Sex change in the Mollusca.

Sex change in the Mollusca is almost exclusively protandric (male to female), and has only been reported among gastropods and bivalves. The adaptive value of protandry in these two classes most likely relates to the limited availability of females, and the consequent size-independent nature of male reproductive success (versus the size-dependent nature of reproductive success in females). In two well studied distantly related prosobranch gastropod superfamilies, Calyptracea and Patellacea, individuals of some species respond to local ecological changes by altering the age at which they change sex, although the critical ecological changes appear to be different. The physiological switch that activates sex change also appears to be different: it is found in the cemtral nervous system of the calyptracean Crepidula fornicata, and actually within the gonads of the patellacean Patella vulgata. Although the taxonomic breadth of studies on sex change is necessarily limited, and many questions remain to be answered, research on the Mollusca has produced a remarkable range of perspectives on sex change - from evolutionary to proximal; further research will benefit greatly from this breadth of knowledge.