{ "id": "ep0rxkaf3w", "slug": "cnidaria", "title": "Cnidaria", "updated": "2026-10-21", "topic_path": [ { "id": "life", "label": "Life and health", "api_url": "https://www.edgepedia.org/api/v1/topics/life" }, { "id": "life.animals", "label": "Animals", "api_url": "https://www.edgepedia.org/api/v1/topics/life.animals" }, { "id": "life.animals.invertebrates", "label": "Invertebrates", "api_url": "https://www.edgepedia.org/api/v1/topics/life.animals.invertebrates" }, { "id": "life.animals.remaining", "label": "Other invertebrate lineages", "api_url": "https://www.edgepedia.org/api/v1/topics/life.animals.remaining" }, { "id": "life.animals.remaining.cnidarians", "label": "Cnidarians and ctenophores", "api_url": "https://www.edgepedia.org/api/v1/topics/life.animals.remaining.cnidarians" }, { "id": "life.animals.remaining.cnidarians.cnidaria", "label": "Cnidaria", "api_url": "https://www.edgepedia.org/api/v1/topics/life.animals.remaining.cnidarians.cnidaria" }, { "id": "life.animals.remaining.cnidarians.cnidaria-overview", "label": "Cnidarian overview", "api_url": "https://www.edgepedia.org/api/v1/topics/life.animals.remaining.cnidarians.cnidaria-overview" } ], "geo": [ { "id": "geo.nongeo.nodate.life.animals.remaining.cnidarians", "label": "Non-geographic · Not dated: Cnidarians and ctenophores", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.nodate.life.animals.remaining.cnidarians", "path": [ { "id": "geo.nongeo", "label": "Non-geographic", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo" }, { "id": "geo.nongeo.nodate", "label": "Non-geographic · Not dated", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.nodate" }, { "id": "geo.nongeo.nodate.life", "label": "Life and health", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.nodate.life" }, { "id": "geo.nongeo.nodate.life.animals", "label": "Animals", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.nodate.life.animals" }, { "id": "geo.nongeo.nodate.life.animals.invertebrates", "label": "Invertebrates", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.nodate.life.animals.invertebrates" }, { "id": "geo.nongeo.nodate.life.animals.remaining", "label": "Other invertebrate lineages", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.nodate.life.animals.remaining" }, { "id": "geo.nongeo.nodate.life.animals.remaining.cnidarians", "label": "Cnidarians and ctenophores", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.nodate.life.animals.remaining.cnidarians" } ] } ], "excerpt": "Cnidaria, also called Coelenterata by some authors, is the animal phylum of corals, jellyfish, sea anemones, hydrozoans, and myxozoan parasites, defined by cnidae (nematocysts), diploblastic radial bodies, polyp and medusa forms, and over 10,000 aquatic species.", "snippet": "Cnidaria, also called Coelenterata by some authors, is the animal phylum of corals, jellyfish, sea anemones, hydrozoans, and myxozoan parasites, defined by cnidae (nematocysts), diploblastic radial bodies, polyp and medusa forms, and over 10,000 aquatic species.", "node": "life.animals.remaining.cnidarians.cnidaria-overview", "markdown": "# Cnidaria\n\n**Cnidaria** is a phylum of aquatic, mostly marine animals that includes corals, sea anemones, jellyfish, hydrozoans, and microscopic parasites, defined by the possession of cnidae, microscopic stinging organelles produced by no other animals. The phylum contains over 10,000 described species divided into three main clades: Anthozoa, Endocnidozoa, and [Medusozoa](https://www.edgepedia.org/medusozoa).[1](https://www.mdpi.com/journal/diversity/special_issues/1TGYO6NVRE) Cnidarians are diploblastic eumetazoans with tissue-level organization, radial or biradial symmetry, and a carnivorous feeding habit supported by venom-delivering nematocysts.[2](https://animaldiversity.org/accounts/Cnidaria/) • [3](https://www.sciencedirect.com/science/article/pii/S0041010109001329)\n\n| Key fact | Detail |\n|---|---|\n| Defining organelle | Cnidae, especially nematocysts, are produced by all cnidarians and only cnidarians; the synapomorphy of extant cnidarians[3](https://www.sciencedirect.com/science/article/pii/S0041010109001329) • [4](https://onlinelibrary.wiley.com/doi/10.1111/pala.12116) |\n| Species and clades | Over 10,000 species in three main clades: Anthozoa, Endocnidozoa (Myxozoa and Polypodiozoa), and Medusozoa (Cubozoa, Hydrozoa, Scyphozoa, Staurozoa)[1](https://www.mdpi.com/journal/diversity/special_issues/1TGYO6NVRE) • [5](https://pmc.ncbi.nlm.nih.gov/articles/PMC10641506/) |\n| Body organization | Tissue level without true organs; two or three body layers; a gastrovascular cavity with a single opening serving