Stony corals, Scleractinia
Anthozoa is a class of marine invertebrates within the phylum Cnidaria that are unique among cnidarians in that they do not do not have a medusa stage in their development. These exclusively polypoid cnidarians are characterized by a tubular body with tentacles around the mouth and most are sedentary after the larval stage. Anthozoa includes the sea anemones, corals, sea pens, sea pansies, and sea fans, among others.
Anthozoa is the largest of the four classes of Cnidaria with over 6,000 species (France 2004). They are found worldwide in all oceans, from the Arctic to the Antarctic. Anthozoa means "flower animals," which is descriptive of this class of invertebrates.
Anthozoans provide a number of values for human beings. Coral reefs are major tourist attractions and also provide a habitat for fish, mollusks, urchins, and crustaceans that serve as food for people (France 2004). Anthozoans are used in the aquarium trade, to make coral jewelry, and scleractinian skeletons are even used as building materials and in bone grafts (France 2004). Despite these values, various human activities (fishing, development, marine pollution) have had negative effects on coral reefs, with more than half of the world's coral reefs considered to be threatened (France 2004).
Anthozoa is one of four classes of the invertebrate phylum, the others being Hydrozoa (Portuguese Man o' War, Obelia, etc.), Scyphozoa (true jellyfish), and Cubozoa (box jellies). All are aquatic and most are marine. The name of the phylum comes from cnidocytes, which are specialized cells that carry stinging secretions that are the cnidarians' main form of offense or defense and function.
The basic body shape of a cnidarian consists of a sac with a gastrovascular cavity, with a single opening that functions as both mouth and anus. It has radial symmetry, meaning that whichever way it is cut along its central axis (that is, by any plane that passes through its longitudinal axis), the resulting halves would always be mirror images of each other.
Theoretically, members of Cnidaria have life cycles that alternate between asexual polyps (the body as a vase shaped form), and sexual, free-swimming forms called medusae (singular medusa; the body in a bell-shaped form). However, members of Anthozoa live only as polyps. The anthozoa larva, once fusing with the substratum and developing into the polyp stage, grows benthic or sessile, meaning it no longer metamorphoses into the medusal stage. (Members of Scyphozoa live most of their life cycle as medusa, the Hydrozoa live as polyps, medusae, and species that alternate between the two, and those of the class Cubozoa are named for their cube-shaped medusae, which form the dominant part of their life cycle.)
Anthozoans are essentially a tubular sac, with a mouth and tentacles position around the mouth on a flattened upper surface known as an oral disk. As with other cnidarians, the tentacles surrounding the mouth have stinging cells and the mouth is the only entry to the digestive system (France 2004).
Anthozoans are only found in marine environments. They can be found in habitats from the intertidalzone to over 6,000 meters (19,500) feet deep (France 2004). Species may be solitary or colonial. While solitary forms may attach to a hard substrate or burrowed into soft mud or sand on the sea bed, the colonial forms may build massive skeletons, such as the reef-building corals.
Larval forms are free moving, but most adult forms are sedentary; some groups, such as those of the orders Actiniaria (sea anemones), Ceriantharia (tube anemones), and Pennatulacea (sea pens and sea pansies) may show some movement.
Anemones and certain species of coral live in isolation, however most corals form colonies of genetically identical polyps; these polyps closely resemble anemones in structure, although are generally considerably smaller.
Members of most species of Anthozoa are suspension feeders, capturing small planktonic invertebrates, phytoplankton, bacteria, and other suspended organic matter (France 2004). Most common are passive methods of capturing prey, when it comes in contact with the tentacles. All cnidarian species can feed by catching prey with nematocysts, with large sea anemones capable of catching fish, crabs, and bivalves, and corals are capable of catching plankton.
Some of the anthozoan species harbor a type of algae, dinoflagellates called zooxanthellae, in a symbiotic relationship. The reef building corals known as hermatypic corals rely on this symbiotic relationship particularly. The zooxanthellae benefit by using nitrogenous waste and carbon dioxide produced by the host, and the cnidarian gains photosynthetic capability and increased calcium carbonate production in hermatypic corals (Madl and Yip 2000).
Various reproductive strategies are utilized by anthozoans, including production of asexual clones by fission (in longitudinal or transverse direction) and pedal laceration (where pieces of the pedal disk separate and develop new individuals) and fragmentation, and anthozoans may have separate sexes or be hermaphroditic (France 2004). Gametes typically are released through the mouth for external fertilization or eggs may be retained for internal fertilization and embryos released through the mouth (France 2004). The ciliated planula larvae that develop from the embryos may or may not feed (France 2004). Some species can produce larvae asexually (France 2004).
The synchronous release of sperm and eggs by many colonies over a coral reef is one of the most spectacular anthozoan behaviors, with such mass spawning events observed in the scleractinian corals of the Great Barrier Reef, as well as in octocorrallian and zoanthid species (France 2004).
Anthozoans may exhibit aggressive behaviors in defending space from neighboring individuals of the same or different species, including using specialized, extra long tentacles with stinging nematocysts, causing tissue death upon contact (France 2004).
Some species of the subclass Alcyonaria (Octocorrallia) can produce light (bioluminescence) that may even exhibit as a wave of light across a colony (France 2004).
Two subclasses are commonly recognized: Alcyonaria (or Octocorrallia) and Zoantharia (or Hexacorallia). Within these, several orders are recognized.
France (2004) lists three extant orders within Alcyonaira and six within Zoantharia:
The following is an alternate taxonomy and one includes extant and extinct suborders (Fautin and Romano 2000; Chen et al. 1995; France et al. 1996; Myers et al. 2008; Kotrc 2005; Oliver 1996). Fossils resembling anthozoans are known from the Precambrian era more than 540 million years ago, but around 465 million years ago stony coral fossils became common (France 2004).
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