Coral Reef Definition and Biology

Summary

A coral reef is an underwater ecosystem built primarily by colonies of reef-building corals. Coral polyps are animals in the class Anthozoa within the phylum Cnidaria, related to sea anemones and jellyfish. Unlike sea anemones, corals secrete hard calcium carbonate exoskeletons that support and protect them; over generations, these skeletons accumulate to form reef structures. Many reef-building corals live in mutualistic association with microscopic algae called zooxanthellae, which provide energy through photosynthesis and help corals grow in nutrient-poor waters. Coral reefs generally develop best in warm, shallow, clear, sunlit, and well-agitated tropical waters, although deep-water and cold-water reefs also exist. Their structure creates complex habitats that support exceptionally high biodiversity, including fish, mollusks, crustaceans, worms, echinoderms, sponges, tunicates, and other cnidarians. Reefs occupy less than 0.1% of the ocean but provide habitat for at least 25% of marine species. They are also important for fisheries, tourism, and shoreline protection, but are threatened by warming waters, ocean acidification, pollution, nutrient runoff, overfishing, destructive fishing practices, and other human activities.

Key Points

• 1

  Coral reefs are underwater ecosystems formed mainly by colonies of coral polyps and their calcium carbonate skeletons.

• 2

  Corals are cnidarian animals, and many reef-building corals depend on symbiotic zooxanthellae for photosynthetic energy.

• 3

  Reefs thrive mainly in warm, shallow, clear, sunlit, and well-circulated water, though cold-water and deep-water reefs also occur.

• 4

  Despite covering less than 0.1% of the ocean, coral reefs support at least 25% of marine species and provide major ecosystem services.

• 5

  Coral reefs are highly vulnerable to ocean warming, acidification, pollution, nutrient enrichment, overfishing, and destructive human activities.