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47-532: Medusozoa is a clade in the phylum Cnidaria , and is often considered a subphylum. It includes the classes Hydrozoa , Scyphozoa , Staurozoa and Cubozoa , and possibly the parasitic Polypodiozoa . Medusozoans are distinguished by having a medusa stage in their often complex life cycle, a medusa typically being an umbrella-shaped body with stinging tentacles around the edge. With the exception of some Hydrozoa (and Polypodiozoa), all are called jellyfish in their free-swimming medusa phase. The phylum Cnidaria

94-480: A medusa stage in their life cycle. The basic pattern is medusa (usually the adult or sexual phase), planula larva , polyp , medusa. Symmetry is tetramerous, with parts in fours or multiples of four. The mitochondrial DNA molecules are linear rather than circular as in anthozoans and almost all other animals. The cnidae, the explosive cells characteristic of the Cnidaria and used in prey capture and defence, are of

141-479: A "ladder", with supposedly more "advanced" organisms at the top. Taxonomists have increasingly worked to make the taxonomic system reflect evolution. When it comes to naming , this principle is not always compatible with the traditional rank-based nomenclature (in which only taxa associated with a rank can be named) because not enough ranks exist to name a long series of nested clades. For these and other reasons, phylogenetic nomenclature has been developed; it

188-437: 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

235-623: A clade can be described based on two different reference points, crown age and stem age. The crown age of a clade refers to the age of the most recent common ancestor of all of the species in the clade. The stem age of a clade refers to the time that the ancestral lineage of the clade diverged from its sister clade. A clade's stem age is either the same as or older than its crown age. Ages of clades cannot be directly observed. They are inferred, either from stratigraphy of fossils , or from molecular clock estimates. Viruses , and particularly RNA viruses form clades. These are useful in tracking

282-642: A cnidarian because of its possession of nematocysts, but molecular studies using 18S rDNA sequences have placed it closer to Myxozoa . Further studies involving 28S rDNA sequences suggest that it is either part of the hydrozoan clade Leptothecata , or a sister taxon to Hydrozoa, and does not group with myxozoans. Ctenophora [REDACTED] Anthozoa [REDACTED] [REDACTED] Staurozoa [REDACTED] Hydrozoa [REDACTED] Polypodiozoa  ? [REDACTED] Cubozoa [REDACTED] Scyphozoa [REDACTED] Bilateria [REDACTED] Medusozoans differ from anthozoans in having

329-415: A medusa which separates and swims away. Staurozoa is a small group commonly known as stalked jellyfish. The animals remain attached to the substrate by a stalk at the opposite end from the mouth. Staurozoans can be regarded as large polyps that have partially differentiated into sexually mature medusae. These spawn gametes which develop into non-swimming planulae that crawl away to new locations. Hydrozoa

376-399: A process known as strobilation . Cubozoa is a group commonly known as box jellyfish, that occur in tropical and warm temperate seas. They have cube-shaped, transparent medusae and are heavily-armed with venomous nematocysts. Cubozoans have planula larvae, which settle and develop into sessile polyps, which subsequently metamorphose into sexual medusae, the oral end of each polyp changing into

423-422: A revised taxonomy based on a concept strongly resembling clades, although the term clade itself would not be coined until 1957 by his grandson, Julian Huxley . German biologist Emil Hans Willi Hennig (1913–1976) is considered to be the founder of cladistics . He proposed a classification system that represented repeated branchings of the family tree, as opposed to the previous systems, which put organisms on

470-659: A single type, there being nematocysts but no spirocysts or ptychocysts. In contrast, the anthozoan life cycle involves a planula larva which settles and becomes a sessile polyp, which is the adult or sexual phase. There is considerable divergence from the basic life cycle pattern among medusozoans. Scyphozoa is the group commonly known as "true jellyfish" and occur in tropical, temperate and polar seas worldwide. Scyphozoans generally have planula larvae that develop into sessile polyps. These reproduce asexually, producing similar polyps by budding, and then either transform into medusae, or repeatedly bud medusae from their upper surface in

517-490: 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 can cause excruciating pain to humans, sometimes followed by death. Other cnidarians, such as

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564-431: A specific population of stem cells , the interstitial cells (I-cells) located within the body column. Developing nematocytes first undergo multiple rounds of mitosis without cytokinesis , 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

