A Study of Defensive Mechanisms Employed by Two Species of Nudibranchs using Toxicity and Unpalatability Analyses

Nudibranchs are marine invertebrates that have developed an intriguing defense mechanism, including warning coloration and the use of chemicals accumulated through their sponge diet. The goal of this study was to determine whether the strength of chemical defenses differs between dietary and accumulated secondary metabolites for two species: Glossodoris vespa and Ceratosoma brevicaudatum. First, NMR spectroscopy was used to not only identify specific compounds in the mantle (outer covering) and the viscera (gut) but also to analyze the possibility of nudibranch species transporting more toxic compounds for defensive purposes. Next, toxicity (brine shrimp) and palatability (Palaemon shrimp) assays were used to examine whether accumulated compounds differ in anti-predator activity. The results of this study show increased toxicity in the mantle compared to the viscera for both species. and while both species exhibited the possibility of selective sequestration, Glossodoris vespa hinted that nudibranchs may have other methods of chemical sequestration including chemical modification that would explain why more toxic and unpalatable compounds are found in the mantle. However, there was no significant change in unpalatability between the mantle and the viscera. Finally, comparisons between genera that have mantle dermal formations along the mantle rim (Glossodoris) and those that have mantle dermal formations concentrated in the mantle horn (Ceratosoma) show that despite varying classes of dietary chemicals and selectivity of sequestration, both species exhibited a chemical arsenal in the mantle that was more toxic than dietary metabolites, suggesting that toxicity is an important part of their defensive strategy