Sperm precedence in the hermaphroditic freshwater snail, Helisoma trivolvis.

Our primary research goal was to determine the nature of sperm precedence in the freshwater snail, Helisoma trivolvis. Sperm precedence has been suggested as a way for female partners to choose among mates after mating has occurred, sometimes referred to as cryptic female choice, one aspect of sexual selection. Most studies of sexual selection have focused on gonochoristic species (with two separate sexes) as opposed to hermaphroditic species like the sperm storing simultaneous hermaphrodite H. trivolvis. Sperm precedence refers to the nonrandom differences in fertilization success after copulation with multiple sperm donors has occurred. Because H. trivolvis store sperm, we predicted that sperm from the first mate would fill the spermatheca and thus second sperm success would be low. We investigated whether the first sperm contributor or second sperm contributor fertilized more offspring of H. trivolvis, using albinism as genetic marker to determine paternity. To ensure a varied genetic background, albino snails used in the study were descendants from F2 albinos resulting from crosses between pigmented wild Helisoma trivolvis and lab-reared albinos. Albino snails were isolated until they reached maturity and were mated first with a pigmented snail and then an albino snail (or the reverse). Egg masses were collected at 1, 3, 5, and 11 weeks after mating and embryos were examined under an inverted microscope at 40x. The presence or lack of eye pigmentation was used to identify paternity in these snail embryos. As expected, we found strong first mate precedence in H. trivolvis overall; about 85% of eggs were fertilized by the first partner, although there were individual differences in precedence. In addition, there was significantly lower first sperm precedence when an albino snail was the first mate, suggesting a slight preference for pigmented partners