611-429: A suffix added should be e.g. "dracohortian". A clade is by definition monophyletic , meaning that it contains one ancestor which can be an organism, a population, or a species and all its descendants. The ancestor can be known or unknown; any and all members of a clade can be extant or extinct. The science that tries to reconstruct phylogenetic trees and thus discover clades is called phylogenetics or cladistics ,

658-401: A vastly expanded minicollagen genes repertoire. In the sea anemone Nematostella vectensis , some minicollagens display a differential expression pattern in different cnidocytes subtypes. Cnidocytes are single-use cells that need to be continuously replaced throughout the life of the animal with different mode of renewal across species. In Hydra polyps , cnidocytes differentiate from

705-432: Is a large group of solitary and colonial cnidarians from both marine and freshwater environments worldwide. Hydrozoans exhibit the greatest variety of life cycles among medusozoans, with either the polyp or the medusa stage being missing in some groups. In general, medusae are budded laterally from polyps, become mature and spawn, releasing gametes into the water. The planulae may settle to become polyps or continue living in

752-499: Is also used with a similar meaning in other fields besides biology, such as historical linguistics ; see Cladistics § In disciplines other than biology . The term "clade" was coined in 1957 by the biologist Julian Huxley to refer to the result of cladogenesis , the evolutionary splitting of a parent species into two distinct species, a concept Huxley borrowed from Bernhard Rensch . Many commonly named groups – rodents and insects , for example – are clades because, in each case,

799-476: Is an evolutionary tradeoff as cnidarian venom systems are known to reduce the cnidarian's reproductive fitness and overall growth. 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, and biopesticides . Animals in

846-420: 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 was estimated at the stylet tip which they write is in the range of technical bullets. Few papers have modeled

893-476: Is in turn included in the mammal, vertebrate and animal clades. The idea of a clade did not exist in pre- Darwinian Linnaean taxonomy , which was based by necessity only on internal or external morphological similarities between organisms. Many of the better known animal groups in Linnaeus's original Systema Naturae (mostly vertebrate groups) do represent clades. The phenomenon of convergent evolution

940-452: Is possibly the case for other jelly species and hence one cannot generally infer nematocyst static traits to prey size. Cnidae are "single use" cells, and thus represent a large expenditure of energy to produce. In Hydrozoans , 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

987-515: Is responsible for many cases of misleading similarities in the morphology of groups that evolved from different lineages. With the increasing realization in the first half of the 19th century that species had changed and split through the ages, classification increasingly came to be seen as branches on the evolutionary tree of life . The publication of Darwin's theory of evolution in 1859 gave this view increasing weight. In 1876 Thomas Henry Huxley , an early advocate of evolutionary theory, proposed

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1034-489: Is still controversial. As an example, see the full current classification of Anas platyrhynchos (the mallard duck) with 40 clades from Eukaryota down by following this Wikispecies link and clicking on "Expand". The name of a clade is conventionally a plural, where the singular refers to each member individually. A unique exception is the reptile clade Dracohors , which was made by haplology from Latin "draco" and "cohors", i.e. "the dragon cohort "; its form with

1081-444: Is widely accepted as being monophyletic and consisting of two clades, Anthozoa and Medusozoa. Anthozoa includes the classes Hexacorallia , the hard corals, and Octocorallia , the soft corals, as well as Ceriantharia , the tube-dwelling anemones. There is strong support for this group having been the first to branch off from the ancestral line. Medusozoa includes the classes Staurozoa , Cubozoa , Scyphozoa and Hydrozoa , but

1128-650: The Latin form cladus (plural cladi ) is used rather than the English form. Clades are the fundamental unit of cladistics , a modern approach to taxonomy adopted by most biological fields. The common ancestor may be an individual, a population , or a species ( extinct or extant ). Clades are nested, one in another, as each branch in turn splits into smaller branches. These splits reflect evolutionary history as populations diverged and evolved independently. Clades are termed monophyletic (Greek: "one clan") groups. Over

1175-476: 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 cutaneous tissue. This prevents the cnidarian from stinging itself although sloughed off cnidae can be induced to fire independently. Over 30 types of cnidae are found in different cnidarians. They can be divided into the following groups: Cnidocyte subtypes can be differentially localized in

1222-557: The pharynx epithelium and within mesenteries . 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 genomes , suggesting a diversification through gene duplication and subfunctionalization. Anthozoans display less capsule diversity and a reduced number of minicollagen genes, and medusozoans have more capsule diversity (about 25 types) and

1269-414: The animal. In the sea anemone Nematostella vectensis , 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 layer of the tentacles and body column, as well as on

