Even though they do not contribute to reef building, soft corals are important members of the coral reef system. They are close relatives of hard corals, differing primarily in their lack of a hard external skeleton. The size of individual soft coral polyps rarely exceeds 0.2 inch (5 mm), but they live in colonies that grow up to 3 feet (about 1 m) tall. Despite the fact that they lack calcium carbonate skeletons, soft corals do not lack support tissue. In their mesogleal layer, they contain collagen fibers (similar to the material that forms tendons and ligaments in mammals) that enable these colonies to maintain a plantlike shape. Many of the soft corals live in areas of the reef that experience wave action and turbulence, and their flexible structure permits them to bend with the water movements. In other species, the internal skeleton includes some calcium carbonate spicules, which give these soft corals a slightly stiffer structure.
Soft coral animals are usually suspension feeders that gather their food with eight feathery tentacles. Tentacles capture bits of plankton and direct them through the mouth into the gastrovascular cavity. In soft corals, the gastrovascular cavity is divided into sections by eight walls of tissue. Each section is lined with the digestive cells. Most soft coral colonies have a system of internal tubes that distribute nutrients to all individuals. To keep neighbors from infringing on their space, many species of soft corals protect themselves by producing chemicals that repel sponges and algae. Some soft corals also secrete toxins to ward off fish and other predators. Toxin-producing species display bright colors that warn other animals that they are dangerous and should be avoided. In general, the more fragile and easily harmed a soft coral species, the more predator-repelling toxin it contains.
Colonies of soft corals occur in a bewildering array of colors, including hues of red, blue, orange, and yellow. Many live on ledges where they extend into the flowing water to capture food. One common group, the leather corals (Sarcophyton), develop distinctive mushroom shapes. The spicules in these corals are densely arranged and mixed with collagen, making them tough enough to withstand the fastest waters without damage. Thick stalks anchor colonies of leather corals to the bottom, and polyps extend from broad caps at the anterior end of the stalks. When they are not feeding, the polyps pull back into the caps for protection. Leather corals are usually located in energetic areas like reef drop-offs and along the walls of channels.
There are hundreds of other species of soft corals. Deadman’s fingers (Briareum asbestinum) grows in vertical stalks that may be 1.5 inches (3 cm) thick and 6 inches (15 cm) tall. When its polyps are extended and feeding, the corals resemble brown, fuzzy branches. Carnation coral (Dendronephthya), pictured in the lower color insert on page C-2, appears as soft and delicate as the flower for which it is named. Several species of soft corals are known as sea fans because of their characteristic flat, fan-shaped branchlets, as shown in Figure below. Sea plume (Muriceopsis flavida) forms featherlike colonies very similar to those of the sea feathers(Pseudopterogorgia acerosa).
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| Sea fans are soft corals that have delicate, fanlike shapes. |