Marine Biology: A Very Short Introduction (Very Short Introductions) by Mladenov Philip V

Author:Mladenov, Philip V. [Mladenov, Philip V.]
Language: eng
Format: mobi, epub
Publisher: Oxford University Press, USA
Published: 2013-09-25T16:00:00+00:00

20. Aerial view of coral reefs of Heron Island, Great Barrier Reef, Australia

Physical requirements

Notwithstanding their importance, coral reefs occupy a very small proportion of the planet’s surface—about 284,000 square kilometres—roughly equivalent to the size of Italy. This is because the physical requirements of the main reef-building animals—the corals—are very specific.

Reef-building corals thrive best at sea temperatures above about 23°C and few exist where sea temperatures fall below 18°C for significant periods of time. Thus coral reefs are absent at tropical latitudes where upwelling of cold seawater occurs, such as the west coasts of South America and Africa. Corals also require lots of light to thrive, so they are generally restricted to areas of clear water less than about 50 metres deep.

Reef-building corals are very intolerant of any freshening of seawater below a salinity of about 30 and so do not occur in areas exposed to intermittent influxes of freshwater, such as near the mouths of rivers, or in areas where there are high amounts of rainfall run-off. This is why coral reefs are absent along much of the tropical Atlantic coast of South America, which is exposed to freshwater discharge from the Amazon and Orinoco Rivers.

Finally, reef-building corals flourish best in areas with moderate to high wave action, which keeps the seawater well aerated, brings in a constant supply of food for the corals, and removes light-blocking sediment from the surface of the corals.

Spectacular and productive coral reef systems have developed in those parts of the Global Ocean where this special combination of physical conditions converges, such as in the Caribbean Sea; the many islands of Indonesia, the Philippines, the South Pacific, and the tropical Indian Ocean; in the Red Sea; and off the northeast and northwest coasts of Australia (see Figure 21).

Biology of corals

Reef-building corals, also known as scleractinian or stony corals, are colonial animals related to sea anemones. Each colony consists of thousands of individual animals called polyps (see Figure 22). Colonies grow through asexual reproduction—the polyps repeatedly bud off new polyps, creating an expanding layer of genetically identical polyps which share a common stomach cavity. As the colony grows, the polyps extract calcium from the surrounding seawater to secrete a sizeable calcium carbonate skeleton which is external to the polyps themselves. Depending on the species, polyps are positioned within individual cups in the skeleton, or in rows within long grooves in the skeleton. The polyps can retract into the skeleton for protection.

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