Opisthobranchs have developed novel systems of defense. The most primitive groups still depend on shells in one way or another. A great variety of chemical products has been found in these animals. For example, Thompson (1998) described the presence of acid secretions in some opisthobranchs when they are bothered. However, the most common chemical system of defense is the production of secondary metabolites like the terpenoids. These compounds are obtained from their food-sponges, marine algae, and tunicates among others-and- are later transformed into similar compounds with more or less activity than the original ones.

Hypselodoris ghiselini

These metabolites of defense are normally distributed in exposed areas of the animals and in some cases they are concentrated in specialized structures such as mantle glands. Aeolid nudibranchs normally feed on sea anemones, hydrozoans, and other cnidarians which have nematocysts called stinging cells.

Navanax inermis	Flatworm mimicking opisthobranchs

These opisthobranchs store non-fired nematocysts in specialized sacs at the ends of their papillae on their backs, and use them for their own defense.

Elysia pusilla

The glowing colors of many opisthobranchs play an important defensive role and signal the presence of toxic or dissuasive chemicals. Many other organisms such as flatworms, sea cucumbers, marine crustaceans, and marine mollusks have mimicked these patterns to confuse potential predators. Many opisthobranchs such as Elysia pusilla (above) have cryptic colorations that help them blend in with their surroundings.