Reef Ecology Guide Phuket Thailand - Cnidarians 1

All members of the phylum Cnidarians are characterized by tentacles armed with a myriad of stinging cells. Each of these cells is equipped with a minute safety pin (cnidocil), which acts as a trigger mechanism.

Whenever potential prey touches the tentacles, cnidociles get bent, which in turn leads to an instantaneous avalanche of stinging cells being ejected.

In this process, a bundle of knife-like appendages extend, cutting into the prey's skin. Immediately thereafter a long filament from inside the cell penetrates the cut and injects a toxin to paralyze or kill the prey.

All this is microscopically small and happens a million times in a few seconds.

Millions upon millions of tiny coral polyps are the life blood of the reef. The skeletons they secrete by countless generations form the solid limestone foundation or platform on which the reef flourishes.

An individual coral polyp consists of a fleshy sack topped with a ring of tentacles around a central mouth opening. It sits in a limestone skeletal case, which is actually secreted by the polyp. Members of the colony are linked together by living tissue. Therefore nutrients captured by a section of the colony can be shared around.

Stony corals have brownish colored, unicellular algae living within the tissues of the polyps. The algae use sunlight and carbon dioxide to produce carbon enriched organic compounds (sugar). These in turn are leaked to the polyp and may provide as much as 98 percent of its nutritional requirements.

Stony corals typically grow in clear, sunlit waters. Some are also found in relatively low light levels. They grow slowly and their diet may be supplemented by absorbing nutrients directly from the water or by feeding on bacteria and organic debris.

Soft corals and their relatives are a prominent part of the reef scene. In some places they may form extensive growth, like vast fields of undersea wildflowers. Structurally they are similar to stony corals.

Both contain colonies of polyps that gather planktonic food. However, as their name suggests, soft corals lack a hard skeleton. Instead, the supporting stem consists of fleshy tissue that is reinforced by a matrix of microscopic calcareous particles.

Soft corals have tiny side branches on the tentacles in contrast to the smooth tentacles of hard corals. Moreover, soft corals generally have eight tentacles compared to six or multiples of six found in hard corals.

Most soft corals are various shades of green, yellow and brown. The color is largely attributable to the presence of single celled algae that live within their tissue.