cnidosac

Cnidosac refers to a specialized organ found in certain opisthobranch molluscs, most notably aeolid nudibranchs, that houses nematocysts (the stinging capsules) stolen from cnidarian prey. The term describes a sac-like structure at the end of each ceras, the finger-like projections on the nudibranch’s back, which is connected to the digestive tract by a short duct.

In aeolid nudibranchs, the animal sequesters nematocysts from consumed cnidarians, such as hydroids or soft corals, and transfers them to the cnidosac without digesting them. The stored nematocysts remain functional and can be discharged through the ceratal epidermis when the slug is threatened, providing a chemical defense against potential predators. The nudibranch itself is protected from these stings by a combination of physiological barriers and selective use of the stored capsules.

The presence of cnidosacs represents a form of defense-based trophic specialization: the animal repurposes cnidarian defenses for its own protection. Not all nudibranchs possess cnidosacs; their occurrence is characteristic of many aeolids but absent in other nudibranch groups and related molluscs. The study of cnidosacs contributes to understanding of predator–prey interactions, chemical ecology, and kleptoplastic-like strategies in marine invertebrates.

Cnidosacs have been observed in diverse marine environments, from temperate to tropical waters, where aeolid nudibranchs feed on cnidarians and employ this adaptation for survival.