A cnidocyte (also known as a cnidoblast or nematocyte) is an explosive

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...

containing one large secretory

organelle In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...

called a cnidocyst (also known as a cnida () or nematocyst) that can deliver a sting to other organisms. The presence of this cell defines the

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...

Cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that th ...

(

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...

sea anemone Sea anemones are a group of predation, predatory marine invertebrates of the order (biology), order Actiniaria. Because of their colourful appearance, they are named after the ''Anemone'', a terrestrial flowering plant. Sea anemones are classifi ...

hydra Hydra generally refers to: * Lernaean Hydra, a many-headed serpent in Greek mythology * ''Hydra'' (genus), a genus of simple freshwater animals belonging to the phylum Cnidaria Hydra or The Hydra may also refer to: Astronomy * Hydra (constel ...

jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrella- ...

, etc.). Cnidae are used to capture prey and as a defense against predators. A cnidocyte fires a structure that contains a

toxin A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849– ...

within the cnidocyst; this is responsible for the stings delivered by a cnidarian.

Structure and function

Each cnidocyte contains an organelle called a cnida, cnidocyst, nematocyst, ptychocyst or spirocyst. This organelle consists of a bulb-shaped capsule containing a coiled hollow tubule structure attached to it. An immature cnidocyte is referred to as a cnidoblast or nematoblast. The externally oriented side of the cell has a hair-like trigger called a cnidocil, which 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 a few

microsecond A microsecond is a unit of time in the International System of Units (SI) equal to one millionth (0.000001 or 10−6 or ) of a second. Its symbol is μs, sometimes simplified to us when Unicode is not available. A microsecond is equal to 1000 n ...

s, and is able to reach

acceleration In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by the ...

s of about 40,000 ''g''. Recent research suggests the process occurs in as little 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 capture the immobilized prey. Recently, in two sea anemone species (''

Nematostella vectensis The starlet sea anemone (''Nematostella vectensis'') is a species of small sea anemone in the family Edwardsiidae native to the east coast of the United States, with introduced populations along the coast of southeast England and the west coast ...

'' and ''Anthopleura elegantissima''), the type I neurotoxin protein Nv1 was shown to be localized in ectodermal gland cells in the tentacles, next to but not in nematocytes. Upon encounter with a crustacean prey, nematocytes discharge and pierce the prey, and Nv1 is massively secreted into the extracellular medium by the nearby gland cells, thus suggesting another mode of entry for toxins.

Cnidocyte capsule composition

The cnidocyte capsule is made of novel Cnidaria-specific genes which combine known protein domains. Minicollagen genes are one of the major structural components of the capsule. They are very short genes containing the characteristic collagen-triple helix sequence, as well as polyproline domains and cystein-rich domains. Trimers of minicollagen proteins assemble through their terminal cystein-rich domain, forming highly organized and rigid supra-structures. Minicollagen 1 Ncol-1 polymers assemble on the inner shell while the outer capsule is composed of polymerized NOWA (Nematocyst Outer Wall Antigen) proteins. Nematogalectin, minicollagen Ncol-15 and chondroitin are novel proteins used to build the tubule shaft. In piercing cnidocytes, the novel protein spinalin is used to make the spines present at the base of the shaft.

Discharge mechanism

The cnidocyst capsule stores a large concentration of

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

s, which are released from the capsule into the

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The ...

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 Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane. It is also defined as the measure of the tendency of a solution to take in a pure ...

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.

Prey detection

Cnidae are "single use" cells, and this costs a lot of energy. In

Hydrozoa Hydrozoa (hydrozoans; ) are a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline water. The colonies of the colonial species can be large, and in some cases the specializ ...

ns, in order to regulate discharge, cnidocytes are connected as "batteries", containing several types of cnidocytes connected to supporting cells and neurons. The supporting cells contain chemosensors, which, together with the

mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, ...

on the cnidocyte (cnidocil), allow only the right combination of stimuli to cause discharge, such as prey swimming, and chemicals found in prey

cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...

or cuteous tissue. This prevents the cnidarian from stinging itself although sloughed off cnidae can be induced to fire independently.

