Ultrastructure of the spermatozoa of three species of Anomalodesmata (Mollusca, Bivalvia) and phylogenetic implications

Anomalodesmata is the rarest and most specialized group of the class Bivalvia. In general, scarce data on sperm ultrastructure of anomalodesmatans are available in the literature. Aiming to contribute to an understanding of spermatozoon morphology in this group, the present study describes the ultrastructure of male gametes of three species-Cardiomya cleryana (Orbigny 1842) (Cuspidariidae), Pandora brevirostris Guller and Zelaya 2016 (Pandoridae) and Thracia similis Couthouy 1839 (Thraciidae). C. cleryana has an acrosome located at the anterior region, and the mitochondria are symmetrically arranged, while the spermatozoa of P. brevirostris and T. similis have unique characteristics, in particular the association of the acrosome with the midpiece and asymmetric mitochondria. This position of the acrosome appears to be peculiar among Anomalodesmata and is unknown for non-anomalodesmatan bivalves. The present ultrastructural description of spermatozoa contributes to differentiate the studied species, bringing new insights about the sperm features of anomalodesmatan species and shares several characteristics with that of other species of the two lineages proposed by molecular phylogeny.