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

Abstract

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.

Investigating the Bivalve Tree of Life – an exemplar-based approach combining molecular and novel morphological characters

“…Hemidonacidae, which had been grouped with Cardiidae into Cardioidea in earlier classifications (e.g. Beesley et al 1998), does not group with Cardiidae in any analysis (see also Taylor et al 2007b;Sharma et al 2012), a result supported by studies of sperm ultrastructure (Healy et al 2008b). Chamidae has appeared as the sister-group of Cardiidae in previous analyses mostly using parsimony Giribet and Distel 2003) and in the Bayesian combined analysis of nine genes + morphology (Fig.…”

“…31). Hemidonacidae has appeared as sister to a clade including Veneridae sensu lato and Chamidae in previous analyses (Taylor et al 2007b) and has been difficult to position even with sperm ulstrastructure (Healy et al 2008b). …”

Comparative study of sperm ultrastructure of Donax hanleyanus and Donax gemmula (Bivalvia: Donacidae)

“…These studies could provide novel and welcome information relevant to the understanding of taxonomic and phylogenetic relationships within this superfamily (Healy ; Healy et al . ). Further TEM sperm studies of Tellinoidea species could also be helpful in exploring possible connections with Arcticoidea, Galeommatoidea and freshwater Dreissenoidea (Healy ).…”

“…), Donax lubrica collected in Digha, India (Pal ) and Donax deltoides from Australia (Healy et al . ). In addition, the sperm ultrastructural features of Iphigenia brasiliana were studied by Matos et al .…”

“…According to Healy et al . (), spermatozoa ultrastructural studies of the family Donacidae seem desirable, if only to reach a better evaluation of the range of sperm features and probably to evaluate the monophyletic status of the family.…”