Cnida — the stinging organelles of Cnidaria

Structure and mechanism

Each cnida sits inside a specialised cell called a cnidocyte (also called a cnidoblast or nematocyte). A typical cnida is a pressurised capsule that contains a tightly coiled tubular filament. The capsule wall is sealed by an operculum and the cell bears a sensitive hair-like trigger. When the trigger is mechanically or chemically stimulated, the operculum opens and the tubule everts explosively, a process driven by rapid changes in internal pressure and osmotic gradients. The everted tubule may have barbs or adhesive material and can inject chemical compounds into the target. In functional terms, a cnida behaves like a miniature, single-use harpoon or glue-sprayer: once discharged the capsule must be replaced by the cell.

Main types

• Nematocysts — the most widespread type. These typically have a barbed or hollow tubule that can penetrate and deliver toxins to immobilize prey or deter predators.
• Ptychocysts — found mainly in certain tube-dwelling anemones; these produce sticky threads used for constructing tubes and for adhering to substrates or captured items.
• Spirocysts — largely adhesive rather than penetrative; these tubules expand and wrap around small prey to entangle it rather than inject venom.

Biological roles and examples

Cnidae serve several ecological functions. Predatory cnidarians use nematocysts to seize plankton and small animals, allowing otherwise sessile organisms such as corals and anemones to supplement nutrition. Mobile medusae and box jellies employ dense batteries of cnidae across their tentacles to capture larger, more active prey. Cnidae also play roles in defence, in competition among benthic organisms, and in attachment or tube-building where adhesive types are present. Their presence and arrangement on the body are important diagnostic characters in taxonomy and ecology.

Chemical effects and human relevance

The material delivered by many nematocysts includes proteinaceous venoms and other bioactive compounds. These neurotoxins and irritants vary widely in potency between species: most cause only local pain or irritation in humans, while some, notably certain cubozoans (box jellies), produce systemic effects that can be life-threatening. Encounters with cnidarians are a frequent medical and public-safety concern in coastal regions, and their toxins are studied for pharmacological properties as well as for antivenom development.

Notable facts and distinctions

Although often called "stinging cells," the functional unit is the stinging capsule (the cnida) housed within the cell. The capsule itself is an organelle-like structure and is generally single-use: after discharge the producing cell regenerates a replacement. No cnidarian lineage is known to have completely lost cnidae, which remain the defining feature of the phylum. Many aspects of cnida diversity, deployment patterns and venom composition continue to be active areas of research in developmental biology, ecology and toxinology. For further context and detailed images see general references on the phylum and on functional morphology in cnidarians.