Phylum Cnidaria Class Anthozoa Number of families 130
Exclusively polypoid cnidarians. Tubular body with hollow tentacles around the mouth; has a pharynx that opens into a digestive cavity subdivided by infoldings of the gut wall. May be solitary or colonial, with or without an internal or external skeleton.
Photo: Jeweled anenome (Corynactis californica) (©Shedd Aquarium. Photo by Patrice Ceisel. Reproduced by permission.)
Evolution and systematics
Anthozoa is the largest class in the phylum Cnidaria, with over 6,000 extant species divided among nine orders and classified in two subclasses. The nine orders are as follows:
Subclass Octocorallia (= Alcyonaria)
• Order Pennatulacea, the sea pens and sea pansies; 16 families
• Order Helioporacea, the blue corals; two families
• Order Alcyonacea, the soft corals, sea fans, and sea whips; 29 families
Subclass Hexacorallia (= Zoantharia)
• Order Actiniaria, the sea anemones; 42 families
• Order Scleractinia (= Madreporaria), the true (stony or hard) corals; 25 families
• Order Corallimorpharia, the mushroom (false) corals (also known as mushroom anemones or disc anemones); 4 families
• Order Zoanthidea (= Zoanthinaria), the zoanthids; four families
• Order Antipatharia, the black (thorny) corals and wire corals; five families
• Order Ceriantharia, the tube anemones; three families
Some authors classify the Ceriantharia and Antipatharia together in a third subclass, the Ceriantipatharia, but genetic evidence does not support this grouping.
Molecular evidence places the Anthozoa as the earliest branch of the phylum Cnidaria. Since anthozoans exist ex clusively as benthic (sea bottom) polyps, whereas the remaining cnidarians have life cycles that alternate between benthic polyp and pelagic medusoid stages, this evidence indicates that the polyp stage is the ancestral condition among Cnidaria. Within the Hexacorallia, additional evidence suggests that ac-tiniarians, and more recently corallimorpharians, may have evolved from a scleractinian ancestor, accompanied by the loss of a skeleton.
Fossils resembling anthozoans, including sea pens, are known from as early as the Precambrian eon (>540 million years ago [mya]), but it wasn't until the Ordovician period (about 465 mya) that stony coral fossils became common. Two groups of corals dominant among Ordovician fossils—the Tabulata and Rugosa—became extinct at the end of the Permian period (about 248 mya). The earliest scleractinian fossils come from the middle Triassic period (about 230 mya).
Anthozoans are polypoid cnidarians. As in all cnidarians, the body wall is composed of two cell layers—the outer ecto-dermis (or epidermis) and the inner gastrodermis—separated by a layer of gelatinous material known as mesoglea or mes-enchyme. Tentacles bearing stinging cells surround the mouth, which is the only opening to the digestive system. A polyp is essentially a tubular sac, with the mouth and tentacles on a flattened upper surface called the oral disk. The mouth leads to a pharynx, a short tube projecting into the closed gut (the coelenteron or gastrovascular cavity). The pharynx typically has one or more ciliated grooves (siphono-glyphs) that funnel water into the coelenteron. The coelen-teron is subdivided into chambers by vertical septa (or mesenteries), infoldings of the gut wall that may or may not
attach to the pharynx. Below the pharynx, the free edges of the septa are thickened to form septal, or mesenterial, filaments that contain cells involved in digestion, including ne-matocysts. The tentacles are hollow and continuous with the coelenteron.
Octocorals have eight pinnate (parts arranged on each side of a common axis) tentacles and eight septa; hexacorals have septa and tentacles—usually simple—in multiples of six. Ce-rianthids have two circlets of tentacles, one around the mouth and the other around the edge of the oral disc. There are a number of other details of polyp anatomy that are used to distinguish the orders of Anthozoa.
