COLLIN, Rachel; Smithsonian Tropical Research Institute: Effects of Conspecific Associations on Size at Sex Change in Calyptraeid Gastropods

Sex change is widespread among both vertebrates and invertebrates. In marine molluscs sex change is often labile and is thought to be largely the result of interactions with conspecifics. Calyptraeid gastropods have been the subject of several previous studies that concluded that the animal�s social environment exerts some influence on their protandrous sex change. I conducted field surveys and laboratory experiments to examine the effects of conspecifics on the size of sex change in calyptraeid gastropods. I use data on the size, sex, and grouping of individuals of 27 populations of 19 species to test three hypotheses about variation in size at sex change: (1) sex ratio is biased towards the first sex. (2) The ratio of the size at sex change to the maximum size is invariant across species. (3) species that form groups or stacks have more variation in size at sex change than species that stack less frequently. I found that sex ratios are slightly more often male biased than they are female biased, that (size at sex change) /(maxiumum size) is not invariant, and that social species show more variation in size at sex change than solitary species. Three species that vary in natural field densities, sex ratio, and mode of development were chosen as subjects of laboratory experiments. Despite these natural history differences, response to conspecific interactions were similar. Juveniles raised in isolation pass through a male phase and they change sex at the same size as the first male to change in a pair of males. Sex change is delayed to a larger size when males are kept with females as compared to those kept alone or with another male. In all species individuals with high initial growth rates change sex sooner than those with slower initial growth rates and but there was no significant change in growth rate during sex change.