A cnidocyte (also known as a cnidoblast) is a type of cell containing a large secretory

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

called a ''cnidocyst'', that can deliver a sting to other organisms as a way to capture prey and defend against predators. A cnidocyte explosively ejects the

toxin A toxin is a naturally occurring poison produced by metabolic activities of living cells or organisms. They occur especially as proteins, often conjugated. The term was first used by organic chemist Ludwig Brieger (1849–1919), derived ...

-containing cnidocyst which is responsible for the stings delivered by a cnidarian. The presence of this cell defines the phylum

Cnidaria Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...

, which also includes the

coral Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...

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

s, hydrae, and

jellyfish Jellyfish, also known as sea jellies or simply jellies, are the #Life cycle, medusa-phase of certain gelatinous members of the subphylum Medusozoa, which is a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animal ...

. Cnidocytes are single-use cells that need to be continuously replaced.

Structure and function

Each cnidocyte contains an organelle called a cnidocyst, which consists of a bulb-shaped capsule and a hollow, coiled tubule that is contained within. Immature cnidocytes are referred to as cnidoblasts or nematoblasts. The externally oriented side of the cell has a hair-like trigger called a cnidocil, a mechano-chemical 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 microseconds, and is able to reach accelerations of about 40,000 ''g''. Research from 2006 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 Neurotoxins are toxins that are destructive to nervous tissue, nerve tissue (causing neurotoxicity). Neurotoxins are an extensive class of exogenous chemical neurological insult (medical), insultsSpencer 2000 that can adversely affect function ...

protein Nv1 was shown to be localized in ectodermal gland cells in the tentacles, next to but not in nematocysts. Upon encounter with a crustacean prey, nematocysts 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 gene products which combine known protein domains. Minicollagen gene products (proteins) 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 cysteine-rich domains. Trimeres of mini collagen proteins assemble through their terminal cysteine-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. Nemato Galectin, minicollagen Ncol-15 and chondroitin are novel proteins used to build the tubule shaft. In piercing cnidocytes, the novel protein 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; it has 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 it ...

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 convent ...

s, which are released from the capsule into the

cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...

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 (chemistry), 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 soluti ...

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. That force is to be calculated as the mass of the mechanism's stylet multiplied by its acceleration. The pressure that is generated by this impact into its prey is to be calculated by the stylet's force divided by its area. Researchers have calculated an ejected mass of 1 nanogram, an acceleration of 5,410,000 g and a stylet tip radius of 15 ± 8 nm. Therefore, a pressure of more than 7

GPa Grading in education is the application of standardized measurements to evaluate different levels of student achievement in a course. Grades can be expressed as letters (usually A to F), as a range (for example, 1 to 6), percentages, or as num ...

was estimated at the stylet tip which they write is in the range of technical bullets.

Fluid dynamics in nematocyst discharge

Few papers have modeled the discharge aside from direct observation. Observational studies typically used a tentacle solution assay with a chemical stimulant to create discharge and cameras to record them. One in 1984 and another in 2006 as imaging technology improved. One study involved

computational fluid dynamics Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and data structures to analyze and solve problems that involve fluid dynamics, fluid flows. Computers are used to perform the calculations required ...

where variables such as barb plate size, prey cylindrical diameter and fluid medium Reynolds number were manipulated. Observational studies indicate that velocities of the barb/stylet decrease throughout the discharge. As such, the incredible maximum acceleration is achieved at the beginning. Dynamic traits such as maximum discharge velocities and trajectory patterns may not correspond to static traits such as tubule lengths and capsule volumes. Therefore, caution is appropriate when using medusan nematocyst assemblages as indicators of prey selection and trophic role. This is possibly the case for other jelly species and hence one cannot generally infer nematocyst static traits to prey size.

Prey detection

Cnidae are "single use" cells, and thus represent a large expenditure of energy to produce. In

Hydrozoa Hydrozoa (hydrozoans; from Ancient Greek ('; "water") and ('; "animals")) is a taxonomy (biology), taxonomic class (biology), class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline wat ...

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 located on sensory neurons that convert mechanical pressure into action potential, electrical signals tha ...

