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

Worldwide.

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

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.

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)

Cnidaria is prey of:
omnivores
coral feeders
Chaetognatha
Polychaeta
Engraulidae
Actinopterygii
Scaridae
Cheloniidae
Decapoda
Isopoda
Amphipoda
Pycnogonidae
Tanaidae
Asteroidea
Echinoidea
Gastropoda
Scaphopoda
Neoloricata
Priapula
Ophiuroidea
Pollachius pollachius
Urophycis regia
Urophycis tenuis
Urophycis chuss
Gadidae
Melanogrammus aeglefinus
Leucoraja ocellata
Mustelus canis
Squalus acanthias
Cynoscion
Pomatomus saltatrix
Odontoceti
Centropyge potteri
Caretta caretta
Fratercula cirrhata
Pomacanthus annularis

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

This list may not be complete but is based on published studies.

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.

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.

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)

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)

Barcode of Life Data Systems (BOLD) Stats
                                        

Specimen Records:	7,930	Public Records:	2,530
Specimens with Sequences:	3,647	Public Species:	652
Specimens with Barcodes:	2,643	Public BINs:	455
Species:	1,245
Species With Barcodes:	723

Access Published & Released Data: Download FASTA File

Collection Sites: world map showing specimen collection locations for Cnidaria

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)