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Corals are made of millions of tiny, dead and alive, sac-shaped creatures called polyps. Large group of polyps (of the same species) forms a colony.
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Type of coral can be determined based on the size, shape and color of the colony.
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Polyps have hard, limestone skeleton at the base of the body which fuses with skeletons of other polyps and leads to the formation of colony. Merging of nearby colonies results in the formation of reef.
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Billions of colorful algae, called zooxanthellae, that live in symbiosis with corals, are responsible for the beautiful colors of the coral reefs.
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There are three type of coral reefs: barrier reef, fringing reef and atolls. Barrier reef is usually located in deep water, away from the shore. Fringing reef is fringe-shaped and located in the shallow water. Atolls are island-shaped. They grow on the edges of the lagoons.
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Coral reef is the living community and the largest living structures on Earth that are created by animals.
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Corals grow very slowly, only 0.2 to 0.8 inches per year.
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One of the oldest and largest coral reef on the planet is Great Barrier Reef in Australia. It was created 5 to 10 thousand years ago. It occupies 2600 miles of sea floor, crosses 500 islands and consists of 900 smaller reefs.
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Coral reefs increase stability of the sea bed, protect beaches from the waves and erosion (hence the name "barrier"), provide valuable shelter for fish, marine mammals and various invertebrates and serves as spawning area for many fish.
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Corals feed at night.
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Corals collect zooplankton and small fish with a help of tentacles that are located around mouth opening. They also obtain nutrients from the algae (with whom they live in symbiosis).
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Corals are often restricted to the shallow water which provides plenty of sun, required for the process of photosynthesis (synthesis of sugar from the sun, water and carbon dioxide) that algae perform.
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Besides hard corals that are inevitable part of coral reefs, there are also soft corals that cannot build reef due to lack of hard skeleton.
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Once per year all corals in the reef release gametes (reproductive cells) at the same time. Corals also reproduce asexually through budding and division.
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25% of marine creatures depend of the coral reefs despite the fact that corals cover less than 1% of the sea floor.
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