Attractin, a water-borne peptide pheromone in Aplysia

The opisthobranch mollusk Aplysia is a simultaneous hermaphrodite that does not usually fertilize its own eggs. It is solitary during most of the year, but moves into breeding aggregations during the summer reproductive season. The aggregations contain both mating and egg-laying animals and are associated with masses of egg cordons. The egg cordons are a source of both contact and water-borne pheromones that attract animals to the area and induce or facilitate reproductive activity. Three characteristics of the Aplysia reproductive system make it ideal for the isolation and characterization of a water-borne peptide pheromone: (a) egg laying can be induced by injecting atrial gland extract into the hemocoel; (b) the resulting eggs are packaged into a long string or "cordon" which has a large surface area; and (c) the cordon is a source of water-borne pheromones. By using atrial gland extract to induce egg laying, it is possible to control the number of animals laying eggs, the timing and synchronization of egg laying, the general area in which the eggs are deposited and, to some extent, the purity of the seawater in which they are deposited. In terms of the resulting "cordon-derived" pheromones, this means that it is possible to control the timing of pheromone secretion, the amount of pheromone secreted, and the relative amount of contaminants in the seawater. This approach was used in the studies described in this paper to isolate and characterize a peptide that is a water-borne pheromonal attractant in Aplysia. To the best of our knowledge, it is the first water-borne peptide pheromone characterized in mollusks and in invertebrates. Some of these studies have been reported previously (Painter et al., 1998).