RESEARCH NOTE EVIDENCE OF SEXUAL SHAPE DIMORPHISM IN VIVIPARUS (GASTROPODA: VIVIPARIDAE)

Sexual dimorphism can be defined as a systemic difference in phenotypic traits between individuals of different sex in the same species (Shine, 1989). While this difference is common across living organisms (Abouheif & Fairbairn, 1997), Purchon (1977) believed it to be rare in molluscs. However, as understanding of intraspecific variation increased and techniques for finer-scale morphological analyses developed, so did the number of examples of sexual dimorphism in the phylum. Two important kinds of dimorphism are differences in either shape or size, and both are exhibited in the ovoviviparous freshwater bivalves and gastropods. Unionoid mussels often show marked dimorphism (Jass & Glenn, 2004; Eads, Bogan & Levine, 2006), especially in the ventral posterior portion of the shell, as a means of making room for the marsupium (Kotrla & James, 1987). Some freshwater snails exhibit shell differences between the sexes (e.g. Melanoides tuberculata which broods its young in a modified cephalic ‘pouch’, some species of Lavigeria which brood in the pallial oviduct and Vinundu westae which is an egg-layer) while others such as Tarebia granifera do not (Brande, Turner & Heller, 1996; Michel, 2004). Another group where sexual dimorphism has been explored in detail is the genus Viviparus, a holarctic group of freshwater brooding snails of the family Viviparidae.