A unique characteristic of cnidarians is the cnida, a complex intracellular capsule containing an eversible hollow tubule that can be released to sting or trap prey. Eversible means that the structure can turn inside out. There are three basic types of cnidae, and all can be found in the class An-thozoa. Nematocysts, which contain toxins and are typically armed with spines for penetrating the tissues of other organ isms, are possessed by all anthozoans. Spirocysts are sticky rather than penetrating and are found only in the hexacoral-lians. Ptychocysts are unique to the cerianthid tube anemones and are used to construct their tubes.
Species in the orders Scleractinia, Octocorallia, and An-tipatharia produce skeletons. In the Scleractinia, the living tissue essentially lies above an external skeleton made of calcium carbonate secreted by the ectodermis. Scleractinian skeletons can take on a variety of shapes, generally described as massive, columnar, encrusting, branching, leaflike or platelike. It is these skeletons that form the framework of tropical coral reefs. Antipatharians secrete an internal horny skeleton that is flexible, black in color, and equipped with thorns on its surface. Octocorals secrete an internal skeleton composed of a protein called gorgonin, calcium carbonate, or a combination of both. Octocorallian and antipatharian skeletons are usually branching treelike or whiplike forms. Unlike other octoco-rals, helioporaceans produce a massive skeleton that is blue in color and resembles those found in the stony corals. Oc-
Anthozoa anatomy. (Illustration by Christina St. Clair)
Septal filament Gastrovascular cavity
Gonads secondary septa Epidermis
Anthozoa anatomy. (Illustration by Christina St. Clair)
tocorals also secrete small calcareous sclerites (hardened plates) of a variety of shapes and colors that are embedded in the mesenchyme and that may give a spiny or scaly appearance to the colony. Octocorallian soft corals lack a supporting internal skeleton and can inflate or deflate the fleshy colony by funneling water into or out of their polyps. With the exception of the zoanthid genus Gerardia, species in the remaining anthozoan orders do not secrete a skeleton. A few actiniarians secrete a chitinous tube, and one genus is able to form a chitinous coiled shell, similar in shape to a snail shell, that is inhabited by a hermit crab. Cerianthids build soft, feltlike tubes from fired ptychocysts.
Anthozoans may be either solitary or colonial. In colonial species, the polyps are united by living tissue, the co-enenchyme, and their gastrovascular cavities are joined by canals or tubes. Actiniarians and ceriantharians are exclusively solitary, and the octocorals and antipatharians are exclusively colonial, but the remaining orders have both types of morphologies. Solitary polyps are commonly 0.5-2 in (1-5 cm) in diameter at the oral disk, but the largest species grow to 3 ft (1 m) across. Polyps of colonial species are typically much smaller (<0.4 in (5 mm)), but the colonies themselves can be quite large. Octocorallian and antipatharian colonies may grow >8 ft (2.5 m) tall, and some scleractinian corals may reach a size of 19.5 ft (6 m) in height and width. Colonial anthozoans may reach a great age. For example, many octoco-rals are 100 or more years old, and the deep-sea zoanthid Gerardia has been estimated to be 1800 years old.
Anthozoans are found worldwide in all oceans.
Anthozoans are restricted to marine habitats, but can be found from the intertidal zone to over 19,500 ft (6,000 m) deep. Solitary forms may be attached to a hard substrate or burrowed into soft bottom mud or sand. Colonial forms grow as an encrusting or stoloniferous form on other substrates, or build massive skeletons; tree-like colonial forms are attached by the base of the main stem. Sea pen colonies are anchored into soft bottoms by the base of the primary polyp. Reef-building corals are most typically found in clear, shallow, warm tropical waters, although a few species are known from the cold, dark deep sea.
Several octocorallian species can produce light when contact with the colony is made. This bioluminescence may be in the form of a bright green flash from a polyp, or a wave of light across the colony as polyps flash sequentially from the point of contact. It is likely the light is meant to startle visual predators.
