Earth, Climate & Environment

Coral Reef

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 mollusks. Built up layer by layer by living corals growing on top of the skeletons of past generations, coral reefs grow upward at rates of 1 to 20 cm (0.4 to 7.8 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 20° C (68° F).

Coral reefs are ecosystems with well-defined structures that involve both photosynthetic algae and consumers. 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 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 colorful butterfly fish, as well as sea urchins, sea cucumbers, brittle stars, and numerous species of mollusks, 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.

Coral reefs are of three types: fringing reef, barrier reef, and atoll. Fringing reefs extend outward 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 with a shallow lagoon.