dcsimg
Image of <i>Corynactis californica</i> Carlgren 1936
Life » » Animals

Cnidarians

Cnidaria

Brief Summary

    Cnidaria: Brief Summary
    provided by wikipedia

     src= Pacific sea nettles, Chrysaora fuscescens

    Cnidaria (/naɪˈdɛəriə/) is a phylum containing over 10,000 species of animals found exclusively in aquatic (freshwater and marine) environments: they are a predominantly marine species. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. They have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms have a single orifice and body cavity that are used for digestion and respiration. Many cnidarian species produce colonies that are single organisms composed of medusa-like or polyp-like zooids, or both (hence they are trimorphic). Cnidarians' activities are coordinated by a decentralized nerve net and simple receptors. Several free-swimming species of Cubozoa and Scyphozoa possess balance-sensing statocysts, and some have simple eyes. Not all cnidarians reproduce sexually, with many species having complex life cycles of asexual polyp stages and sexual medusae. Some, however, omit either the polyp or the medusa stage.

    Cnidarians were formerly grouped with ctenophores in the phylum Coelenterata, but increasing awareness of their differences caused them to be placed in separate phyla.[when?] Cnidarians are classified into four main groups: the almost wholly sessile Anthozoa (sea anemones, corals, sea pens); swimming Scyphozoa (jellyfish); Cubozoa (box jellies); and Hydrozoa, a diverse group that includes all the freshwater cnidarians as well as many marine forms, and has both sessile members, such as Hydra, and colonial swimmers, such as the Portuguese Man o' War. Staurozoa have recently been recognised as a class in their own right rather than a sub-group of Scyphozoa, and the parasitic Myxozoa and Polypodiozoa were only firmly recognized as cnidarians in 2007.

    Most cnidarians prey on organisms ranging in size from plankton to animals several times larger than themselves, but many obtain much of their nutrition from dinoflagellates, and a few are parasites. Many are preyed on by other animals including starfish, sea slugs, fish, turtles, and even other cnidarians. Many scleractinian corals—which form the structural foundation for coral reefs—possess polyps that are filled with symbiotic photo-synthetic zooxanthellae. While reef-forming corals are almost entirely restricted to warm and shallow marine waters, other cnidarians can be found at great depths, in polar regions, and in freshwater.

    Recent phylogenetic analyses support monophyly of cnidarians, as well as the position of cnidarians as the sister group of bilaterians. Fossil cnidarians have been found in rocks formed about 580 million years ago, and other fossils show that corals may have been present shortly before 490 million years ago and diversified a few million years later. However, molecular clock analysis of mitochondrial genes suggests a much older age for the crown group of cnidarians, estimated around 741 million years ago, almost 200 million years before the Cambrian period as well as any fossils.

    license
    cc-by-sa-3.0
    copyright
    Wikipedia authors and editors
    original
    visit source
    partner site
    wikipedia
    ID
    8bfa0b38426ba3d63a249ff52b148c27
    Brief Summary
    provided by EOL authors
    Cnidarians are a diverse group of aquatic animals. More than 9,000 species are part of the Phylum Cnidaria, and all species are aquatic. Cnidarians are widespread in marine habitats and less common in fresh water. This interesting group of invertebrates includes many charismatic organisms such as hydras, sea fans, jellyfishes, sea anemones, corals, and the Portuguese man-of-war. Cnidarians all have some type of specialized stinging cell organelle. Cnidarians' bodies typically take one of two forms: the polyp or the medusa. While the polyp form is adapted for a sedentary or sessile lifestyle, the medusa form is adapted for floating or free-swimming. Sea anemones and corals (class Anthoza) are all polyps. True jellyfishes (class Scyphozoa) are all medusae, though some have a polyp larval stage. Notably, some hydroids (class Hydrozoa) alternate between polyp and medusa forms throughout their lives. National Biological Information Infrastructure (NBII) at http://www.nbii.gov
    license
    cc-publicdomain
    copyright
    Public Domain
    bibliographic citation
    Aquatic Invertebrates. Available from National Biological Information Infrastructure (NBII) at http://www.nbii.gov
    author
    Tracy Barbaro (tbarbaro)
    original
    visit source
    partner site
    EOL authors
    ID
    15312676

Comprehensive Description

Distribution

    Distribution
    provided by EOL staff

    Worldwide.

    license
    cc-by-nc-sa-3.0
    copyright
    Soulanille, Elaine
    author
    Soulanille, Elaine
    partner site
    EOL staff
    ID
    eolspecies:nid:2449:tid_chapter:27

