Mechanism of postcopulatory sexual selection in Rapana venosa

Postcopulatory sexual selection is a common phenomenon in polyandry species. The mechanism of postcopulatory sexual selection is complicated, and it is the key and difficult topics in sexual selection. Rapana venosa is one of the most important commercial molluscs in China, whose females mate with multiple males. In this study, paternity analysis was performed in three families of R. venosa using microsatellite markers and behavior observation. Multiple paternity was observed in all the three families, suggesting that R. venosa was genetic polyandry. There was no correlation between six morphological parameters of males and paternity ratios, and no correlation between the mating duration and paternity ratios. The results showed that the first mating male had most progeny and the last mating male had least progeny. R. venosa exhibited first-male sperm precedence. This pattern of sperm precedence was consistent with the hypothesis of "topping off", in which a female accepts a large amount of sperm from her first mate, and, if additional space remains in her spermathecae, she then seeks additional mates until she has no need for further sperm. This study provides important information for the studies on mechanism of postcopulatory sexual selection in marine invertebrates.