ASYMMETRY OF MALE AND FEMALE MATING IN MIXED-SPECIES GROUPS OF APLYSIA-FASCIATA POIRET AND APLYSIA-BRASILIANA RANG (MOLLUSCA, GASTROPODA)

Aplysia fasciata and A. brasilianca are found on opposite sides of the Atlantic Ocean, but are morphologically and behaviorally very similar . To determine the relationship between these two species, mating patt erns were examined in single-species and mixed-species groups of 2 and 4 individuals. In single species groups, A. brasiliana spent less tim e mating than did A. fasciata. However, the relative distribution of t ime spent mating as a male, as a female, or in both sexual roles simul taneously was virtually identical in both species. The overall time sp ent mating in mixed-species groups was similar to that seen in single- species groups of A, brasiliana, and was significantly less than in si ngle-species groups of A, fasciata. In mixed-species groups there were relative increases in the likelihood for A, fascinta to mate as males , and for A. brasiliana to mate as females. Our data indicate that whe n animals with differing sexual drives are in contact, the animal with the stronger sexual drive generally mates as a male. This suggests th at the level of mating is set by a drive to mate as a male, while fema le mating is largely nonselective and passive. The data also indicate that A. fasciata and A. brasiliana are either somewhat different popul ations of the same species, or are very closely related species.