cnidocyte

Cnidocytes are specialized stinging cells unique to the phylum Cnidaria, used for prey capture and defense. They occur on the outer epidermis and, in many species, on tentacles around the mouth.

Structure: Each cnidocyte contains a cnidocyst, a capsule-like organelle that houses a thread. The thread is coiled inside the capsule and can bear barbs or spines. The cell has a hair-like trigger, the cnidocil, on its surface. When stimulated, the capsule rapidly opens and the thread everts explosively, within a few milliseconds. The ejected thread can penetrate prey or deliver venom; some threads are barbed, others are smooth.

Types: The most common are nematocysts, which inject venom via the thread; spirocysts produce a sticky, adhesive thread for attachment rather than injection; ptychocysts are specialized to secrete tubular structures used by tube-dwelling anemones to build protective tubes.

Discharge and venom: The venom can cause paralysis, tissue damage, or pain in prey or potential predators; potency varies by species and cnidocyte type.

Distribution and function: They help immobilize prey, subdue predators, and can deter handling by humans. Among cnidarians, cnidocytes are the defining stinging cells and are replenished as needed.

Development: Cnidocytes arise from cnidoblasts during growth and development, enabling cnidarians to continually renew their stinging cell population.