Emesis, learned aversion, and chemical defense in octocorals: a central role for prostaglandins?

Prostaglandin A2 and its ester derivatives comprise as much as 8% of the wet tissue weight of some octocoral species such as Plexaura homomalla (phylum Cnidaria, class Anthozoa, subclass Octocorallia). These high levels of prostaglandins, although initially palatable to fish, may function as defensive toxins by inducing emesis and learned aversions in potential predators. As the fish Fundulus heteroclitus and Halichoeres garnoti gain experience through the course of experiments, they increasingly reject foods containing emetic prostaglandins, but do not alter their acceptance of untreated control foods. Emesis is also induced in fish by consumption of tissue or lipid extracts from the subtropical whip coral Leptogorgia virgulata (subclass Octocorallia, order Gorgonacea). The emetic properties of L. virgulata induce learned aversions in the fish Micropterus salmoides and Morone saxatilis. Extracts of L. virgulata do not contain high levels of prostaglandins but do, however, contain other metabolites that appear to mimic the effects of eicosanoids. Some nonprostanoid secondary metabolites may induce emesis by stimulating prostaglandin biosynthesis in the gastric mucosa of predators.