One of the most spectacular of anthozoan behaviors is the synchronous release of sperm and eggs by many colonies over a wide area of coral reef. Mass spawning events have been observed in octocorallian and zoanthid species, but are best known in the scleractinian corals of the Great Barrier Reef, Australia. In some cases, many species release gametes on the same night, and there is considerable evidence that hybridization between species may occur. Synchrony is achieved by timing gamete release with the lunar cycle, and many corals spawn shortly after sunset following a full moon. Eggs and sperm may be released separately into the water column; or, in some hermaphroditic species, may be combined into gamete bundles that float to the surface and break apart there. During these mass spawning events, huge slicks of gametes and developing larvae can be observed on the water surface, attracting a variety of predators that feed on the spawned gametes.
Anthozoans engage in aggressive interactions to defend space from neighboring individuals of the same or different species. A number of specialized structures may be used to repel encroachers. Sea anemones may possess acrorhagi, inflated saclike structures bearing nematocysts that ring the collar below the tentacles. Acrorhagi can be elongated to come into contact with an intruder, whereupon they cause tissue death. Scleractinian, octocorallian, and antipatharian corals may develop specialized tentacles ("sweeper tentacles") after prolonged contact with foreign species. These tentacles are five to ten times longer than normal feeding tentacles and have a greater number of stinging nematocysts. The tentacles search and sweep an area around the polyp and cause tissue death in neighboring species upon contact. Similar structures found in some sea anemones are called "catch tentacles." Some scleractinian corals and corallimorpharians also may extrude their septal filaments to digest the neighboring species' tissues.
Most anthozoans are suspension feeders whose diet consists of small planktonic invertebrates, phytoplankton, bacterio-plankton, or other suspended organic matter. Their methods of prey capture are generally, though not exclusively, passive. Drifting prey may be captured when it comes in contact with the extended tentacles of anthozoan polyps. Prey capture may also involve the firing of cnidae. Many scleractinians produce a slimy mucus that covers the polyp and traps floating and sinking food particles. The mucus is moved around by cilia (small hairlike projections), and eventually enters the mouth. Large sea anemones may feed on crabs, bivalves and fishes, while ptychodactarian anemones have been observed preying upon gorgonian octocorals. In 1997, colonies of the soft coral Gersemia antarctica were first reported to bend over and feed in the soft sediments of the Antarctic. Researchers proposed that this feeding strategy may be employed where suspended organic material is in low supply, as it is in the deep sea. Some anthozoans also may absorb dissolved organic matter directly from the seawater into their cells.
Another source of nutrition for many anthozoans comes from symbiotic photosynthetic algae living within cells of the gastrodermis. Like other plants, these algae produce energy-rich organic molecules through photosynthesis; between 20% and 95% of this production is transferred to the host. These
symbionts are usually dinoflagellates, called "zooxanthellae"; however, green algae symbionts ("zoochlorellae") are also known. Most shallow-water, reef-building corals contain zooxanthellae, and they also may be found in tropical gor-gonians, anemones, and zoanthids. The zooxanthellae are additionally thought to increase the rate at which scleractinian reef corals produce their calcium carbonate skeleton.
The predators of anthozoans include nudibranchs, sea stars, crabs, polychaetes, and fishes.
Anthozoans display a wide range of reproductive strategies. Asexual clones may be produced in a variety of ways. Polyps may undergo fission in either a longitudinal or transverse direction. Many sea anemones produce clones by pedal laceration, wherein pieces of the pedal disk tear off or break free and develop into new individuals. The growth of antho-zoan colonies may be considered a mode of asexual reproduction. After a free-living larva settles, it metamorphoses into a polyp that repeatedly divides to give rise to additional polyps, all of which remain connected by living tissue. In some species, budded polyps may be released from the parent colony, and these then settle and develop a new colony. An-thozoans, particularly colonial species, also may reproduce by fragmentation. For many scleractinian corals, damage caused by storms or strong wave action may produce fragments that lead to new colonies.
