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Reef Ecology Guide Phuket Thailand - Cnidarians 1

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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.

STONY CORALS (Scleractinia)
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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.

  FORMOSA STAGHORN CORAL (Acropora formosa)

FORMOSA STAGHORN CORAL at the Similan Islands

Very common, often the dominant species of lagoons and fringing reefs. Colonies arborescent, forming thickets.

Occur in different colors like cream, brown or blue. Branch ends often pale.

  SOLID TABLE CORAL (Acropora clathrata)

SOLID TABLE CORAL often seen at the Similan Islands

The branches of this species form an almost solid plate.

It has a light skeleton, which allow them to grow quickly and to overcome their neighbors.

  MUSHROOM CORAL (Fungia fungitis)

MUSHROOM CORAL comonly found at the Similan Islands

Very common species, not attached, on upper reef slopes exposed to surge.

The ridges have triangular teeth.

  BRAIN CORAL (Leptoria phrygia)

BRAIN CORAL, A vommon Similan Island Corral type

Mostly occurring in single colonies down to 15 m depth.

The colonies

  ORANGE CUP CORAL (Tubastrea coccinea)

ORANGE CUP CORAL, very butifull to see at the Similans

The commonest cup coral seen by divers because it grows in areas exposed to currents and not in caves like the other genus members.

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SOFT CORALS & SEA FANS (Alcyoniidae)
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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.

  GIANT SEA FAN  (Anella mollis)

GIANT SEA FAN at Similan Island No. 7

This species has a flexible uni-plane skeleton.

Normally found growing on drop-offs, reaching far out into the open water.

  KNOTTED FAN CORAL (Melithaea ochracea)

KNOTTED FAN CORAL, usually found at the Similans

The colonies are fan-shaped with a thick main stem, dividing in many sub-branches.

The polyps are distributed over the entire surface. Coloration is variable, mainly deep red with white polyps.

Grows on slopes and drop-offs, also on deeper reef terraces. Feeds on micro plankton.

  RED WHIP CORAL (Junceella sp.)

RED WHIP CORAL Similan Islands

Whip-like gorgonians with heavily calcified horny axis. Found in groups on reef slopes.

Sexes are separate, but also asexual reproduction by budding. Buds fall down and grow to become neighbors of the parents.

  KLUNZINGERS SOFT CORAL (Dendronephthya klunzingeri)


Wide spread species of soft coral. Red calcareous sclerites show as loosely distributed spines in the translucent body wall.

Coloration pink to violet. On hard substrate, nocturnal plankton feeder.

  BLACK CORAL (Antipathes sp.)

BLACK CORAL, found on the deeper dives at the Similan Islands

They show a variety of branched or whip-like forms, which resemble gorgonians.

The name is derived from the color of the inner skeleton. Living colonies never appear black under water.

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