Chemical ecology of the nudibranch Glossodoris pallida: Is the location of diet-derived metabolites important for defense?

The nudibranch mollusc Glossodoris pallida possesses the diet-derived terpenoid compounds scalaradial, desacetylscalaradial, and deoxoscalarin. In this study, we determined whether the compounds were located strategically in the nudibranch body for defense against predators, as well as whether they were present in mucus secreted by the nudibranch. Analysis of the distribution of metabolites in the different body parts indicated an accumulation of compounds in the border of the mantle (where the mantle dermal formations are located), as well as in the mucus secretion. Several experiments were conducted in order to establish a relationship between the location of the compounds and the chemical defense of the nudibranch, using artificial diets as well as animals with and without the borders of their mantles. The compounds from the mantle border deterred feeding by crabs (Leptodius spp.) and reef fishes when tested at their natural concentrations. When the mantle borders were removed from some nudibranchs, these animals were significantly more susceptible to predation by reef fishes than were animals with their mantle borders intact. While we were able to demonstrate that the diet-derived compounds in the mantle border function in the defense of the nudibranch, both in the laboratory against the crabs and in the field against natural assemblages of predatory reef fishes, our experiments with artificial diets did not indicate that the location of the compounds was critical for defense. Food cubes with pieces of mantle border tissue or extracts of mantle border tissue distributed uniformly throughout the cubes or along one border were equally effective at deterring feeding by reef fishes relative to control cubes. The location of diet-derived compounds in the mantle borders may also serve other functions in addition to defense.