acrosomal vesicle was reported by Sousa & Oliveira (1994) nor can any trace of it be seen in their micrographs of mature or developing sperm (that all appear to show a homogeneously textured basal ring). Surprisingly, Sousa & Oliveira (1994) did not comment on the difference in acrosomal morphology between D. trunculus and the species examined by Hodgson et al. (1990). Healy (1995b) noted the difference and suggested the possible taxonomic utility of sperm in future studies of the Donacidae , as has already been demonstrated by Hodgson et al. (1990) for the three South African species examined by them (the close sperm similarity between D. madagascariensis and D. sordidus being linked to reports of hybridization in areas where these species occur together – see Discussion in Hodgson et al., 1990). A sixth species of Donax , the Indian Ocean D. lubricus Hanley, 1845 was examined briefly by Pal (1992, 1996), but the descriptions and micrographs provide incomplete data. As in other examined species of Donax , the acrosomal vesicle of D. lubricus is seated in a broad anterior depression of the nucleus, and the subacrosomal material is organized as an axial rod. However, in contrast to other species of Donax , but similar to groups such as the Mactroidea ( Longo & Anderson, 1969; Hylander & Summers, 1977; Healy, 1995b; J. M. Healy, P. M. Mikkelsen & R. Bieler, unpubl. data), the anterior portion of the acrosomal vesicle appears to be largely devoid of contents. A more detailed study of this species would seem warranted.

Despite the spermatozoal differences between the two apparent ‘groups’ of Donax , all Donax sperm show two consistent differences from Hemidonax pictus : (1) the acrosomal vesicle is seated in a marked depression of the nuclear apex ( Hemidonax : apex flat or showing minimal depression); and (2) the subacrosomal material is organized into a well-formed axial rod, often showing longitudinal-fibrous structure ( Hemidonax : subacrosomal material consisting of diffuse granules). Further work on other species of Donacidae seems warranted, if only to reach a better appreciation of the range of sperm morphologies within this family (and possibly to assess the monophyletic status of the family). Available sperm ultrastructural data thus do not support the decision of some authors to allocate Hemidonax to the Donacidae ( Lamy, 1917; Thiele, 1934; Keen, 1969; Vokes, 1980; Abbott & Dance, 1982) or Donacoidea ( Scarlato & Starobogatov, 1979 ). The rather fragmentary sperm ultrastructural data for other Tellinoidea (see Sousa, Corral & Azevedo, 1989; Healy, 1995b; Sousa et al., 1998) likewise suggest no connection with Tellinidae , Semelidae or Scrobiculariidae . The suggestion by Schneider & Carter (2001) of a close relationship between Hemidonax and the Psammobiidae (Tellinoidea) also does not seem likely, at least based on the light microscopic results of Guerra, Campos & Esponda (1994) showing an elongate, rod-shaped nucleus in Gari solida (Gray, 1828) . Certainly this hypothesis needs to be tested using TEM of psammobiid sperm.