Nomarski micrograph of a Ruthenium-red stained nematocyst from Aiptasia pallida, the pale anemone. A micrograph, microphotograph or photomicrograph is a Photograph or similar image taken through a Microscope or similar device to show Aiptasia is a Genus of a Symbiotic Cnidarian belonging to the class Anthozoa ( Sea anemones, Corals) 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. This article is about the class of Biotoxins For other uses see Venom (disambiguation and Venomous (disambiguation. The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called A phylum ( Plural: phyla) is a Taxonomic rank between Kingdom and above Class. Cnidaria (naɪˈdɛəriə is a phylum containing some 9000 Species of Animals found exclusively in aquatic mostly marine, environments Corals are Marine organisms from the class Anthozoa and exist as small Sea anemone –like Polyps typically in colonies of many Sea anemones are a group of water dwelling predatory animals of the order Actiniaria; they are named after the Anemone, a terrestrial Flower Hydra is a Genus of simple fresh-water animals possessing radial symmetry. Jellyfish are free-swimming members of the phylum Cnidaria. They have several different basic morphologies that represent several different cnidarian classes including the ). 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. Sessile is a term in Biology with two distinct meanings In botany and medicine In Botany, sessile means "without a stalk Fish are aquatic Vertebrate animals that are typically ectothermic (previously Cold-blooded) covered with scales, and equipped with two Structure of crustaceans As Arthropods crustaceans have a stiff Exoskeleton, which must be shed to allow the animal to grow ( Ecdysis or molting A cnidocyte fires a structure that contains the toxin, from a characteristic sub-cellular organelle called a cnidocyst (or cnida or nematocyst). In Cell biology, an organelle (pronunciation /ɔː(rgəˡnɛl/ is a specialized subunit within a cell that has a specific function and is usually separately enclosed This is, for instance, responsible for the stings delivered by jellyfish.

Structure and action

A discharged nematocyst seen under a scanning electron microscope. The scanning electron microscope ( SEM) is a type of Electron microscope that images the sample surface by scanning it with a high-energy beam of Electrons

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]. To help compare Orders of magnitude of different Times this page lists times between 10&minus6 seconds and 10&minus5 seconds (1 micro g-force (also G-force, g-load) is a measurement of an object's Acceleration expressed in g s 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.

Discharge mechanism

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. Calcium (ˈkælsiəm is the Chemical element with the symbol Ca and Atomic number 20 An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge The cytoplasm is the contents of a cell that is enclosed within the Plasma membrane. 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. Osmotic pressure is the hydrostatic pressure produced by a difference in concentration between solutions on the two sides of a surface such as a semipermeable membrane This increase in water volume in the cytoplasm forces the coiled nematocyst to eject rapidly. The coiled nematocyst is a hollow tube that exists inside the cell in an "inside out" condition. Imagine a rubber glove with only the fingers inside out and tucked into the palm of the glove. If you blow into the cuff of the glove, the fingers will pop out quickly creating a glove into which you can put your hand. The pressure of water flowing into the cnidocyte forces the water into the tubular nematocyst causing it to right itself as it comes rushing out of the cell with enough force to impale a prey organism.

Prey detection

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. A chemosensor, also known as chemoreceptor, is a Sensory receptor that transduces a chemical signal into an Action potential. A mechanoreceptor is a Sensory receptor that responds to mechanical pressure or distortion In Biology, the term cuticle or cuticula is given to a variety of tough but flexible non-mineral outer coverings of an organism or part of an organism that

Types of nematocysts

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.

Nematocyst toxicity

Nematocysts are very efficient weapons. A single nematocyst has been shown to suffice to paralyze a small arthropod (Drosophila larva). Arthropods are Animals belonging to the Phylum Arthropoda (from Greek ἄρθρον arthron, " Joint " Drosophila is a Genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" A larva ( Latin; plural larvae) is a juvenile form of Animal with indirect development, undergoing Metamorphosis (for example The most deadly cnidocytes (to humans, at least) are found on the body of a box jellyfish. Box jellyfish are invertebrates belonging to the class Cubozoa, named for their Cube -shaped medusae. 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. Chironex fleckeri, (commonly known as the "box jellyfish" "marine stinger" or "sea wasp" is a highly venomous jellyfish of the class The Australian Institute of Marine Science (AIMS is a state-of-the-art tropical marine research centre located primarily at Cape Ferguson 50km south of Townsville 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. The lion's mane jellyfish ( Cyanea capillata) is the largest known species of Jellyfish. The Adventure of the Lion's Mane, one of the 56 Sherlock Holmes short stories written by British author Sir Arthur Conan Doyle, is one of 12 stories in the cycle Sherlock Holmes is a famous fictional detective of the late nineteenth and early twentieth centuries who first appeared in Publication in 1887 The Portuguese Man O' War ( Physalia physalis) also known as the blue bubble, blue bottle, man-of-war, or the Portuguese man of war On the other side, aggregating sea anemones may have the lowest sting intensity, perhaps due to the inability of the nematocysts to penetrate the skin, providing only a feeling of that similar to touching sticky candies to human fingers. A school (from Greek σχολεῖον - scholeion) is an Institution designed to allow and encourage Students (or "pupils" 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 toxin ( Greek:, toxikon, lit (poison for use on arrows is a Poisonous substance produced by living cells or organisms that is active at very low Scorpions are eight-legged Carnivorous Arthropods They are members of the order Scorpiones within the class Arachnida. Spiders are Predatory Invertebrate Animals that have two body segments, eight legs no chewing mouth parts and no wings 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. A phylum ( Plural: phyla) is a Taxonomic rank between Kingdom and above Class. The phylum Ctenophora (tɨˈnɒfərə commonly known as comb jellies, is a phylum that includes the Sea gooseberry ( Pleurobrachia pileus) and

References

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

External links

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