cnidocyst

A cnidocyst, commonly called a nematocyst, is the defining stinging organelle inside the cnidocytes, the specialized cells that form the stinging cells of cnidarians such as jellyfish, corals, sea anemones, and hydras. Each cnidocyst consists of a capsule filled with venomous fluid and an elongated, coiled tubule. The capsule is protected by a surrounding cell membrane and is connected to a cnidocyte basement by a trigger apparatus.

Discharge is initiated when the cnidocil, a hairlike sensory structure on the cnidocyte, is stimulated by physical contact or chemical cues. This triggers a rapid increase in internal pressure, causing the capsule to evaginate the tubule in a fraction of a second. The tubule everts and penetrates or adheres to the target, delivering venom. The discharge mechanism is extremely fast and can inject toxins that immobilize prey or deter predators. Some tubules are barbed or spined, aiding penetration and retention.

There are several nematocyst types, differing in tubule shape, discharge pattern, and ecological function. Some are specialized for piercing and injecting venom into prey, while others may trap or adhere prey through adhesive or entangling mechanisms. The diversity of nematocysts within a cnidarian supports efficient prey capture and defense in a variety of habitats.

Cnidocysts are produced by cnidocytes embedded in the animal’s epidermis or gastrodermis. The venom they deliver contains a mix of proteins and peptides, contributing to the potency of stings in natural interactions and, in some species, medical significance for humans. They are a central adaptation in the biology and ecology of cnidarians.