Many species of the opisthobranch mollusk Aplysia form breeding aggregations during the reproductive season. The aggregations contain both mating and egg-laying animals, and are associated with egg cordons. Although pheromones play a significant role in developing and maintaining the aggregations, little is known about the active factors. Behavioral studies have shown that egg-laying animals are more attractive than nonlaying animals, have shorter latencies to mating, and induce conspecifics to lay eggs. As a first step toward isolating and chemically characterizing the active factors, we examined the relative importance of the egg layer and egg cordon as sources of pheromonal activity in Aplysia brasiliana. T-maze experiments showed that both animal-derived and cordon-derived factors are attractive, and that the animal-derived factors are specifically associated with egg layers. Extracts of the atrial gland--an exocrine organ secreting into the oviduct--increased the attractiveness of nonlaying animals when placed in the surrounding seawater, suggesting that the "cordon-derived" aggregation pheromones may be products of the atrial gland. Mating studies showed that both animal-derived and cordon-derived factors induce mating, and that the animal-derived factors are associated with both egg layers and nonlayers. In contrast, neither animal-derived nor cordon-derived factors induced egg laying. Comparable results were obtained with either one or two animals in the chamber, suggesting that the accessibility of a potential mate did not influence the results. The lack of effect may result from the low-probability nature of egg-laying activity.

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