Larval release behaviors in the Caribbean spiny lobster, Panulirus argus: role of peptide pheromones.

Larval release in the Caribbean spiny lobster Panulirus argus is highly synchronous and is controlled by a "pumping pheromone" released from the hatching eggs. The pheromone induces a parent female to undergo stereotypical larval release behaviors, including rapid abdominal extensions and pleopod pumping. These behaviors help to break open the egg membranes and result in the synchronous release of larvae. Based on previous studies on larval release in brachyuran crabs, we hypothesized that larval release behaviors are induced by pheromones composed of small peptides. We quantified pleopod pumping activity upon exposure to a range of synthetic peptides to identify compounds that will induce larval release behaviors. Chemically cued pumping behavior was described in terms of the threshold concentration for response, maximum percentage response, and effective concentration range. Pleopod pumping behavior was evoked by di- and tripeptides with a neutral amino acid at the amino terminus and a basic amino acid at the carboxy terminus and also by the basic-basic dipeptide Lys-Arg. All carboxy-terminal arginine peptides tested produced a significant pumping response, with the exception of Trp-Ile-Arg. Response concentration thresholds ranged from 10(-9) M for the most potent peptide (Gly-Arg) to 10(-4) M for the least potent (Gly-His-Lys). The maximum percentage of lobsters responding was largely independent of the threshold concentration and ranged from 24.3 to 58.3%. Effective concentration ranges for the peptides were variable from 1 to 4 orders of magnitude. Pumping response usually declined with increasing concentration beyond the concentration that evoked the maximum response of the peptides. Our results support the conceptual model that larval release in subtidal crustaceans is controlled by small peptides that act as pheromones.