According to sex allocation theory, hermaphrodites are expected to allocate most of their reproductive resources to the female function when there is a low level of mate competition and to shift them to the male function when mate competition increases: sex allocation theory assumes that there is a trade-off between sexual functions. Although several studies highlight some adjustments of sex allocation according to mating opportunities, empirical support for the trade-off between sexual functions is surprisingly scarce. Here we argue that this lack of support for a trade-off might partially depend on the fact that sex allocation studies often overlook gender-related traits other than gamete production. We investigated whether parental care (a putative female behavior) and motility (a proxy for mate searching, and a putative male behavior) varied plastically according to mating opportunities in the hermaphroditic polychaete worm Ophryotrocha diadema. We found that parental care was higher under low mating opportunities while motility increased under high mating opportunities, and that the two behaviors were negatively correlated with each other – i.e. there was a trade-off between them. We also observed the behavior of a separate-sex species closely-related to O. diadema – Ophryotrocha labronica. We found that males moved more than females in the separate-sex species and that mothers performed more parental care than fathers in both Ophryotrocha species. Our results provide convincing evidence in support of a trade-off between sexual functions and highlight the importance of investigating sex-allocation adjustments in reproductive traits others than gametes.