General · EdgepediaSeptember 17, 20267 min read

Cnidaria

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.¹ Cnidarians are diploblastic eumetazoans with tissue-level organization, radial or biradial symmetry, and a carnivorous feeding habit supported by venom-delivering nematocysts.²^•³

Key fact	Detail
Defining organelle	Cnidae, especially nematocysts, are produced by all cnidarians and only cnidarians; the synapomorphy of extant cnidarians³^•⁴
Species and clades	Over 10,000 species in three main clades: Anthozoa, Endocnidozoa (Myxozoa and Polypodiozoa), and Medusozoa (Cubozoa, Hydrozoa, Scyphozoa, Staurozoa)¹^•⁵
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 nets²
Body forms	Sessile polyp and free-swimming medusa; sexual reproduction produces a ciliated, free-swimming planula larva²
Nematocyst size	Capsule length about 20–200 μm, secreted by the Golgi apparatus of a specialized cell (nematoblast)³
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
Ecological role	Reef-building corals provide the habitat framework for at least 25% of all marine species⁷

Body plan and life cycle

Cnidarians 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

The 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 Ancestral-state reconstruction from genome-scale phylogenies depicts the ancestral cnidarian as a solitary, non-symbiotic polyp that lacked a medusa stage.⁸

Cnidae and nematocysts

If collar cells 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

Terminology 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 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^•¹⁰ Upon an appropriate stimulus, the tubule that was coiled inside the capsule everts and is emitted from one end as the capsule opens.³ Live and super-resolution imaging of the model sea anemone Nematostella vectensis has defined the step-by-step biomechanical sequence of this operation.¹¹

The capsule wall is built from minicollagens, proteins whose coordinated evolution with the nematocyst has been traced in Hydra and other cnidarians.¹² 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

Classification and phylogeny

The phylum comprises three major clades: Anthozoa (classes Octocorallia and Hexacorallia), Endocnidozoa (classes Myxozoa and Polypodiozoa), and Medusozoa (classes Cubozoa, Hydrozoa, Scyphozoa, and Staurozoa).⁵ Phylogenomic analyses support the monophyly of Medusozoa, Anthozoa, Octocorallia, Hydrozoa, and a clade consisting of Staurozoa, Cubozoa, and Scyphozoa.¹³ Some recent proposals would elevate Octocorallia and Hexacorallia to classes and split the phylum into Endocnidozoa, Medusozoa, and Anthozoa.¹⁴

Myxozoans, 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^•¹⁴

The phylum's position in the tree of animals remains debated. Recent phylogenomic data suggest Cnidaria is the sister group of Bilateria, with ctenophores as the sister group of the rest of Metazoa, replacing older hypotheses such as Coelenterata (Cnidaria sister to Ctenophora).⁴

Fossil record and origins

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 The oldest known fossil cnidarians occur in 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 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 Some Cambrian fossils from major lineages appear remarkably similar to extant forms.¹³

Ecological role and human connections

Cnidarians 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 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 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

Jellyfish 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 Several divergent animal lineages, including some nudibranchs, sequester nematocysts from cnidarian prey and incorporate the organelles into their own bodies for defense.⁹

References

Topic: Encyclopedia › Life and health › Animals › Invertebrates › Other invertebrate lineages › Cnidarians and ctenophores › Cnidaria › Cnidarian overview

Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026

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