Cnidaria

Lore & Background

Cnidarians mostly 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 used for digestion and respiration. Many cnidarian species produce colonies that are single organisms composed of medusa-like or polyp-like zooids, or both. Cnidarians' activities are coordinated by a decentralized nerve net and simple receptors. They also have rhopalia, involved in gravity sensing and sometimes chemoreception. Several free-swimming species of Cubozoa and Scyphozoa possess balance-sensing statocysts, and some have simple eyes. Not all cnidarians reproduce sexually, but many have complex life cycles of asexual polyp stages and sexual medusae stages. Some omit either the polyp or the medusa stage, and the parasitic classes evolved to have neither form. Cnidarians were formerly grouped with ctenophores in the phylum Coelenterata, but increasing awareness of their differences caused them to be placed in separate phyla. Most cnidarians are classified into four main groups: Anthozoa (sea anemones, corals, sea pens); Scyphozoa (jellyfish); Cubozoa (box jellies); and Hydrozoa (a diverse group including all freshwater cnidarians and many marine forms). Staurozoa have recently been recognised as a class in their own right, and the highly derived parasitic Myxozoa and Polypodiozoa were firmly recognized as cnidarians only in 2007.

Reader's Guide

Cnidarians are more complex than sponges, about as complex as ctenophores, and less complex than bilaterians. They are distinguished from all other animals by having cnidocytes that fire harpoon-like structures mainly used to capture prey. In some species, cnidocytes can also be used as anchors. Cnidarians have only one opening in their body for ingestion and excretion—they do not have a separate mouth and anus. Like sponges and ctenophores, cnidarians have two main layers of cells sandwiching a middle layer of jelly-like material called mesoglea. 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 symbiotic 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 filled with symbiotic photosynthetic 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. Cnidarians are a very ancient phylum, with fossils found in rocks formed about 580 million years ago during the Ediacaran period. Molecular clock analysis of mitochondrial genes suggests an even older age for the crown group of cnidarians, estimated around 741 million years ago.

Did You Know?

• The term cnidaria derives from the Ancient Greek word knídē (κνίδη 'nettle'), signifying the coiled thread reminiscent of cnidocytes.
• Cnidarians have only one opening in their body for ingestion and excretion—they do not have a separate mouth and anus.
• Fossils of cnidarians have been found in rocks formed about 580 million years ago during the Ediacaran period.
• Molecular clock analysis of mitochondrial genes suggests the crown group of cnidarians originated around 741 million years ago.
• The highly derived parasitic Myxozoa and Polypodiozoa were firmly recognized as cnidarians only in 2007.