Cnidae

A cnidocyte (also known as a cnidoblast or nematocyte) is an explosive cell containing one giant secretory organelle or cnida (plural cnidae) that defines the phylum Cnidaria (corals, sea anemones, hydrae, jellyfish, etc.). Cnidae are used for prey capture and defense from predators. Despite being morphologically simple, lacking a skeleton and many species being sessile, cnidarians prey on fish and crustaceans. A cnidocyte fires a structure that contains the toxin, from a characteristic subcellular organelle called a cnidocyst (also known as a cnida or nematocyst). This is responsible for the stings delivered by a cnidarian.

Each cnidocyte contains an organelle called a cnida or cnidocyst (e.g. nematocyst, ptychocyst or spirocyst), which comprises a bulb-shape capsule containing a coiled hollow tubule structure attached to it. The immature cnidocyte is referred to as a cnidoblast. The externally oriented side of the cell also has a hair-like trigger called a cnidocil. Cnidocil is a mechano and chemo-receptor. When the trigger is activated, the tubule shaft of the cnidocyst is ejected and in the case of the penetrant nematocyst, the forcefully ejected tubule penetrates the target organism. This discharge takes no more than a few microseconds, and is able to reach accelerations of about 40,000 g. Recent research suggests the process to occur as fast as 700 nanoseconds, thus reaching an acceleration of up to 5,410,000 g. After penetration, the toxic content of the nematocyst is injected into the target organism, allowing the sessile cnidarian to devour it.

Cnidae capsule stores a large concentration of calcium ions, which are released from the capsule into the cytoplasm of the cnidocyte when the trigger is activated. This causes a large concentration gradient of calcium across the cnidocyte plasma membrane. The resulting osmotic pressure causes a rapid influx of water into the cell. This increase in water volume in the cytoplasm forces the coiled cnidae tubule to eject rapidly. Prior to discharge the coiled cnidae tubule exists inside the cell in an "inside out" condition. The back pressure resulting from the influx of water into the cnidocyte together with the opening of the capsule tip structure or operculum, triggers the forceful eversion of the cnidae tubule causing it to right itself as it comes rushing out of the cell with enough force to impale a prey organism.

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