Differences in gonadal sex allocation in a simultaneous hermaphrodite commensal flatworm under field conditions

Abstract

The theory of sex allocation in simultaneous hermaphrodites predicts that resource investment between male and female reproductive functions varies with the size of the mating group. To test this theory in commensal flatworms that are simultaneous hermaphrodites, in the present study, we evaluated the variations in the sizes of male and female gonads in Temnocephala iheringi (Platyhelminthes, Rhabdocoela), a species that inhabits the pallial cavity of the golden apple snail Pomacea canaliculata (Mollusca: Gastropoda). We evaluated two snail populations, each naturally exhibiting different infestation intensities of T. iheringi, assuming distinct mating group sizes. We found that the total size of the gonads of flatworms from both snail populations was similar, but that the proportion of the male gonad was higher in flatworms from the snail population with higher infestation intensity (larger mating group) than in those from the snail population with lower infestation intensity. Additionally, the proportion of the female gonad was lower in flatworms from the snail population with higher infestation intensity. These findings support the theory of sex allocation and, for the first time, demonstrate differences in gonadal sex allocation in a commensal organism in its natural environment.