Visceral Sex, Hermaphroditism, and Protandry in a Population of the Freshwater Bivalve Elliptio complanata

Sexual allocation was examined in a dense (28 /m2) population of Elliptio complanata (Lightfoot) in Lac de l'Achigan, Québec. The gonads of individuals in this population contained mixtures of male and female tissue with 6% containing >10% and <90% female tissue. The sex ratio was determined by microscopic examination of gonads and was found to be skewed toward males. The sexual composition of the population varied significantly with body size. Protandry was indicated because the gonads of 80% of animals <50 mm in shell-length were composed primarily of male tissue, while larger animals had a significantly higher proportion of females and hermaphrodites. The fraction of the population producing eggs or glochidia increased with the fraction of the gonad composed of female tissue, such that animals whose gonads contained <40% female tissue had a low probability of egg production. There was no significant tendency for dioecious organisms to be found in denser aggregations than hermaphrodites.