Offshore coral reefs and islands

 
 

The most important reef-building organisms are the stony corals and certain calcareous algae. Although many stony corals are solitary, most are colonial and consist of a number of individuals (polyps), all descendents of one ancestor. All corals have the same basic body structure, two layers of cells making up a hollow cylindrical body with a ring of tentacles surrounding a mouth. These tentacles are armed with stinging cells (nematocysts) which capture small planktonic animals that drift too close.

Inside the body of the hermatypic (reef-building) corals live single-celled algae (zooxanthellae) which are essential for the deposition of the coral's limestone skeleton. They need light for photosynthesis which limits the depth to which reef-building corals can grow.

The islands are composed of coral sand derived from the reefs through the erosion by waves, the boring action of some worms, and the grazing activities of parrot fish and sea urchins. Over the past 5,000 years this sand has been deposited by currents and become consolidated as beach rock, now overlain with steep coral sand beaches.

Different parts of a coral reef are subjected to different environmental conditions, particularly light and exposure to wave action. These conditions determine the type of coral that can survive and give rise to the zonation pattern shown in the diagram below:

 
   
   
   
 
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reef
 
 

The high diversity of corals provides a wide range of habitats for other reef species which include tube-living fan worms, boring bivalves, barnacles and numerous crustaceans, in particular the pistol shrimp whose "cracking" can clearly be heard underwater.

Fish occupy a variety of ecological niches within the reef community. Damsel fish mark out, tend and actively defend their own coral/algal patch; surgeon fish feed on soft algae; butterfly fish feed on the coral polyps or plankton; cardinal fishes live in crevices and among the branches of corals, and parrot fish bite off lumps of live coral and digest its organic content. Clown fish live in the protection of sea anemones, fish hunt along the reef edge, these include the sharks and rays. During the summer the largest fish of all, the whale shark (92), may be observed cruising around the islands with its mouth wide open; but don't worry, it only feeds on plankton.

The islands are small and low, not more than 3-4 m above sea level and mostly covered by small salt-tolerant shrubs. They support the nesting of internationally important colonies of terns and turtles. The animals are distributed to minimize the competition and conflicts over available space. Bridled terns are found in the densely vegetated pans of the islands, whereas the lesser crested terns prefer the more open areas inland from the sandy beach. Large colonies, in excess of 10,000 birds, nest on the islands of Karan and Kurain, which means that a significant proportion of the world population of these birds is nesting here. These birds are fish eaters with sardines, anchovy, flying fish and juvenile mackerel making up the largest proportion of their diet. Along the beaches the ghost crab and the land hermit crab may be found. Under the bushes are the holes of the resident house mouse population.

Two species of turtle, the green (93) and the hawksbill (94), are regularly seen in the Gulf waters. The green turtle is the more common of the two and may be seen foraging on the seagrass beds inside the Berri Oil field and the shallow sea north of the Abu Ali Island. They nest in the summer months, the females returning to the same beaches as they themselves hatched from, to lay their eggs. A nest is dug at night and, if she is not disturbed, a clutch of about 100 eggs will be laid before the hole is refilled. About 60 days later all the hatchlings from a single nest emerge together and race down to the sea to avoid the waiting predators.

The sex of the young is determined by the temperature at which the egg was incubated. Females develop when it is warmer, the pivotal temperature varying between species but being in the region of 29-30°C. The capture or killing of turtles, their eggs or their young is prohibited. The animals remain threatened due to very high mortality rates in their early years, from predation and accidental mortality through drowning in trawl nets. The loss of breeding or feeding habitats would pose a serious threat to the continued existence of these animals in Gulf waters.

 
   
   
   
   

Click below for images of animals inhabiting offshore coral reefs and islands within the Sanctuary:

The island of Karan
Parrot fish
Clown fish and sea anemone
Surgeon fish
Cardinal fish
Grouper
Lion fish
Lesser crested tern colony, Karan
Bridled tern
White-cheeked tern
Hermit crab
House mice
Green turtle near Karan
Turtle nesting, Karan
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