Reproduction and Sex in the Androdioecious Barnacle Scalpellum scalpellum: Darwin's Model for Studying Reproductive System Evolution.

males are not hermaphrodites arrested in development but the result of a much more profound evolutionary history. Settlement experiments showed that all cyprids are capable of settling and developing into hermaphrodites. Development into males happens only in cyprids attached in receptacles on adult hermaphrodites and must therefore be governed by environmental sex determination (ESD), presumably induced by some chemical factor(s) present only in the receptacle area. We observed mating between hermaphrodites and between a hermaphrodite and its dwarf males. Hermaphrodite to hermaphrodite mating resembles that seen in balanomorphan barnacles, except that adjacent specimens often check their environment with their cirri for the presence of a mating partner. Dwarf male mating is by means of a unique penis structure, made almost exclusively of cuticle and extending from inside the male and bending down into the brood chamber of its partner. This male penis is much larger (relatively to body size) than the structurally very different penis of the hermaphrodite individuals. The hermaphrodite recognizes when the dwarf male extends its penis and arrests its cirral motion so as not to damage or disturb its tiny partner. For the adult hermaphrodites we showed that their allocation of resources to male and female functions is in agreement with predictions from sex allocation theory. As predicted, solitary hermaphrodites allocated fewer resources to male function than those settled gregariously, where one or several hermaphrodite partners are within mating distance. The solitary individuals had both a shorter penis and less developed testes than the gregarious ones. As also predicted, solitary hermaphrodites were more likely to carry males than the gregarious ones.