CNIDARiANS - Anthozoans

A leather sea anemone (Photo: Simeon Daple.)

ANTHOZOA

Anthozoans are exclusively marine, polypoid cnidarians. They include the familiar sea anemones, and other anemone-like groups with skeletons (such as the "stony" scleractinian corals) and without skeletons (such as tube anemones), as well as sea pens, sea fans, blue coral, and black coral. Anthozoans occur from the intertidal zone to the depths of the trenches (to 6000 m). In excess of 6000 species currently exist (Hyman 1940), comprising about two-thirds of extant cnidarian species (Dunn 1982); some anthozoans, such as the scleractinian corals, have a rich fossil history (Wells and Hill 1956).
As in all cnidarians, the body wall is composed of two cell layers—the outer ectodermis (or epidermis) and the inner gastrodermis—separated by a layer of gelatinous material known as mesoglea or mesenchyme. 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 (siphonoglyphs) that funnel water into the coelenteron. The coelenteron 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 nematocysts. The tentacles are hollow and continuous with the coelenteron.Most anthozoan orders contain exclusively colonial species.
Anthozoans may be either solitary or colonial. In colonial species, the polyps are united by living tissue, the coenenchyme, 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.


Stichodactyla mertensii (Photo: Selma P.)

Feeding ecology and diet

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 gorgonians, anemones, and zoanthids. The zooxanthellae are additionally thought to increase the rate at which scleractinian reef corals produce their calcium carbonate skeleton.



Reproductive biology

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 anthozoan 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. Anthozoans, 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 gastrodermis 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 spawners"), 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 planula 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.
There Anthozoa includes Cnidaria that have no jellyfish stage. This is the largest class of cnidarians, containing over 6,000 species. A gullet extends for a short distance into the gastrovascular cavity, and septa are present, which increase the surface for digestion and absorption. Anthozoa are flower animals, including a great many beautiful and colorful organisms, e.g., the sea anemone, sea pansy, sea fan, and coral. Anthozoans are colonial or solitary organisms.