A Quantitative Assessment of Spermatozoan Morphology in Nutricola confusa and Nutricola tantilla (Bivalvia: Veneridae)

The brooding bivalves, Nutricola tantilla and N. confusa overlap in their geographic distributions, habitats, and modes and timing of reproduction. Based on previous studies we infer that males of both species release spermatozoa into the water column; while females retain developing embryos in a brood chamber. Females release fully formed juveniles and there is no pelagic larval stage. We hypothesized that the muco-ciliary processes of particle selection and retention may act on differences in sperm morphology and contribute to reproductive isolation. We extracted sperm cells from both species and quantified nine linear measurements: the lengths of the acrosome, nuclear, midpiece and tail regions, and five different width measurements. We found significant differences in the lengths of the acrosome, midpiece, and tail. We also found that N. confusa produces dimorphic sperm and this is the first report of sperm dimorphism in the Veneroidea. Despite the significant differences in lengths, it is likely that other prezygotic mechanisms are responsible for reproductive isolation.