Types of cnidae

Over 30 types of cnidae are found in different cnidarians. They can be divided into the following groups: # Penetrant: The penetrant or stenotele is the largest and most complex nematocyst. When discharged, it pierces the skin or

chitin Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...

ous exoskeleton of the prey and injects the venomous fluid, hypnotoxin, that either paralyzes the victim or kills it. # Glutinant: a sticky surface used to stick to prey, referred to as ptychocysts and found on burrowing (tube) anemones, which help create the tube in which the animal lives # Volvent: The volvent or desmoneme is a small and pear-shaped nematocyst. It contains a short, thick, spineless, smooth and elastic thread tube forming a single loop and closed at the far end. When discharged, it tightly coils around the prey. They are the smallest nematocysts. A lasso-like string is fired at prey and wraps around a cellular projection on the prey, which are referred to as spirocysts. Cnidocyte subtypes can be differentially localized in the animal. In the sea anemone ''

'', the majority of its non-penetrant sticky cnidocytes, the spirocytes, are found in the tentacles, and are thought to help with prey capture by sticking to the prey. By contrast, the two penetrant types of cnidocytes present in this species display a much broader localization, on the outer

epithelial Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellula ...

layer of the tentacles and body column, as well as on the

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struc ...

epithelium and within

mesenteries In zoology, a mesentery is a membrane inside the body cavity of an animal. The term identifies different structures in different phyla: in vertebrates it is a double fold of the peritoneum enclosing the intestines; in other organisms it forms co ...

. The diversity of cnidocytes types correlates with the expansion and diversification of structural cnidocyst genes like minicollagen genes. Minicollagen genes form compact gene clusters in Cnidarian

genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...

s, suggesting a diversification through gene duplication and subfunctionalization.

Anthozoan Anthozoa is a subphylum of marine invertebrates which includes the sea anemones, stony corals and soft corals. Adult anthozoans are almost all attached to the seabed, while their larvae can disperse as part of the plankton. The basic unit of t ...

s display less capsule diversity and a reduced number of minicollagen genes, and medusozoans have more capsule diversity (about 25 types) and a vastly expanded minicollagen genes repertoire. In the

'', some minicollagens display a differential expression pattern in different cnidocytes subtypes.

Cnidocyte development

Cnidocytes are single-use cells that need to be continuously replaced throughout the life of the animal with different mode of renewal across species.

Modes of renewal

Hydra Hydra generally refers to: * Lernaean Hydra, a many-headed serpent in Greek mythology * ''Hydra'' (genus), a genus of simple freshwater animals belonging to the phylum Cnidaria Hydra or The Hydra may also refer to: Astronomy * Hydra (constel ...

polyps A polyp in zoology is one of two forms found in the phylum Cnidaria, the other being the medusa. Polyps are roughly cylindrical in shape and elongated at the axis of the vase-shaped body. In solitary polyps, the aboral (opposite to oral) end is ...

, cnidocytes differentiate from a specific population of

stem cells In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...

, the interstitial cells (I-cells) located within the body column. Developing nematocytes first undergo multiple rounds of

mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...

without

cytokinesis Cytokinesis () is the part of the cell division process during which the cytoplasm of a single eukaryotic cell divides into two daughter cells. Cytoplasmic division begins during or after the late stages of nuclear division in mitosis and meios ...

, giving rise to nematoblast nests with 8, 16, 32 or 64 cells. After this expansion phase, nematoblasts develop their capsules. Nests separate into single nematocytes when the formation of the capsule is complete. Most of them migrate to the tentacles where they are incorporated into battery cells, which hold several nematocytes, and

neurons A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...

. Battery cells coordinate firing of nematocytes. In the

hydrozoan Hydrozoa (hydrozoans; ) are a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline water. The colonies of the colonial species can be large, and in some cases the specialized ...

jellyfish ''

Clytia hemisphaerica ''Clytia hemisphaerica'' is a small hydrozoan-group cnidarian, about 1 cm in diameter, that is found in the Mediterranean Sea and the North-East Atlantic Ocean. ''Clytia'' has the free-swimming jellyfish form typical of the Hydrozoa, as well ...

'', nematogenesis takes place at the base of the tentacles, as well as in the

manubrium The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. Sha ...

. At the base of the tentacles, nematoblasts proliferate then differentiate along a proximal-distal

gradient In vector calculus, the gradient of a scalar-valued differentiable function of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p is the "direction and rate of fastest increase". If the gradi ...

, giving rise to mature nematocytes in the tentacles through a conveyor belt system. In the Anthozoan sea anemone ''

'', nematocytes are thought to develop throughout the animal from epithelial progenitors.

Cnidocyst maturation

The nematocyst forms through a multi-step assembly process from a giant post-Golgi vacuole.

Vesicle Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like features o ...

s from the

Golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles ins ...

first fuse onto a primary vesicle: the capsule primordium. Subsequent vesicle fusion enables the formation of a

tubule In biology, a tubule is a general term referring to small tube or similar type of structure. Specifically, tubule can refer to: * a small tube or fistular structure * a minute tube lined with glandular epithelium * any hollow cylindrical body stru ...

outside of the capsule, which then invaginates into the capsule. Then, an early maturation phase enables the formation of long arrays of barbed spines onto the invaginated tubule through the condensation of spinalin proteins. Finally, a late maturation stage gives rise to undischarged capsules under high

through the synthesis of poly-γ-glutamate into the matrix of the capsule. This trapped osmotic pressure enables rapid thread discharge upon triggering through a massive osmotic shock.

