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Nomarski micrograph of a Ruthenium-red stained nematocyst from Aiptasia pallida, the pale anemone. The red dye stains the polyanionic venom proteins found inside the capsule of this partially discharged nematocyst.

A cnidocyte, cnidoblast or nematocyte, is a type of venomous cell unique to the phylum Cnidaria (corals, sea anemones, hydrae, jellyfish, etc.). It evolved in these aquatic animals as a means for them to catch prey and defend themselves from predators, despite being morphologically simple, lacking a skeleton and usually being sessile; they prey on fish and crustaceans. A cnidocyte fires a structure that contains the toxin, from a characteristic sub-cellular organelle called a cnidocyst (or cnida or nematocyst). This is, for instance, responsible for the stings delivered by jellyfish.

A discharged nematocyst seen under a scanning electron microscope.

Each cnidocyte cell contains an organelle called a cnidocyst, which comprises a bulb-shape capsule containing a coiled hollow thread-like structure attached to it. The externally-oriented side of the cell also has a hair-like trigger called a cnidocil. When the trigger is activated, the shaft of the cnidocyst penetrates the target organism, and the hollow thread is everted into it. This discharge is one of the fastest biological processes, takes no more than a few microseconds, and reaches accelerations of about 40,000g^[1]. However, it has recently been shown to occur as fast as 600 nanoseconds, thus reaching an acceleration around five million G's. After penetration, the toxic content of the nematocyst is injected into the target organism. The rapid activity of the injected neurotoxins immediately paralyzes the mobile prey, thus allowing the sessile cnidarian to devour it.

The nematocyst 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 nematocyst to eject rapidly.

A nematocyte is able under some conditions to fire independently, but this presents several problems for the cnidarian. First, it must avoid stinging itself. Second, it must replace cnidocytes after discharge, as they are "single use" cells, and this costs a lot of energy. In order to regulate discharge, cnidocytes are connected as "batteries", containing several types of nematocytes connected to supporting cells and neurons. The supporting cells contain chemoreceptors, which, together with the mechanoreceptor on the cnidocyte (cnidocil), allow only the right combination of stimuli to cause discharge, such as prey swimming, and chemicals found in prey cuticle or skin.

Over 30 types of nematocytes are found in different cnidarians. They can be divided into the following groups:

1. Penetrant: A harpoon-like structure used to penetrate
2. Glutinant: Sticky surfaces used to stick to prey
3. Volvent: A lasso-like string that is fired at prey and wraps around a cellular projection on the prey
4. Ptychocyst: A special type of nematocyte found on burrowing (tube) anemones, which help create the tube in which the animal lives.

Depending on the species, one or several types can appear simultaneously on the organism.

Nematocysts are very efficient weapons. A single nematocyst has been shown to suffice to paralyze a small arthropod (Drosophila larva). The most deadly cnidocytes (to humans, at least) are found on the body of a box jellyfish. One member of this family, the sea wasp, Chironex fleckeri, is "claimed to be the most venomous marine animal known," according to the Australian Institute of Marine Science. It causes excruciating pain to humans, often followed by death, sometimes within two or three minutes. The chance of survival if stung while swimming alone is "virtually zero." Other cnidarians, such as the jellyfish Cyanea capillata (the "The Adventure of the Lion's Mane" made famous by Sherlock Holmes) or the hydrozoan Physalia physalis (Portuguese Man o' War, "Bluebottle") can cause extremely painful and sometimes fatal stings. On the other side, aggregating sea anemones may have the lowest sting intensity, perhaps due to nematocysts not penetrating the skin, providing only a feeling of that similar to touching sticky candies to human fingers. Besides feeding and defense, sea anemone colonies use cnidocytes to sting one another in order to win space.

Venom from animals such as cnidarians, scorpions and spiders may be species-specific. A substance that is weakly toxic for humans or other mammals may be strongly toxic to the natural prey or predators of the venomous animal. Such specificity has been used to create new medicines and bioinsecticides.

Animals in the phylum Ctenophora ("sea-gooseberries" or "comb jellies") are transparent and jelly-like but have no nematocysts, and are harmless to humans.

1. ^ G. Kass-Simon and A.A. Scappaticci, Jr. The behavioral and developmental physiology of nematocysts. Can. J. Zool. Vol. 80, 2002, pp 1772-1794

• Dangerous marine animals of Northern Australia: the Sea Wasp Australian Institute of Marine Science; dangers of box jellyfish
• Nematocysts Firing Movie