Body size-dependent gender role in a simultaneous hermaphrodite freshwater snail, Physa acuta

We examined whether gender role in the simultaneous hermaphrodite freshwater snail, Physa acuta, is determined by relative body size in a manner predicted by the size-advantage model. We observed the body-size combinations of pairs in the laboratory by using field-collected populations. Smaller individuals tended to play the "male" role (sperm donor), and larger snails the "female" (sperm recipient). Next, we analyzed the mating behaviors involved in gender-role decision in snail pairs of three different body-size combinations, using "large" and "small" snails. Smaller snails were more likely to approach the partner as a male in different-size combination (large/small), whereas frequent initial approaches as a male and rejection behavior as a female were observed in the large/large combination. Third, we examined the body size preference when a snail can freely choose the partner from two other individuals of different body sizes (large/large/small or large/small/small). Small individuals had a significant tendency to act as the male and positively selected large snails as the female partner in both triple combinations. However, the large individual acted as both the male and the female with nearly equal frequency. In the size-differing pairings, copulations occurred after fewer male approaches and fewer rejections than in pairings involving two large snails, suggesting that body size difference is one of the behavioral solutions in gender conflict. Clear gender-role switching associated with body size was not seen. Smaller snails thus have a tendency to play the male role more frequently but adopt both gender roles when their body size is sufficiently large.