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 cutaneous 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: # Nematocyst (Penetrant or Piercing): The penetrant or stenotele is the largest and most complex nematocyst. When discharged, it pierces the skin or

chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...

ous exoskeleton of the prey and injects the venomous fluid, hypnotoxin, that either paralyzes the victim or kills it. # Ptychocysts (Glutinant or Adhesive): 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 # Spirocysts (Volvent or Ensnaring): The volvent or desmoneme is a small and pear-shaped cnidocyte. 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 cnidocytes. 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 spherocytes, 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 a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...

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

pharynx The pharynx (: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates ...

epithelium and within

mesenteries In human anatomy, the mesentery is an organ that attaches the intestines to the posterior abdominal wall, consisting of a double fold of the peritoneum. It helps (among other functions) in storing fat and allowing blood vessels, lymphatics, a ...

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

genome 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 genes, other functional regions of the genome such as ...

s, suggesting a diversification through gene duplication and subfunctionalization. Anthozoans display less capsule diversity and a reduced number of mini collagen 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

In Hydra polyps, cnidocytes differentiate from a specific population of

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

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

mitosis Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...

without

cytokinesis Cytokinesis () is the part of the cell division process and part of mitosis 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 ...

, giving rise to nematoblast nests with 8, 16, 32 or 64 cells. After this expansion phase, nematoblasts develop their capsules. Nests separate into single nematocysts 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 nematocysts, and

neurons A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

. Battery cells coordinate firing of nematocysts. In the

hydrozoan Hydrozoa (hydrozoans; from Ancient Greek ('; "water") and ('; "animals")) is 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 sp ...

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

manubrium The sternum (: sternums or sterna) 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, human lung, lungs, and ma ...

. 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 f of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p gives the direction and the rate of fastest increase. The g ...

, 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. Furthermore, a single regulatory gene that codes for the transcription factor ZNF845 also called CnZNF1 promotes the development of a cnidocyte and inhibits the development of a RFamide producing neuron cell. This gene evolved in the stem cnidarian through domain shuffling.

Cnidocyst maturation

The nematocyst forms through a multi-step assembly process from a giant post-Golgi vacuole. Vesicles 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 Cell (biology), cells. Part of the endomembrane system in the cytoplasm, it protein targeting, packages proteins ...

first fuse onto a primary vesicle: the capsule primordium. Subsequent vesicle fusion enables the formation of a tubule 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 ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

(''

Drosophila ''Drosophila'' (), from Ancient Greek δρόσος (''drósos''), meaning "dew", and φίλος (''phílos''), meaning "loving", is a genus of fly, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or p ...

larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

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

Australian Institute of Marine Science Australian(s) may refer to: Australia * Australia, a country * Australians, citizens of the Commonwealth of Australia ** European Australians ** Anglo-Celtic Australians, Australians descended principally from British colonists ** Aboriginal Aus ...

. 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'') is one of the Largest organisms#Cnidarians (Cnidaria), largest known species of jellyfish. Its range is confined to cold, boreal ecosystem, boreal waters of the Arctic Ocean, Arctic, northern Atla ...

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

Sherlock Holmes Sherlock Holmes () is a Detective fiction, fictional detective created by British author Arthur Conan Doyle. Referring to himself as a "Private investigator, consulting detective" in his stories, Holmes is known for his proficiency with obser ...

) or the siphonophore ''Physalia physalis'' (

Portuguese man o' war The Portuguese war (''Physalia physalis''), also known as the man-of-war or bluebottle, is a marine hydrozoan found in the Atlantic Ocean and the Indian Ocean. It is the only species in the genus ''Physalia'', which in turn is the only genus in ...

, "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. Despite their effectiveness in prey-predator interactions, there is an evolutionary tradeoff as cnidarian venom systems are known to reduce the cnidarian's reproductive fitness and overall growth.

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 (biology), order Scorpiones. They have eight legs and are easily recognized by a pair of Chela (organ), grasping pincers and a narrow, segmented tail, often carried in a characteristic forward cur ...

s and

spider Spiders (order (biology), order Araneae) are air-breathing arthropods that have eight limbs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude spider silk, silk. They are the largest order of arachnids and ran ...

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 ) is 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"), and they are ...

("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 process in symbiosis, symbiotic relationships whereby plastids, notably chloroplasts from algae, are sequestered by the host. The word is derived from ''Kleptes'' (κλέπτης) which is Greek language, Greek ...

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

Notes

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 web, url=http://jellieszone.com/nematocysts.htm , title=Nematocysts , first=David , last=Wrobel , website=The JelliesZone , 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