When is a male not a male? Sex recognition and choice in two sex-changing species

For dioecious species, choosing a mate of the same sex can have reproductive costs. For sex-changing animals, however, a lack of sex recognition may not carry a reproductive cost, as pairs that were initially same-sex can become opposite-sex pairs as one partner changes sex. The strength of sex discrimination in sex changers, then, should depend on the duration of mating associations and whether the time of sex change is influenced by social situation ("flexible" sex change). We studied two species of marine snails that change sex from male to female, one with flexible sex change and long-term or permanent mating associations (Crepidula fornicata) and one with short-term pairings and relatively fixed time of sex change (Crepidula convexa), to determine whether either species exhibits sex recognition and whether members of C. convexa show stronger sex discrimination. In laboratory experiments, small males, the choosing animals, were placed with either a male or a female conspecific (no-choice experiments) or given a choice of a male or female (choice experiments). We controlled for shell length in all experiments, as relative size may influence sex change or choice. Males of both species paired more often with females than males, but, as predicted, males of C. convexa showed stronger discrimination: When given a choice, no C. convexa male paired with another male. In contrast, some C. fornicata males always chose other males even when given the choice of a female. These results suggest that sex recognition can be adaptive even for sex changers but demonstrate that the level of sex recognition will depend on other aspects of reproductive behavior.