Habitat

    Habitat
    provided by EOL staff

    Aquatic: mostly marine, though there are some freshwater species. Cnidarians include benthic, pelagic, and epibiont taxa.

    license
    cc-by-nc-sa-3.0
    copyright
    Soulanille, Elaine
    author
    Soulanille, Elaine
    partner site
    EOL staff
    ID
    eolspecies:nid:2449:tid_chapter:29

Trophic Strategy

    Trophic Strategy
    provided by EOL staff

    Most Cnidarians are either active or passive predators, capturing other animals with their nematocyst-lined tentacles. Many cnidaria living in well-lit habitats get much or most of their food from the mutalistic zooxanthellae or zoochlorellae within their gastrodermal cells.

    license
    cc-by-nc-sa-3.0
    copyright
    Soulanille, Elaine
    author
    Soulanille, Elaine
    partner site
    EOL staff
    ID
    eolspecies:nid:2449:tid_chapter:33

Associations

    Associations
    provided by EOL staff

    The gastrodermal cells of many cnidarians contain microscopic mutualistic algae, usually “zooxanthellae” (gold-brown) but in some Hydra and anemones the algae are green “zoochlorellae.” The cnidarian host provides habitat, protection, CO2, and nutrients to the algae. Photosynthate (sugars produced by photosynthesis) from the algae can supply as much as 90% of the cnidarian’s nutrition. (Ruppert, Fox, & Barnes 2004)

    license
    cc-by-nc-sa-3.0
    copyright
    Soulanille, Elaine
    author
    Soulanille, Elaine
    partner site
    EOL staff
    ID
    eolspecies:nid:2449:tid_chapter:14419

General Ecology

    Predators
    provided by EOL staff

    The predators of corals include certain species of fish, gastropods, and sea stars. Jellyfish don’t have many predators, but among them are ocean sunfish, marine turtles, and some humans.

    license
    cc-by-nc-sa-3.0
    copyright
    Soulanille, Elaine
    author
    Soulanille, Elaine
    partner site
    EOL staff
    ID
    eolspecies:nid:2449:tid_chapter:10115

Reproduction

    Reproduction
    provided by EOL staff

    Sexual and asexual reproduction are common among cnidarians, and there are many species that can reproduce via both methods. Asexual reproduction occurs by cloning and includes budding, fragmentation, and fission. Sexual reproduction occurs by external fertilization when adults – which are usually gonochoric (separate sexes), though some taxa are hermaphroditic – spawn gametes into the water. (Ruppert, Fox, & Barnes 2004)

    license
    cc-by-nc-sa-3.0
    copyright
    Soulanille, Elaine
    author
    Soulanille, Elaine
    partner site
    EOL staff
    ID
    eolspecies:nid:2449:tid_chapter:35

Evolution

    Systematics and Taxonomy
    provided by EOL staff

    Anthozoa

    • Exclusively marine
    • No medusa stage
    • Includes sea anemones, corals, sea fans, sea pens, sea pansies

    Scyphozoa

    • Exclusively marine
    • Lifecycle includes conspicuous medusa phase (most of the “jellyfish”)
    • Includes box jellies, stalked jellies, flag-mouth jellies, root-mouth jellies

    Hydrozoa

    • Marine species as well as freshwater species
    • Most species are colonial and lifecyles may include polyp, medusae, or both.
    • Colonial species include hydroids, Portuguese man-of-war, fire and rose corals. Solitary species include a few jellies and freshwater Hydra.

    (Ruppert, Fox, & Barnes 2004)

    license
    cc-by-nc-sa-3.0
    copyright
    Soulanille, Elaine
    author
    Soulanille, Elaine
    partner site
    EOL staff
    ID
    eolspecies:nid:2449:tid_chapter:38

Risks

    Risk Statement
    provided by EOL staff

    The larval stage of cnidarians can cause a condition known as seabather's eruption. This should not be confused with cercarial dermatitis, which is caused by certain schistosomatid trematode flatworms (e.g., Austrobilharzia variglandis) that normally use birds and mammals other than humans as their definitive hosts. The areas of skin affected by seabather's eruption is generally under the garments worn by bathers and swimmers where the organisms are trapped after the person leaves the water. In contrast, cercarial dermatitis occurs on the exposed skin outside of close-fitting garments.

    (Centers for Disease Control Parasites and Health website)

    license
    cc-by-nc-sa-3.0
    copyright
    Shapiro, Leo
    author
    Shapiro, Leo
    author
    Soulanille, Elaine
    partner site
    EOL staff
    ID
    eolspecies:nid:2449:tid_chapter:41

Education Resources