Sex, size, and position: A test of models predicting size at sex change in the protandrous gastropod Crepidula fornicata

The optimal size for sex-changing organisms to change sex can be predicted based on mate availability and the relationship between body size and reproductive output for each sex. Sex-changing invertebrates also show individual variation in size at sex change that may be due to population structure and environmental heterogeneity. Protandry in slipper-shell gastropods of the genus Crepidula is a common textbook example of sex change, yet it has not been studied within the framework of recent sex allocation theory. I developed three models that predict the size at sex change using estimates of immediate reproductive potential based on individual size and interactions with conspecifics. These models were evaluated for small mating groups that structure the population as well as for the population as a whole. All three models predict that sex change should occur at smaller sizes than observed in the field. High male growth rates suggest that males sacrifice immediate optimality in favor of increased lifetime fitness. Although the models made better predictions when evaluated for groups than they did when evaluated for the whole population, discriminant analysis showed that an individual's sex could be predicted as well by its size alone as by the combination of each group member's size.