Cnidosacs

Cnidosacs are specialized defensive organs found in many aeolid nudibranchs (sea slugs). Each cnidosac houses nematocysts, the stinging cells stolen from cnidarian prey, which the animal can use for protection.

Mechanism and structure: During feeding, aeolid nudibranchs typically ingest cnidarians such as hydroids or anemones. The cnidarian tissue is digested, but the nematocysts are spared and transported to the cnidosacs located at the tips of the cerata (the dorsal projections along the body). Within the cerata, specialized cells called cnidophages accumulate and store the nematocysts in the cnidosac, keeping them intact and functional for later use.

Function and significance: The stored nematocysts can be deployed in defense against predators, providing an effective deterrent by delivering stinging cells from the nudibranch’s cerata. This phenomenon, often described as kleptocnidae, represents a form of cellular kleptoplasty where defensive cells are repurposed rather than digested. The cnidosac system illustrates a unique ecological adaptation in which a predator co-opts prey defenses for its own protection.

Distribution and variation: CNidosacs are characteristic of many aeolid nudibranchs but are not universal among all sea slugs. The specific cnidarian sources and the number of cerata, and thus cnidosacs, vary among species, reflecting differences in diet and ecological niche.