Lowered nutritional quality supplements nudibranch chemical defense

Reduced palatability through lowered nutritional content is a common defense for modular or gelatinous organisms, but commonly thought unavailable for active, mobile animals. However, nudibranchs - a group of brightly colored, shell-less snails - may supplement their chemical defenses via increased water and ash content. Among 33 gastropod species, nudibranchs differed from prosobranch snails by containing approximately half the digestible organic material per unit wet weight, indicating their caloric value per unit volume will likewise be less than that of shelled snails. Nudibranchs also allocated proportionally less organic material to exterior body regions than to the viscera. Because exterior regions are more easily regenerated and often contain nematocysts or high concentrations of chemical defense, the areas of slug bodies most vulnerable to attack are the best defended, the least valuable to potential predators, and the least costly for nudibranchs to replace. To test how these differences in organic content affect palatability to predators, I offered to Cancer productus, a common molluscivorous crab, pellets differing in organic content (12% or 22% of wet weight) and secondary chemistry (presence or absence of Archidoris montereyensis extract). In no-choice bioassays, chemical extracts reduced feeding by approximately 50%, while lowered organic content reduced feeding by approximately 19%. The effects of organic content did not significantly interact with those caused by chemical extracts, suggesting that, in some contexts, reduced nutritional quality offers a somewhat independent form of antipredator deterrence. Although the nutritional context of defenses is often considered in studies of plants and other organisms suffering partial predation, this study shows that such a multi-factor approach should be more commonly extended to mobile animals.