Use of ingested algal diterpenoids by Elysia halimedae Macnae (Opisthobranchia : Ascoglossa) as antipredator defenses

Marine mollusks of the Order Ascoglossa (= Sacoglossa) are specialized herbivores of tropical green seaweeds and other seaweeds (diatoms, red algae, seagrasses). In this study, we investigated the feeding relationship between the ascoglossan Elysia halimedae Macnae and its preferred food, the seaweed Halimeda macroloba Decaisne. The toxic diterpenoid metabolites of many species of Halimeda have been shown to act as feeding deterrents toward nonspecialized grazers such as herbivorous fishes. However, E. halimedae feeds only on species of Halimeda and prefers young plant growth, which contains the highest concentrations of the Halimeda compounds. Our observations suggest that E. halimedae modifies the major diterpenoid from H. macroloba and sequesters and stores this modified compound for its own defense. When irritated, Elysia secretes large amounts of mucus containing this defensive compound. The compound occurs in Elysia as ≈ 7% of the whole animal dry mass. Similar high concentrations of the compound are found in the egg masses of E. halimedae. We isolated and determined the structure of the ascoglossan compound and tested the compound in field assays toward carnivorous and herbivorous fishes on Guam. This compound is a significant feeding deterrent at naturally occurring concentrations toward potential predators. Thus, E. halimedae is adapted to feed on toxic algae and utilizes the algal chemical defenses for its own defense against predation.