1316-414: 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

1363-408: 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 cnidae tubule to eject rapidly. Prior to discharge the coiled cnidae tubule exists inside

1410-460: 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

1457-468: 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 where variables such as barb plate size, prey cylindrical diameter and fluid medium Reynolds number were manipulated. Observational studies indicate that velocities of

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1504-438: The earliest known animal predator from the late Ediacaran . Burgessomedusa from the mid- Cambrian Burgess Shale is the oldest known free-living medusa (commonly known as jellyfish). The affinities of the class Polypodiozoa , containing the single species Polypodium hydriforme , have long been unclear. This species is an endoparasite of fish eggs and has a peculiar life cycle. It has traditionally been considered to be

1551-510: 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 osmotic pressure 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. Nematocysts are very efficient weapons. A single nematocyst has been shown to suffice in paralyzing

1598-546: The group consists of a common ancestor with all its descendant branches. Rodents, for example, are a branch of mammals that split off after the end of the period when the clade Dinosauria stopped being the dominant terrestrial vertebrates 66 million years ago. The original population and all its descendants are a clade. The rodent clade corresponds to the order Rodentia, and insects to the class Insecta. These clades include smaller clades, such as chipmunk or ant , each of which consists of even smaller clades. The clade "rodent"

1645-452: The immobilized prey. Recently, in two sea anemone species ( Nematostella vectensis 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

1692-430: 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. The cnidocyst capsule stores a large concentration of calcium ions , which are released from

1739-629: The jellyfish Cyanea capillata (the " Lion's Mane " made famous by Sherlock Holmes ) or the siphonophore Physalia physalis ( Portuguese man o' war , "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

1786-590: The last few decades, the cladistic approach has revolutionized biological classification and revealed surprising evolutionary relationships among organisms. Increasingly, taxonomists try to avoid naming taxa that are not clades; that is, taxa that are not monophyletic . Some of the relationships between organisms that the molecular biology arm of cladistics has revealed include that fungi are closer relatives to animals than they are to plants, archaea are now considered different from bacteria , and multicellular organisms may have evolved from archaea. The term "clade"

1833-518: The latter term coined by Ernst Mayr (1965), derived from "clade". The results of phylogenetic/cladistic analyses are tree-shaped diagrams called cladograms ; they, and all their branches, are phylogenetic hypotheses. Three methods of defining clades are featured in phylogenetic nomenclature : node-, stem-, and apomorphy-based (see Phylogenetic nomenclature§Phylogenetic definitions of clade names for detailed definitions). The relationship between clades can be described in several ways: The age of

1880-609: The nearby gland cells, thus suggesting another mode of entry for toxins. 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 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

1927-546: The relationships between these are unclear. Analysis using ribosomal RNA subunits suggests that within Medusozoa, Staurozoa was the first group to diverge, with Cubozoa and Scyphozoa forming a clade, a sister group to Hydrozoa. Further study involving the order of mitochondrial genes supports this view, and their possession of linear mitochondrial genomes is striking evidence of the monophyly of medusozoans. The stem group of Medusozoa also includes Auroralumina attenboroughii ,

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1974-513: The spread of viral infections . HIV , for example, has clades called subtypes, which vary in geographical prevalence. HIV subtype (clade) B, for example is predominant in Europe, the Americas and Japan, whereas subtype A is more common in east Africa. Cnidocyte Each cnidocyte contains an organelle called a cnida, cnidocyst, nematocyst, ptychocyst or spirocyst. This organelle consists of

2021-454: The stem cnidarian through domain shuffling. The nematocyst forms through a multi-step assembly process from a giant post-Golgi vacuole. Vesicles from the Golgi apparatus 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

2068-410: 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

2115-480: The tentacles through a conveyor belt system. In the Anthozoan sea anemone Nematostella vectensis , 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

2162-447: The tentacles where they are incorporated into battery cells, which hold several nematocytes, and neurons . Battery cells coordinate firing of nematocytes. In the hydrozoan jellyfish Clytia hemisphaerica , nematogenesis takes place at the base of the tentacles, as well as in the manubrium . At the base of the tentacles, nematoblasts proliferate then differentiate along a proximal-distal gradient , giving rise to mature nematocytes in

2209-413: The water column as medusae. Clade In biological phylogenetics , a clade (from Ancient Greek κλάδος (kládos)  'branch'), also known as a monophyletic group or natural group , is a grouping of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree . In the taxonomical literature, sometimes

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