Do "sperm trading" simultaneous hermaphrodites always trade sperm?

Sperm trading can be a mechanism to solve the conflict over sex roles in hermaphrodites with copulation, sperm competition, and sperm digestion. If present, sperm donation depends on sperm receipt, resulting in conditional reciprocal inseminations. Conditional reciprocity can involve three traded commodities: penis intromissions on a yes-or-no basis, intromission durations (indicating ejaculate size), or sperm transfer. If present, animals that refuse to donate (cheaters) should be deserted by their partner or receive smaller ejaculates. We tested whether any of these commodities is traded in the simultaneously hermaphroditic sea slug, Chelidonura sandrana. Matings involve a stereotypic single sex role alternation. As a result, reciprocity was more frequent than expected under random mating, supporting trading of penis intromissions. Contrary to the predictions under trading of ejaculate sizes, intromission durations were not balanced between mating partners. To test trading of sperm transfer itself, we interrupted the sperm groove that transports sperm into the penis during copulation. Treated animals (experimental cheaters) could still copulate normally but could not transfer sperm. When paired to a cheater, sham-treated control animals did not respond to the absence of sperm receipt, neither by desertion nor by shortening intromission. We thus reject trading of insemination durations and sperm transfer in C. sandrana. Although trading of intromissions may be present, we discuss why reciprocity in this species may also be a by-product of the mutual willingness of both partners to donate as well as receive sperm, and not an indicator of sperm trading. Copyright 2005.