Anthozoans may be gonochoristic (having separate sexes) or hermaphroditic. In colonial species, gonochoristic colonies are composed entirely of male or female polyps, whereas hermaphroditic colonies may have both male and female polyps ("monoecious") or hermaphroditic polyps. Polyps lack well-defined gonads; rather, the gametes accumulate in the gastro-dermis of some or all mesenteries. The gametes are typically shed into the gastrovascular cavity and are either released through the mouth for external fertilization ("broadcast spawn-ers"), or eggs are retained for internal fertilization and the embryos released through the mouth at a later time ("brooders"). Brooding species may hold the embryos internally within the gastrovascular cavity or externally in a coat of mucus on the polyp's surface. Anthozoan embryos develop into ciliated plan-ula larvae that may or may not feed, and that can stay in the water column for days to weeks. Contrary to initial assumptions, some species also can produce larvae asexually.
Some of the largest concentrations of anthozoans, in terms of both numbers and species diversity, occur on coral reefs. Such human activities as fishing, coastal development, terrestrial runoff, and marine pollution have had dramatic negative impacts on coral reefs, as have coral diseases that have increased in frequency and severity over the last decade of the twentieth century. Despite a ban in most countries, cyanide is still commonly used to collect reef fishes for the aquarium trade. The cyanide stuns the target fishes, allowing for easy collection, but kills many of the anthozoans and other invertebrates living on the reef. In 1998, the World Resources Institute estimated that more than half of the world's coral reefs are threatened by these and other human activities. As of 2002, however, only two anthozoan species were listed on the International Union for Conservation of Nature and Natural Resources (IUCN) Red List of Threatened Species: the broad sea fan, Eunicella verrucosa, and the starlet sea anemone, Nematostella vectensis. All scleractinian corals, antipatharian black corals, and octocorallian blue corals and organ-pipe corals, are listed on Appendix II of the Convention on International Trade in Endangered Species (CITES). This listing means that "trade must be controlled in order to avoid utilization incompatible with their survival." A number of other treaties have been established to protect coral reef organisms, such as the International Coral Reef Initiative and the U.S. Coral Reef Initiative. Most recently, there has been concern over the impact of deep-sea fisheries trawling on slow-growing deep-water corals. Norway and Australia have created conservation areas to protect reefs within their territorial waters.
Coral reefs, which are largely a framework of scleractin-ian skeletons glued together by sponges and other organisms, are a major tourist destination and source of recreation. Corals provide a habitat for a variety of organisms that humans use for food, including fishes, urchins, mollusks and crustaceans. It is estimated that approximately 50% of U.S. federally-managed fisheries depend on coral reefs for part of their life cycle, at an annual worth of over $100 million. An-thozoans of all orders (except Antipatharia) are sold in the aquarium trade, and octocorallian and antipatharian skeletons are used to make coral jewelry. Scleractinian skeletons are used as building material and in bone grafts, as the structure of the coral skeleton is similar to that of human bone. Black
coral skeleton was once thought to have medicinal properties, and the name "Antipathes" is a Latin word that means "against disease." A variety of natural products have been isolated from anthozoans for commercial use, from suntan lo tions to antifoulants. In particular, octocorals produce a range of bioactive compounds; some of these have been harvested for molecular biological and pharmaceutical applications, including anticancer and anti-inflammatory agents.
1. Cauliflower coral (Pocillopora damicornis); 2. Red coral (Corallium rubrum)-, 3. Red soft tree coral (Dendronephthya hemprichi); 4. American tube dwelling anemme (Ceriantheopsis americanus); 5. Sea pansy (Renilla reniformis); 6. Goniastrea aspera; 7. Rubber coral (Palythoa caesia); 8. Mushroom coral (Fungia scutaria); 9. Black coral (Antipathella fiordensis). (Illustration by Emily Damstra)
1. Giant green anemone (Anthopleura xanthogrammica), 2. Elephant ear polyps (Amplexidiscus fenestrafer), S. Magnificent sea anemone (Heter-actis magnifica), 4. Deep water reef coral (Lophelia pertusa), S. Frilled anemone (Metridium senile), B. Starlet sea anemone (Nematostella vecten-sis), l. Elkhorn coral (Acropora palmata), B. Acropora millepora, 9. Close-up of A. palmata. (Illustration by Emily Damstra)
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