Coral Reef

I

Introduction

Coral Reef, ridge or elevated part of a relatively shallow area of the seafloor, approaching the sea's surface. It is formed by a rocklike accumulation of calcareous (calcium-containing) exoskeletons of coral animals, calcareous red algae, and molluscs. Built up layer by layer by living corals growing on top of the skeletons of past generations, coral reefs grow upwards at rates of 1 to 100 cm (0.4 to 40 in) per year. Coral reefs are tropical, extending to about 30° north and south of the equator and forming only where surface waters are never cooler than 16° C (61° F). They are, therefore, very vulnerable to climate changes. In 2001, the United Nations Environment Programme’s World Conservation Monitoring Centre (UNEP-WCMC) produced the first world atlas of coral reefs, identifying their biological diversity as well as the dangers presented by climate changes and human impact. According to their data, coral reefs occupy about 284,300 sq km (109,740 sq mi) worldwide—much less than previously thought.

II

Forms of Life

Coral reefs are ecosystems with well-defined structures that involve both photosynthetic plants and consumers in the sense explained in the article on Ecology. The outer layer of a reef consists of living polyps of coral. Within the coral animals live single-celled, round algae called zooxanthellae. Below and surrounding the polyps is a calcareous skeleton, both living and dead, that contains filamentous green algae. Other species of algae, both fleshy and calcareous, grow in the surface of old skeletal deposits. These algae and other associated plants make up most of the primary producers.

The photosynthetic zooxanthellae and filamentous green algae transfer some food energy directly to the coral polyps. Coral animals also feed at night on zooplankton, which they capture with their tentacles. Coral animals prey on zooplankton not so much for the calories but for scarce nutrients, especially phosphorus. Through digestion, coral animals release these nutrients to the algae. Coral and algae then apparently cycle these nutrients between them, reducing nutrient loss to the water.

Herbivorous fish, such as the colourful butterfly fish, as well as sea urchins, sea cucumbers, brittle-stars, and numerous species of mollusc, feed on algae. Hiding in the numerous caves and crevices of a reef are predatory animals such as small crabs, wrasses (long, spiny-finned fishes), moray eels, and sharks. The numerous microhabitats and the productivity of the reefs support a great diversity of marine life.

III

Kinds of Reef

Coral reefs are of three types: fringing reef, barrier reef, and atoll. Fringing reefs extend outwards from the shore of an island or mainland, with no body of water between reef and land. Barrier reefs occur farther offshore, with a channel or lagoon between reef and shore. Atolls are coral islands, typically consisting of a narrow, horseshoe-shaped reef enclosing a shallow lagoon.

IV

Coral Bleaching

Coral reefs have recently been affected by bleaching, that is, the discoloration or loss of symbiotic zooxanthellae. In 1979 and 1980, several incidents of coral bleaching occurred at reefs around Okinawa, Easter Island, north-east Australia, and the Caribbean Sea. A more extensive outbreak of bleaching occurred in 1982 and 1983, including reefs off east Africa, Indonesia, and the west coast of Central and South America. Bleaching incidents even more widespread and damaging occurred over the three-year period from 1986 to 1988, including areas such as Taiwan, Hawaii, Fiji, Mayotte Island, and the entire length of the Great Barrier Reef. In 1998 mass bleaching affected coral reefs worldwide and destroyed about 90 per cent of coral in the central Indian Ocean. In 2002 it was estimated that about 25 per cent of the world’s reefs had irrevocably been lost; however, reefs in some parts of the world, notably in East Asia, were beginning to show tentative signs of recovery.

The cause of these widespread bleaching incidents is unknown; pollution, global warming, and ultraviolet radiation have been suggested as suspects. Although it has not been conclusively shown that any or all of these are the cause of these coral bleaching episodes, recent research indicates that the cause may be unusually warm waters. The optimum temperature for coral growth is between 26° C and 27° C (78.8° F and 80.6° F). Temperatures above 29° C (84.2° F) have been shown to cause stress in corals, and may boost the rate of photosynthesis by the symbiotic zooxanthellae, creating high concentrations of free-radical toxins in the coral tissue. These stressed coral polyps may actively expel zooxanthellae, causing the coral to appear bleached. Other recent research has suggested that the unusually warm water temperatures might actually impair the ability of the symbiotic zooxanthellae to photosynthesize, and that as temperatures rise a protein used by the zooxanthellae is seriously damaged. It has been suggested that once the ability to photosynthesize is degraded, the host coral expels the zooxanthellae.

Bleached corals have difficulty recovering; a reef can take years to recover, and subsequent bleaching incidents may make it impossible. Without their symbiotic zooxanthellae, corals are unable to deposit the calcium carbonate skeleton that makes up the foundation of a coral reef. Not only corals, but all reef organisms could potentially lose their habitat because of bleaching incidents, as the calcium carbonate structure of the reef erodes.