Effects of conspecific associations on size at sex change in three species of calyptraeid gastropods

In marine molluscs, sex change is often labile and is thought to be largely influenced by interactions with conspecifics. Previous studies of calyptraeid gastropods concluded that their social environment influences the timing of protandrous sex change. We conducted field surveys and laboratory experiments to examine the effects of conspecifics on size at sex change in 3 Panamanian calyptraeids. Crepidula cf. onyx, C. incurva Broderip, 1834 and Bostrycapulus calyptraeformis (Des-hayes, 1830) vary in densities, sex ratio and mode of development, which suggests that they might respond to associations with conspecifics in different ways. However, our laboratory experiments showed that the response to interactions with conspecifics is generally similar. In all 3 species, individuals raised in isolation pass through a male phase and males raised alone change sex at the same size as males raised with another male. Both C. cf. onyx and C. incurva change sex at a larger size when kept with a female than when kept alone or with another male. The differences in size at sex change between the treatments is small and the treatment effect explains more of the variation in size at sex change in C. cf. onyx, the more solitary species, than in C. incurva, a species that is usually found in pairs. In all species, individuals with high initial growth rates change sex sooner and at a smaller size than those with slower initial growth rates. Growth rates increase significantly during sex change in C. cf. onyx and C. incurva but not in B. calyptraeformis.