Overview

Brief Summary

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.

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Distribution

Worldwide.

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Ecology

Habitat

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

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Trophic Strategy

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.

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Associations

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)

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Known predators

  • R. Hiatt and D. W. Strasburg, Ecological relationships of the fish fauna on coral reefs of the Marshall Islands, Ecol. Monogr. 30(1):65-127, from p. 125 (1960).
  • M. E. Vinogradov and E. A. Shushkina, Some development patterns of plankton communities in the upwelling areas of the Pacific Ocean. Mar. Biol. 48:357-366, from p. 359 (1978).
  • Link J (2002) Does food web theory work for marine ecosystems? Mar Ecol Prog Ser 230:1–9
  • Opitz S (1996) Trophic interactions in Caribbean coral reefs. ICLARM Tech Rep 43, Manila, Philippines
  • Myers, P., R. Espinosa, C. S. Parr, T. Jones, G. S. Hammond, and T. A. Dewey. 2006. The Animal Diversity Web (online). Accessed February 16, 2011 at http://animaldiversity.org. http://www.animaldiversity.org
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Known prey organisms

Cnidaria preys on:
zooplankton
ciliates
meroplankton
Appendicularia
Doliolidae
Calanoida
Cyclopoidea
phytoplankton
Calanus
Pteropods
Copepoda
Crangon
Mysidae
Decapoda
Gammaridae
Hyperiidae
Caprellidae
Isopoda
Ammodytes marinus
Clupea harengus
Alosa pseudoharengus
Scomber
Peprilus triacanthus

Based on studies in:
Marshall Islands (Reef)
Pacific (Marine)
USA, Northeastern US contintental shelf (Coastal)

This list may not be complete but is based on published studies.
  • R. Hiatt and D. W. Strasburg, Ecological relationships of the fish fauna on coral reefs of the Marshall Islands, Ecol. Monogr. 30(1):65-127, from p. 125 (1960).
  • M. E. Vinogradov and E. A. Shushkina, Some development patterns of plankton communities in the upwelling areas of the Pacific Ocean. Mar. Biol. 48:357-366, from p. 359 (1978).
  • Link J (2002) Does food web theory work for marine ecosystems? Mar Ecol Prog Ser 230:1–9
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General Ecology

Predators

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.

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Life History and Behavior

Reproduction

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)

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Evolution and Systematics

Evolution

Systematics and Taxonomy

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)

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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage

Barcode of Life Data Systems (BOLD) Stats
                                        
Specimen Records:11,284Public Records:4,771
Specimens with Sequences:5,890Public Species:688
Specimens with Barcodes:4,338Public BINs:682
Species:1,323         
Species With Barcodes:762         
          
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Barcode data

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Locations of barcode samples

Collection Sites: world map showing specimen collection locations for Cnidaria

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Relevance to Humans and Ecosystems

Risks

Risk Statement

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)

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