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 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)
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)
Evolution and Systematics
Systematics and Taxonomy
- Exclusively marine
- No medusa stage
- Includes sea anemones, corals, sea fans, sea pens, sea pansies
- Exclusively marine
- Lifecycle includes conspicuous medusa phase (most of the “jellyfish”)
- Includes box jellies, stalked jellies, flag-mouth jellies, root-mouth jellies
- 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)
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.