Cnidocytes are prey-capture and defensive cells unique to animals of the phylum Cnidaria. Classes Anthozoa - Corals and sea anemones Cubozoa - Sea wasps or box jellyfish Hydrozoa - Hydroids, hydra-like animals Scyphozoa - Jellyfish Cnidaria (from New Latin cnida nematocyst, fr. ...

Mechanism and structure

Cnidocytes contain specific sub-cellular secretion products (organoids) named nematocysts that are used for self-defense and prey capturing. Nematocysts are composed of a coiled thread-like structure attached to the bulb-shaped nematocyst capsule body. The externally-oriented side of a cnidocyst cell also has a hair-like trigger on it. When the trigger is activated the cell "fires" its nematocyst. 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 evert rapidly. Nematocyst discharge is one of the fastest biological processes and takes not more than few microseconds. The discharge is also accompanied by stimulation of specific chemoreactors on the animal's body. General Name, Symbol, Number calcium, Ca, 20 Chemical series alkaline earth metals Group, Period, Block 2, 4, s Appearance silvery white Atomic mass 40. ... The cytoplasm is a jelly-like material, made up of mostly water, that fills the cell. ... An ion gradient is a concentration gradient of ions, it can be called an electrochemical potential gradient of ions across membranes. ... Turgor (also called turgor pressure or osmotic pressure) is the pressure that can build in a space that is enclosed by a membrane that is permeable to a solvent such as water but not to solutes. ...

Some nematocysts puncture the skin or exoskeleton of an organism and inject a paralyzing toxin. Others stick to the surface of the prey item and yet others coil themselves around cellular projections on the prey. An exoskeleton, in contrast to an endoskeleton, is an external anatomical feature that supports and protects an animals body. ... A toxin, in a scientific context, is a biologically produced substance that causes injury to the health of a living thing on contact or absorption, typically by interacting with biological macromolecules such as enzymes and receptors. ...

Types of nematocysts

There are three types of nematocysts. Depending on the species, one or several types can appear simultaneously on the organism:

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 itself around a cellular projection on the prey.

Miscellaneous

The most deadly cnidocytes are found on the box jellyfish's body. 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. They state that it causes excruciating pain to humans, often followed by death, sometimes within two or three minutes, and that the chance of survival if stung while swimming alone is "virtually zero." Orders see text Box jellyfish, also known as sea wasps, are creatures found in Australia and the Philippines that can be extremely deadly, with venom in their tentacles. ...

On the other side, aggregating sea anemones have the lowest sting intensity, perhaps due to nematocysts not penetrating the skin, providing only a feeling of touching sticky candies to human fingers. Besides feeding and defense, sea anemone colonies use cnidocytes to sting one another in order to win space. The aggregating anenome, anthopleura elegantissima, can be found on rocky, tide swept shores. ... Cnidocytes are prey-capture and defensive cells found on animals of the phylum Cnidaria. ...

The Ctenophora ("sea-gooseberries" or "comb jellies") are a group of transparent, jelly-like sea creatures belong to a different phylum; these animals have no nematocysts at all and are harmless to humans. Classes Tentaculata Nuda Ctenophores are jellyfish-like animals commonly called comb jellies, sea gooseberries, sea walnuts, or Venus girdles. ... Phylum is one of the levels of scientific classification of organisms; see the Phylum (biology) article. ...

External links

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