Coral Reefs (Rainforests of The Sea)

Coral reefs are large underwater structures composed of the skeletons of colonial marine invertebrates called coral. The coral species that build reefs are known as hermatypic, or "hard," corals because they extract calcium carbonate from seawater to create a hard, durable exoskeleton that protects their soft, sac-like bodies. Other species of corals that are not involved in reef-building are known as “soft” corals. These types of corals are flexible organisms often resembling plants and trees and include species such as sea fans and sea whips.

Each individual coral is referred to as a polyp. Coral polyps live on the calcium carbonate exoskeletons of their ancestors, adding their own exoskeleton to the existing coral structure. As the centuries pass, the coral reef gradually grows, one tiny exoskeleton at a time, until they become massive features of the marine environment.

Corals are animals and not plants. Corals are known as colonial organisms because many individual creatures live and grow while connected to each other. They are also dependent on one another for survival. Coral polyp bodies are usually clear. The bright colors that characterize many corals are actually various types of algae growing in the polyp’s tissue. The presence of the algae, specifically a type of algae called zooxanthellae, helps the coral in several ways. For one, the algae remove waste from the coral. The algae also use the coral’s waste products for photosynthesis, which is how a plant makes its own food. Byproducts of photosynthesis include oxygen and carbohydrates, which the coral consumes and uses to build reefs. The mutually beneficial relationship between coral and algae is called symbiosis.

Corals reproduce both sexually and asexually. Externally fertilized eggs develop during synchronized spawning. Polyps across a reef simultaneously release eggs and sperm into the water en masse. The release of eggs or planula usually occurs at night. During this process, the larvae may use several different cues to find a suitable location for settlement.

Coral reefs are sometimes known as the “rainforests of the sea.” Nearly a quarter of all the fish in the sea rely on healthy coral reefs. Corals provide habitats for fish and other organisms in the ocean. Reefs are home to a variety of animals, including fish, seabirds, sponges, cnidarians (which includes some types of corals and jellyfish), worms, crustaceans (including shrimp, cleaner shrimp, spiny lobsters, and crabs), mollusks (including cephalopods), echinoderms (including starfish, sea urchins, and sea cucumbers), sea squirts, sea turtles and sea snakes. Aside from humans, mammals are rare on coral reefs, with visiting cetaceans such as dolphins the main exception. A few species feed directly on corals, while others graze on algae on the reef. Reef biomass is positively related to species diversity. The shelter created by these coral colonies makes reefs a vibrant biodiversity hotspot where coral, fish, algae, and hundreds of other species live together in a bustling ecosystem.

Coral reefs form some of the world's most productive ecosystems, providing complex and varied marine habitats that support a wide range of other organisms. Fringing reefs just below low tide level have a mutually beneficial relationship with mangrove forests at high tide level and seagrass meadows in between the reefs protect the mangroves and seagrass from strong currents and waves that would damage them or erode the sediments in which they are rooted, while the mangroves and seagrass protect the coral from large influxes of silt, freshwater, and pollutants. This level of variety in the environment benefits many coral reef animals, which, for example, may feed in the seagrass and use the reefs for protection or breeding.

Coral reefs deliver ecosystem services for tourism, fisheries, and shoreline protection. Coral reefs are fragile, partly because they are sensitive to water conditions. They are under threat from excess nutrients (nitrogen and phosphorus), rising temperatures, pollution, oceanic acidification, collecting live corals for the aquarium market, mining coral for building materials, overfishing (e.g., from blast fishing, cyanide fishing, spearfishing on scuba), sunscreen use, and harmful land-use practices, including runoff and seeps (e.g., from injection wells and cesspools), and a warming climate are some of the many ways that people damage reefs all around the world every day.

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