as mouth and anus; two simple nerve nets[2](https://animaldiversity.org/accounts/Cnidaria/) |\n| Body forms | Sessile polyp and free-swimming medusa; sexual reproduction produces a ciliated, free-swimming planula larva[2](https://animaldiversity.org/accounts/Cnidaria/) |\n| Nematocyst size | Capsule length about 20–200 μm, secreted by the Golgi apparatus of a specialized cell (nematoblast)[3](https://www.sciencedirect.com/science/article/pii/S0041010109001329) |\n| Origin | Molecular evidence dates the phylum to 741 Ma (95% credible region 686–819 Ma), with major taxa diversifying before the Cambrian (543 Ma)[6](https://www.sciencedirect.com/science/article/pii/S1055790311004374) |\n| Ecological role | Reef-building corals provide the habitat framework for at least 25% of all marine species[7](https://repository.library.noaa.gov/view/noaa/746/noaa_746_DS1.pdf) |\n\n## Body plan and life cycle\n\nCnidarians have achieved the tissue level of organization, in which similar cells are grouped into tissues, but true organs do not occur. Their bodies have two or sometimes three layers, and a gastrovascular cavity (coelenteron) has a single exterior opening that serves as both mouth and anus, often surrounded by tentacles. Two simple nerve nets, one epidermal and one gastrodermal, coordinate muscular and sensory functions.[2](https://animaldiversity.org/accounts/Cnidaria/)\n\nThe phylum takes its name from the two basic body forms its members take. Medusae, such as adult jellyfish, are free-swimming or floating, usually umbrella-shaped with four-part (tetramerous) symmetry, the mouth on the concave side and tentacles arising from the rim. Polyps are usually sessile, with tubular bodies attached at one end and a mouth, usually surrounded by tentacles, at the other; polyps may live alone or in colonies whose individuals sometimes specialize for reproduction, feeding, or defense. Reproduction is by asexual budding in polyps and by formation of gametes in medusae and some polyps; individuals may be monoecious or dioecious, and sexual reproduction yields a ciliated, free-swimming planula larva.[2](https://animaldiversity.org/accounts/Cnidaria/) Ancestral-state reconstruction from genome-scale phylogenies depicts the ancestral cnidarian as a solitary, non-symbiotic polyp that lacked a medusa stage.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC5932825/)\n\n## Cnidae and nematocysts\n\nIf [collar cells](https://www.edgepedia.org/collar-cell) and spicules define sponges, nematocysts define cnidarians. Each is a tiny organelle containing a coiled, tubular thread that may bear barbs and is often venomous; hundreds or thousands may line the tentacles or body surface. When a prey animal or predator makes contact, the nematocyst discharges, driving its barbed, poisoned thread into the victim's flesh by means of a rapid increase in hydrostatic pressure. Nematocysts can penetrate human skin, sometimes producing a painful wound and, in extreme cases, death.[2](https://animaldiversity.org/accounts/Cnidaria/)\n\nTerminology matters here: despite common usage, a nematocyst is not a \"stinging cell\" but the capsule made by the cell that delivers the sting. The capsule-producing cells are cnidoblasts (nematoblasts, ptychoblasts, and spiroblasts), with the corresponding mature cells called cnidocytes (nematocytes, ptychocytes, and spirocytes).[3](https://www.sciencedirect.com/science/article/pii/S0041010109001329) Discharge is triggered by stimulation of the cnidocil, a modified cilium on the outside of the cnidocyte, by chemical and/or mechanical mechanisms; in vivo discharge typically requires near-simultaneous chemical and mechanical stimuli.[9](https://onlinelibrary.wiley.com/doi/10.1111/ivb.12154) • [10](https://pmc.ncbi.nlm.nih.gov/articles/PMC2762458/) Upon an appropriate stimulus, the tubule that was coiled inside the capsule everts and is emitted from one end as the capsule opens.[3](https://www.sciencedirect.com/science/article/pii/S0041010109001329) Live and super-resolution imaging of the model sea anemone *Nematostella vectensis* has defined the step-by-step biomechanical sequence of this operation.[11](https://pubmed.ncbi.nlm.nih.gov/35715400/)\n\nThe capsule wall is built from minicollagens, proteins whose coordinated evolution with the nematocyst has been traced in *Hydra* and other cnidarians.[12](https://www.sciencedirect.com/science/article/pii/S0168952508001960) The inventory of cnidae types in a species, called the cnidom, is an essential component of most taxonomic descriptions, though not all members of a species share the same cnidae and capsule size can vary geographically and with individual size.