(order Corallimorpharia) have some of the characteristics of anemones, and others of corals. In fact, however, they look like small anemones. They are mostly colonial, and consist of flat disks, 2-4 inches in diameter, with a smooth, napped or tentacled surface. One genus, Discosonia, is particularly colorful, overgrowing rocks with its bright blue, purple or red disks.
Soft corals and gorgonians
These animals (subclass Alcyonaria) are among the loveliest of the cnidarians. In the clean, plankton-rich waters of Indonesia, soft corals and gorgonians— sea whips or sea fans—are common. Some contain zooxanthellae, but many frequent the deeper parts of the reef, where they filter plankton from the water. Semi-precious pink "coral" is a gorgonian (Corallium), harvested from deep waters off Japan and in the Mediterranean.
Soft corals. Soft corals, as the name suggests, lack the hard limestone skeletons of their reef-building relatives. Instead, the numerous polyps that make up the colony are supported by a fleshy central "body"; in some cases strengthened by spicules, spines of silica or calcium.
Soft corals (order Alcyonacea) have few obvious defense mechanisms, and might seem to be vulnerable to attack by predators and parasites, or to fouling by overgrowth. The animals avoid these problems by secreting various bioactive substances, a kind of chemical defense. Substantial efforts are being made by biochemists and pharmaceutical companies to identify compounds in soft corals—and also sponges—that may have properties useful in medicine. Since many of these compounds have evolved to prevent alien growths, they are receiving attention as potential anti-cancer drugs.
A very common group of soft corals in Indonesia are the leather corals (Lobophyturn, Sarcophyton and Sinularia) so-named because of their color and texture. These corals grow as wrinkled lobes in well-lit, shallow areas of reef. Because of their symbiotic zooxanthellae, they are a dull brown, sometimes with a slightly green or yellow tinge. When their white polyps are extended for feeding they are easy to identify as soft corals, but when their polyps are retracted they could be mistaken for sponges. The leather corals, however, have a much smoother surface than sponges.
Perhaps the most beautiful of the soft corals is Dendronephthya, a soft coral with fuzzy branches of vivid pink, white, orange, red, red-and-white, and a variety of other colors. The main "stem" is normally translucent and contains numerous white spicules, which offer some structural support. Most of the color comes from the polyps, which also contain sharp spicules to deter browsing by fishes. Dendronephthya grows deeper on the reef and in areas of low light, and always where currents can provide it with abundant plankton.
Xenia, particularly common in Indonesia, has perhaps the largest individual polyps of any soft coral, each 6-8 centimeters long. The white (also tan, or light blue) polyps grow in clusters, and the tentacles at the end of each are feathery. These continually open and close, like numerous grasping hands. In Xenia one can easily count eight tentacles, which is one feature that distinguishes soft corals (Octocorallia) from hard corals and anemones (Zoantharia) which have six, or multiples of six, tentacles.
Gorgonians. Gorgonians (order Gorgonaceae) have a strong, horny skeleton, which gives strength and support without sacrificing flexibility. They tend to grow on the deeper parts of the reef, away from strong wave action. They live by filter feeding, and to maximize the water flow across their surfaces always grow at right angles to the prevailing current. Where the tidal current flows along the reef, gorgonians grow with their long axis vertical. Sometimes, however, particularly on some of the big walls in Indonesia, large sea fans can be seen growing horizontally out from the reef wall, to take advantage of the current upwelling.
There" are many species of these animals. Some have a twig-like structure, like a branch from a delicate tree. Many are brightly colored. The sea fans (Melithaeidae and Plexauridae) are flat nets, growing in some cases to three meters across. The skeleton of a sea fan is coated with a kind of "rind," which is sometimes a delicate shade. When you see a big gorgonian it is worth spending a few moments looking closely at its surface because they often host an assortment of small symbiotic animals.
Sea pens. These animals (order Pennatulacea) are filter-feeders related to the gorgonians. Their common name comes from their resemblance to the old-fashioned quill pen. Although common in Indonesia, they are not really reef dwellers, and will usually only be seen by night divers who venture out over mud or sand bottoms. Sea pens, sometimes growing in large fields, rotate gently back and forth with the current, their "feathers" sifting plankton from the current.
Closeup of the lovely soft coral Dendronephthya. In this photograph the strengthening spicules are clearly visible in the animals ' transparent tissue.
The poisonous sea wasp, Chironex. This animal has been responsible for human fatalities in Australian waters.
Black corals. Black coral (subclass Ceriantipatharia, order Antipatharia) looks to the diver like a gorgonian, although it is more closely related to the stony corals and anemones. On deeper dives in Indonesia, one can see wire corals (Cirrhipathes) and black coral bushes (Antipathes).
The polished skeleton of the latter, particularly the thicker branches, is the precious black coral. Black coral is scarce, and its export from Indonesia and import into many other countries is prohibited by laws.
Cerianthids. In some areas, particularly with sandy bottoms, one can find cerianthids (subclass Ceriantipatharia, order Ceriantharia) or tube anemones. These are quite different from true anemones. Cerianthids have fine tentacles arranged in two concentric bands, and a tube— made of fibers created by special nematocysts—into which they can retract if disturbed.
A pair of Chromodoris willani, perhaps mating. All nudibranchs are hermaphrodites, although they require a partner to produce viable off-spring. Bunaken group, Sulawesi.
Fire coral and hydroids
All cnidarians have stinging cells on their tentacles with which they can defend themselves and immobilize their prey. In most cases, however, these are rather weak and are usually not capable of penetrating human skin. One group, class Hydrozoa, which includes the fire corals and their relatives, has members capable of inflicting very painful stings. The notorious Portuguese-man of-war (Physalia physalis) is a hydrozoan, and not, despite its appearance, a jellyfish. Fortunately for divers this animal is more of an open ocean dweller.
Stinging hydroids. A far greater nuisance to divers in Indonesia are the hydroids Aglaophenia and Lytocarpus. Despite their delicate, fern-like appearance, these colonial animals can deliver a burning sting that raises a welt on bare skin. They are fairly common on many Indonesian reefs, and their presence prompts divers to wear Lycra or thin neoprene suits even in the warmest of conditions. The stinging hydroids are sometimes called sea ferns, or sea nettles.
Fire corals. Somewhat less virulent are the fire corals of the genus Millepora, but as their name suggests they too should be treated with respect. These hydrozoans are members of a group called hydrocorals for their superficial resemblance to the true corals. Hydrocorals secrete a limestone skeleton, and form colonies that are usually a dull-yellow brown in color (Millepora) although some species (Distichopora, Stylaster) can be brightly colored.
The unusual blue coral (Heliopora coerulea), is a fire coral which has a skeleton that when dried displays a light blue tint. It is taxonomically distinct, however and has been placed in class Anthozoa with the octocorals.
Some hydrocorals are important
