Sequential hermaphroditism and the size-advantage hypothesis: an experimental test

When members of one sex have a low reproductive success when small and a high reproductive success when large, while members of the opposite sex do relatively better when small and relatively worse when large, sequential hermaphroditism is commonly believed to be favoured by natural selection. This so-called size-advantage hypothesis has not been rigorously tested experimentally. For the hermaphrodite Ophryotrocha puerilis puerilis, a polychaete, in which small individuals are males and large ones females, the hypothesis predicts that reproductive success will increase less with body size for males than for females, eventually promoting sex change in males. Dry body weight of males was not correlated with reproductive rate, whereas there was a positive correlation for females in reproducing pairs. Furthermore, an increment in the size of females affected clutch size and reproductive rate more than did an equal increment in the size of males. Reproductive success of males decreased with size, because females preferred smaller males. At the same time, large males won contests for access to females, although female choice overrode this combat superiority. Therefore, after reaching a certain size a male would not benefit from staying male. Taking into account the relatively low cost of sex change in this species (about 5 days being lost, equivalent to one interbrood interval) sex reversal occurred as predicted by the size-advantage hypothesis.