Cnidocytes

Cnidocytes are specialized stinging cells unique to the animals of the phylum Cnidaria. They house organelles called nematocysts, which are capsule-like structures that discharge a coiled tubule when triggered, often delivering venom or other compounds. The discharge mechanism serves both prey capture and defense.

Each cnidocyte contains a nematocyst capsule under high internal pressure, a coiled tubule, and a sensory trigger known as the cnidocil on the cell surface. When the cnidocil is stimulated by contact or chemical cues, water rapidly enters the capsule, the tubule everts, and the thread penetrates the target tissue.

Cnidocytes are diverse. Most species possess nematocysts, which can be penetrant or spined, while other cnidarians also have spirocysts (adhesive-only organelles) and, in some groups, ptychocysts that reinforce tube structures.

Cnidocytes are distributed throughout the epidermis and, in some animals, the gastrodermis. They originate in cnidoblasts derived from stem cells; maturation produces a mature cnidocyte capable of firing when triggered.

Ecologically, cnidocytes are critical for feeding and defense. The toxins and mechanical force delivered by nematocysts immobilize prey, deter predators, and contribute to the stinging capability of corals, jellyfish, sea anemones, and related organisms.