Coral Reefs

Article | Updated 3 months ago

Coral reef habitats are three-dimensional structures made of plants and animals. Coral reefs are formed as individual coral colonies build hard skeletons (made of calcium carbonate) that support their soft living polyps. Coral reefs provide a place for more corals and other animals to attach, leading to the growth and development of large reef formations.

The living polyps of reef forming corals contain populations of microscopic, single-celled algae known as zooxanthellae. These algae, like other green plants, require sunlight for photosynthesis and, therefore, reef-building corals are typically found in shallow waters. However, in areas where the water is very clear, allowing sunlight to penetrate further, corals can grow at depths of up to 60 metres.

The majority of corals also require warm water to grow and are most common in tropical and sub-tropical waters.

Plants and Animals of Coral Reef Habitats

Organisms that form or live among coral reefs can be divided into two groups. One group consists of species that attach to the seabed, rocks or the reef itself, such as corals, sponges and algae. These are known as sessile organisms and are important, as they provide shelter for a range of other animals to live on or in.

The second group of organisms associated with coral reefs consists of the motile fauna. Such animals have the ability to move around and include certain molluscs (e.g. snails, sea slugs), echinoderms (e.g. sea stars, sea urchins), crustaceans (e.g. crabs, prawns), and fishes. Some of these animals are easily observed, such as fishes, while others are able to hide within the reef or blend into their surroundings (e.g. lobsters).

Methods for Sampling Reef Habitats

Sampling of the plants and animals that live among shallow coral reefs can be done by SCUBA diving or snorkeling. Divers can swim along transects – measuring tapes laid out in a straight line over the reefs – and identify, count or, when an identification cannot be made in the field, collect the individuals of each species they observe. This procedure is useful for sessile animals that are easily observed, such as corals, anemones, and sponges.

For other animals that are motile and/or hide among the corals and plants, counting and visual identification is difficult. To study these animals, it is sometimes necessary to collect pieces of rock or coral and break them open to find the many small animals that are living inside.

Surveying of fishes is best performed by visual identification by experts whilst underwater, as they are fast moving and can hide easily.