A nematocyst is a microscopic, harpoon-like organelle found exclusively in cnidarians, a diverse group of aquatic animals including jellyfish, sea anemones, and corals. These specialized structures are housed within dedicated cells called cnidocytes, primarily located on the tentacles and around the mouths of these organisms. Nematocysts serve a dual purpose: they are essential tools for capturing prey and provide a potent defense mechanism against predators, responsible for the characteristic “sting” of many cnidarians.
Anatomy of a Nematocyst
Each nematocyst begins as a bulb-shaped capsule. Within this capsule, a long, hollow thread lies tightly coiled and inverted, much like a glove finger pushed inward. A hinged lid, the operculum, seals the capsule’s opening, keeping the thread contained until deployment.
On the exterior of the cnidocyte cell, a hair-like projection called the cnidocil acts as a mechanical and chemical trigger. The thread’s base often features prominent barbs or spines, designed to penetrate and anchor into a target. This hollow thread is filled with a complex mixture of venom.
The Firing Mechanism
The firing of a nematocyst is a rapid event, triggered by specific stimuli. When the cnidocil detects touch or chemical cues from potential prey or predators, water rapidly flows into the nematocyst capsule.
This sudden water influx increases internal hydrostatic pressure within the capsule, reaching approximately 150 atmospheres (15 MPa). This pressure forces the operculum open. The compressed, coiled thread then explosively everts, turning inside out as it shoots outward like a miniature harpoon.
This discharge process is one of the fastest biological movements known, occurring in as little as 700 nanoseconds and generating accelerations exceeding 5 million times the force of gravity. The thread’s barbs facilitate its penetration and anchoring into the target. Once embedded, the hollow thread injects its venom directly into the prey or perceived threat.
Purpose and Effects
Nematocysts are essential for cnidarian survival, serving in food acquisition and as a protective measure. For prey capture, the injected venom quickly incapacitates or paralyzes small organisms like fish or crustaceans. This allows the cnidarian, often slow-moving, to secure and consume its meal.
Nematocysts also deter larger predators. The venom causes pain or discomfort, encouraging threats to retreat. Cnidarian venoms are complex mixtures, often containing neurotoxins that affect nerve function and cytotoxins that damage cells and tissues. Effects on target organisms can range from paralysis and tissue damage to severe pain or even death for larger creatures, including humans, depending on the species and venom amount.