Defensive chemicals of the Spanisch dancer nudibranch Hexabranchus sanguineus and its egg ribbons: macrolides derived from a sponge diet

The Spanish dancer nudibranch Hexabranchus sanguineus (Rüppell et Leuckart), a large brightly colored shell-less sea slug (Gastropoda : Opisthobranchia) common to Indo-Pacific coral reefs, derives a potent chemical defense from a sponge that it eats ( Halichondria sp.). In turn, the nudibranch passes defensive compounds to its egg ribbons, which are similarly conspicuous and physically defenseless. Slices of the dorsal mantle tissue of the nudibranch were rejected in laboratory feeding assays employing two common sympatric predators: an Indo-Pacific reef fish, Thalassoma lunare (Linnaeus), and a reef hermit crab, Dardanus megistos (Herbst). The defensive metabolites, a suite of unusual oxazole-containing macrolides, were isolated from the sponge, the nudibranch, and the nudibranch egg masses at 0.14–0.38, 0.14–0.62, and 2.65% of dry weight, respectively, and were effective inhibitors of feeding by T. lunare at minimum concentrations of 0.01–0.02% dry weight of food pellet. The macrolides were concentrated in the dorsal mantle of the nudibranch, which is most vulnerable to predatory attack, and in the combined digestive gland/gonad, site of both sponge digestion and egg production. The most abundant macrolide in the sponge tissue was not present in the nudibranch or its egg masses, suggesting that chemical modification of this compound takes place upon digestion. In a reef environment dominated by visually oriented predators, the striking color pattern and behavioral responses of Hexabranchus may have arisen with a concomitant elaboration of dietarily derived chemical defenses.