Cnidaria  

Collection Sites: world map showing specimen collection locations for Cnidaria

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

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)