COLLIN, R; STRI: Sex Ratio and Patterns of Sex Change in Calyptraeid Gastropods

The size advantage hypothesis predicts that the optimal size at which an individual should change sex is a function of its size and the size and sex of its potential mates. In species that form small mating groups, variation in group composition may result in more variation in size at sex change across the population than in species that do not form mating groups. I use data on the size, sex, and grouping of individuals of 27 populations of 19 species of calyptraeids, a family of protandrous marine gastropods including Crepidula. These data are used to test the following hypotheses about variation in size at sex change: (1) Sex ratio is biased toward the first sex, (2) The ratio of the size at sex change to the maximum size is a life history invariant, (3) Species that form groups or stacks have more variation in size at sex change than species that stack less frequently. There was substantial variation in sex ratio across the different species with sex ratio being related to mode of development, skewed size distribution, and frequency of stacking, but not with maximum body size. Sex ratios were male biased more often than they were female-biased but there were several female biased populations. There was little evidence that the ratio of size at sex change and maximum size is invariant and there is evidence that at least one of the assumptions of this theory is invalid for calyptraeids and probably for other animals. Species that form lager stacks or mating groups had more variation in size at sex change within a population than species that were generally solitary. These results suggest that incorporating information about individual groupings could help refine predictions of life-history theory.