[3](https://www.sciencedirect.com/science/article/pii/S0041010109001329)\n\n## Classification and phylogeny\n\nThe phylum comprises three major clades: Anthozoa (classes [Octocorallia](https://www.edgepedia.org/octocorallia) and [Hexacorallia](https://www.edgepedia.org/hexacorallia)), Endocnidozoa (classes Myxozoa and Polypodiozoa), and Medusozoa (classes Cubozoa, Hydrozoa, Scyphozoa, and Staurozoa).[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC10641506/) Phylogenomic analyses support the monophyly of Medusozoa, Anthozoa, Octocorallia, Hydrozoa, and a clade consisting of Staurozoa, Cubozoa, and [Scyphozoa](https://www.edgepedia.org/scyphozoa).[13](https://pmc.ncbi.nlm.nih.gov/articles/PMC4605497/) Some recent proposals would elevate Octocorallia and Hexacorallia to classes and split the phylum into Endocnidozoa, Medusozoa, and Anthozoa.[14](https://www.marinespecies.org/documents/LifeWatch%20reports/editor%20workshop%20reports/20240227_Cnidaria_Workshop_Report.pdf)\n\nMyxozoans, microscopic endoparasites once classified among unicellular eukaryotes, are now recognized as cnidarians on the basis of genetic relatedness and their nematocysts, historically called \"polar capsules\" in myxozoan research. Despite genome reduction and the evolution of parasitism, myxozoans retain nematocysts structurally and functionally homologous to those of free-living relatives; recent reviews propose synonymizing the myxozoan terms, treating \"polar capsule\" as a taxon-specific nematocyst morphotype and \"polar filament\" as the tubule.[15](https://www.sciencedirect.com/science/article/pii/S0020751920301995) • [14](https://www.marinespecies.org/documents/LifeWatch%20reports/editor%20workshop%20reports/20240227_Cnidaria_Workshop_Report.pdf)\n\nThe phylum's position in the tree of animals remains debated. Recent phylogenomic data suggest Cnidaria is the sister group of [Bilateria](https://www.edgepedia.org/bilateria), with ctenophores as the sister group of the rest of Metazoa, replacing older hypotheses such as [Coelenterata](https://www.edgepedia.org/coelenterata) (Cnidaria sister to [Ctenophora](https://www.edgepedia.org/ctenophora)).[4](https://onlinelibrary.wiley.com/doi/10.1111/pala.12116)\n\n## Fossil record and origins\n\n[Molecular clock](https://www.edgepedia.org/molecular-clock) analyses place the origin of Cnidaria at 741 Ma (95% credible region 686–819 Ma), with the major taxa diversifying before the Cambrian (543 Ma).[6](https://www.sciencedirect.com/science/article/pii/S1055790311004374) The oldest known fossil cnidarians occur in [Ediacaran](https://www.edgepedia.org/ediacaran) strata and are polypoid forms that were nonbiomineralizing or weakly so; possible anthozoans such as *Ramitubus* occur in the Doushantuo Lagerstätte of South China (age poorly constrained, approximately 585 Ma), and conulariid scyphozoans may first appear as early as 635–577 Ma in the Lantian Formation.[4](https://onlinelibrary.wiley.com/doi/10.1111/pala.12116) Four of the five classical cnidarian classes have been identified in the Cambrian fossil record, indicating that considerable diversification had occurred by about 500 million years ago.[16](https://pmc.ncbi.nlm.nih.gov/articles/PMC3062418/) Some Cambrian fossils from major lineages appear remarkably similar to extant forms.[13](https://pmc.ncbi.nlm.nih.gov/articles/PMC4605497/)\n\n## Ecological role and human connections\n\nCnidarians are exclusively carnivorous, using nematocysts to protect themselves and capture prey, although some shallow-water species harbor photoendosymbionts from which they derive fixed carbon.[3](https://www.sciencedirect.com/science/article/pii/S0041010109001329) Scleractinian (stony) corals build reef frameworks that shelter at least 25% of all marine species, and reefs provide shoreline protection, fisheries, and tourism revenue.[7](https://repository.library.noaa.gov/view/noaa/746/noaa_746_DS1.pdf) That role is under pressure: the fourth global coral bleaching event was declared on 15 April 2024, and from early 2023 to mid-2025 bleaching-level heat stress affected 84% of the world's coral reef area across the Pacific, Atlantic, and Indian Oceans.[17](https://www.nesdis.noaa.gov/news/worlds-fourth-mass-coral-bleaching-event-likely-ended-2025)\n\nJellyfish themselves are eaten by more than 150 animal species, including fish, sea turtles, crustaceans, and other jellyfish, and humans have fished for jellies off the coast of China for at least 1,700 years.[18](https://ocean.si.edu/ocean-life/invertebrates/jellyfish-and-comb-jellies) Several divergent animal lineages, including some nudibranchs, sequester nematocysts from cnidarian prey and incorporate the organelles into their own bodies for defense.