Nematocyst toxicity

Nematocysts are very efficient weapons. A single nematocyst has been shown to suffice in paralyzing a small

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

(''

Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species ...

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...

). The most deadly cnidocytes (to humans, at least) are found on the body of a

box jellyfish Box jellyfish (class Cubozoa) are cnidarian invertebrates distinguished by their box-like (i.e. cube-shaped) body. Some species of box jellyfish produce potent venom delivered by contact with their tentacles. Stings from some species, including '' ...

. One member of this family, the sea wasp, ''

Chironex fleckeri ''Chironex fleckeri'', commonly known as the Australian box jelly, and nicknamed the sea wasp, is a species of extremely venomous box jellyfish found in coastal waters from northern Australia and New Guinea to Malaysia, the Philippines and Vietn ...

'', is "claimed to be the most venomous marine animal known," according to the

Australian Institute of Marine Science The Australian Institute of Marine Science (AIMS) is a tropical marine research centre located primarily at Cape Ferguson in the locality of Cape Cleveland, City of Townsville Queensland, Australia. It is around from Townsville. Established ...

. It can cause excruciating pain to humans, sometimes followed by death. Other cnidarians, such as the jellyfish ''

Cyanea capillata The lion's mane jellyfish (''Cyanea capillata''), also known as the giant jellyfish, arctic red jellyfish, or the hair jelly, is one of the largest known species of jellyfish. Its range is confined to cold, boreal waters of the Arctic, northern ...

'' (the " Lion's Mane" made famous by

Sherlock Holmes Sherlock Holmes () is a fictional detective created by British author Arthur Conan Doyle. Referring to himself as a " consulting detective" in the stories, Holmes is known for his proficiency with observation, deduction, forensic science and ...

) or the siphonophore ''Physalia physalis'' (

Portuguese man o' war The Portuguese man o' war (''Physalia physalis''), also known as the man-of-war, is a marine hydrozoan found in the Atlantic Ocean and the Indian Ocean. It is considered to be the same species as the Pacific man o' war or blue bottle, which is ...

, "Bluebottle") can cause extremely painful and sometimes fatal stings. On the other hand, aggregating sea anemones may have the lowest sting intensity, perhaps due to the inability of the nematocysts to penetrate the skin, creating a feeling similar to touching sticky candies. Besides feeding and defense, sea anemone and coral colonies use cnidocytes to sting one another in order to defend or win space.

Venom Venom or zootoxin is a type of toxin produced by an animal that is actively delivered through a wound by means of a bite, sting, or similar action. The toxin is delivered through a specially evolved ''venom apparatus'', such as fangs or a sti ...

from animals such as cnidarians,

scorpion Scorpions are predatory arachnids of the order Scorpiones. They have eight legs, and are easily recognized by a pair of grasping pincers and a narrow, segmented tail, often carried in a characteristic forward curve over the back and always end ...

s and

spider Spiders ( order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude silk. They are the largest order of arachnids and rank seventh in total species ...

s 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, and biopesticides. Animals in the phylum

Ctenophora Ctenophora (; ctenophore ; ) comprise a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), an ...

("sea-gooseberries" or "comb jellies") are transparent and jelly-like but have no nematocysts, and are harmless to humans. Certain types of sea slugs, such as the nudibranch aeolids, are known to undergo kleptocnidy (in addition to

kleptoplasty Kleptoplasty or kleptoplastidy is a symbiosis, symbiotic phenomenon whereby plastids, notably chloroplasts from algae, are sequestered by host organisms. The word is derived from ''Kleptes'' (κλέπτης) which is Greek language, Greek for thie ...

), whereby the organisms store nematocysts of digested prey at the tips of their cerata.

References

External links

Dangerous marine animals of Northern Australia: the Sea Wasp
Australian Institute of Marine Science; dangers of box jellyfish
Nematocysts Firing Movie
* {{Cite journal , url=http://jellieszone.com/nematocysts.htm , title=Nematocysts , first=Dave , last=Wrobel , publisher=JelliesZone , access-date=2010-04-14 , archive-url=https://web.archive.org/web/20100330162532/http://jellieszone.com/nematocysts.htm , archive-date=2010-03-30 , url-status=dead
Portuguese Man-of-War: Real Stories, Real People, Real Encounters.
Animal cells Cnidarian anatomy