Plasticity in sex allocation: comparisons among three species of polychaete worms

Sex allocation in hermaphrodites is plastic. Phenotypic plasticity in sex allocation has species-specific properties in many simultaneous hermaphroditic taxa. Owing to the large biological diversity among the studied taxa, it is not easy to identify which factors promote or constrain plasticity in sex allocation. We compared the ability to adjust sex allocation to mating opportunities in three closely related species of simultaneous hermaphrodites. We found that, in the polychaete worms Ophryotrocha diadema, O. adherens and O. gracilis, adults plastically changed their sex allocation when they were exposed to different levels of mating opportunities. However, the degrees of plasticity differed among species as well as between the male and the female functions. We discussed how the relative costs of male and female functions and ecological factors, such as habitat conditions and the prevailing population density in the wild, may influence the evolution of plasticity in sex allocation in each species.