Relative size influences gender role in the freshwater hermaphroditic snail, Helisoma trivolvis

Simultaneous hermaphrodites have the unique challenge of allocating their available resources to egg and sperm production and behaviorally to a male and/or female mating role. Models that address the influence of body size on sex allocation predict that larger individuals should allocate proportionally more resources to female than male function and that this should translate into corresponding behavioral preferences during mating. We investigated the relationship between size and gender role in the hermaphroditic freshwater snail Helisoma trivolvis. We hypothesized that when 2 H. trivolvis mate, the larger would assume the female role and the smaller the male role. We also predicted that reciprocal mating would be more likely when partners were similar in size. We measured 180 snails, paired them, and observed their sex roles during copulation. The size difference between snails neither influenced the latency to copulation nor predicted whether mating was unilateral or reciprocal. In unilateral matings, the smaller snail acted as the male significantly more often than the larger snail. In order to test the hypothesis that increased activity of smaller snails influences gender role, we also measured movement rates in snails of various sizes but found no relationship between size and activity. These experiments indicate that in H. trivolvis body size does influence gender role in unilateral matings, and enable us to rule out activity as a direct determinant of male gender role. Whether snails mate reciprocally or not may depend on other factors such as previous mating history, time of isolation, or age. Copyright 2008, Oxford University Press.