[9](https://onlinelibrary.wiley.com/doi/10.1111/ivb.12154)\n\n## References\n\n1. [Diversity, Phylogeny and Evolutionary History of Cnidaria, MDPI Diversity special issue](https://www.mdpi.com/journal/diversity/special_issues/1TGYO6NVRE)\n2. [Cnidaria (corals, sea anemones, jellyfish, and relatives), Animal Diversity Web, University of Michigan](https://animaldiversity.org/accounts/Cnidaria/)\n3. [Fautin, D. G. Structural diversity, systematics, and evolution of cnidae, Toxicon](https://www.sciencedirect.com/science/article/pii/S0041010109001329)\n4. [Van Iten, H., Marques, A. C., et al. (2014). Origin and early diversification of the phylum Cnidaria Verrill, Palaeontology](https://onlinelibrary.wiley.com/doi/10.1111/pala.12116)\n5. [Revisiting mitogenome evolution in Medusozoa with eight new mitochondrial genomes, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC10641506/)\n6. [Estimation of divergence times in cnidarian evolution based on mitochondrial protein-coding genes and the fossil record, Molecular Phylogenetics and Evolution](https://www.sciencedirect.com/science/article/pii/S1055790311004374)\n7. [Coral reef ecosystems and threats, NOAA](https://repository.library.noaa.gov/view/noaa/746/noaa_746_DS1.pdf)\n8. [Phylogenomics provides a robust topology of the major cnidarian lineages, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC5932825/)\n9. [Goodheart, G. (2017). Sequestration of nematocysts by divergent cnidarian predators, Invertebrate Biology](https://onlinelibrary.wiley.com/doi/10.1111/ivb.12154)\n10. [Puncture mechanics of cnidarian cnidocysts: a natural actuator, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC2762458/)\n11. [The architecture and operating mechanism of a cnidarian stinging organelle, PubMed](https://pubmed.ncbi.nlm.nih.gov/35715400/)\n12. [Evolution of complex structures: minicollagens shape the cnidarian nematocyst, Trends in Genetics](https://www.sciencedirect.com/science/article/pii/S0168952508001960)\n13. [Phylogenomic Analyses Support Traditional Relationships within Cnidaria, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4605497/)\n14. [Cnidaria editor workshop report, World Register of Marine Species](https://www.marinespecies.org/documents/LifeWatch%20reports/editor%20workshop%20reports/20240227_Cnidaria_Workshop_Report.pdf)\n15. [A comparison of the structure and function of nematocysts in free-living and parasitic cnidarians (Myxozoa), International Journal for Parasitology](https://www.sciencedirect.com/science/article/pii/S0020751920301995)\n16. [Evolutionary crossroads in developmental biology: Cnidaria, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC3062418/)\n17. [World's Fourth Mass Coral Bleaching Event Likely Ended in 2025, NOAA NESDIS](https://www.nesdis.noaa.gov/news/worlds-fourth-mass-coral-bleaching-event-likely-ended-2025)\n18. [Jellyfish and Comb Jellies, Smithsonian Ocean](https://ocean.si.edu/ocean-life/invertebrates/jellyfish-and-comb-jellies)\n\n---\n*Topic: Encyclopedia › Life and health › Animals › Invertebrates › Other invertebrate lineages › Cnidarians and ctenophores › Cnidaria › Cnidarian overview*\n\n*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*\n\n*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*\n\nLicense: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license\n", "same_as": [], "url": "https://www.edgepedia.org/cnidaria", "markdown_url": "https://www.edgepedia.org/cnidaria.md", "license": { "name": "Edgepedia Community License 1.0", "url": "https://www.edgepedia.org/edgepedia/license", "summary": "Free with credit, commercial use included. AI training is open to everyone. For other uses, organizations over USD 100M in revenue or 100M monthly users license separately.", "spdx": "LicenseRef-Edgepedia-Community-1.0" }, "credit": "\"Cnidaria\", Edgepedia (EdgeChat), https://www.edgepedia.org/cnidaria. Edgepedia Community License 1.0.", "credit_md": "\"[Cnidaria](https://www.edgepedia.org/cnidaria)\", Edgepedia (EdgeChat), [https://www.edgepedia.org/cnidaria](https://www.edgepedia.org/cnidaria). [Edgepedia Community License 1.0](https://www.edgepedia.org/edgepedia/license).", "credit_html": "\"Cnidaria\", Edgepedia (EdgeChat), https://www.edgepedia.org/cnidaria. Edgepedia Community License 1.0.", "speakable": "Cnidaria, also called Coelenterata by some authors, is the animal phylum of corals, jellyfish, sea anemones, hydrozoans, and myxozoan parasites, defined by cnidae, diploblastic radial bodies, polyp and medusa forms, and over 10,000 aquatic species." }