Coral Reefs

Emperor angelfish (Pomacanthus imperator) at a cleaning station amongst soft corals (Dendronephthya sp), where small cleaner shrimps operate. Egypt, Red Sea.
Georgette Douwma/Photographer's Choice RF/Getty Images

There are hundreds of coral species found in the world’s oceans. There are two types of corals: hard, or stony, corals and soft corals, such as sea fans and gorgonians. Only hard corals build reefs.

While the majority of coral reefs are found in tropical and sub-tropical water within the latitudes of 30 degrees north and 30 degrees south, there are also deep water corals in colder regions.

Formation of Coral Reefs

The builders of coral reefs are tiny animals called polyps, which are classified in the phylum Cnidaria.

The polyps feed with tentacles, which they extend at night to catch tiny floating animals, or zooplankton.

As these polyps thrive, grow, then die, they leave their limestone (calcium carbonate) skeletons behind. The limestone is colonized by new polyps. Therefore, a coral reef is built up of layers of these skeletons covered ultimately by living polyps.

The reef-building or hermatypic corals can form a wide range of shapes. Coral reefs may be branched, table-like, or look like massive cups, boulders or knobs.

Three Major Types of Reefs

  • Fringing reefs, which grow close to the coast in shallow waters.
  • Barrier reefs, which are large, continuous and are separated from land by a lagoon (the Great Barrier Reef is the largest example of a barrier reef.)
  • Atolls, which are ring-shaped and located near the sea surface on top of underwater islands or inactive volcanoes.

Both tropical and cold-water corals can form reefs.

Tropical Coral Reefs

Tropical corals are inhabited by small algae called zooxanthellae. These algae provide the coral's brilliant color and use sunlight to perform photosynthesis, providing the coral with necessary oxygen. The algae's need for the sun's energy is the reason these types of corals are found in shallower water that can be penetrated by sunlight.

A flourishing tropical reef is made up of many different plant and animal communities. It is estimated that 800 different coral species are involved in building tropical reefs.

Tropical coral reefs, in particular, are important for recreation, tourism and as a food source for not only their marine inhabitants but humans. The largest and most well-known example of a tropical reef is the Great Barrier Reef in Australia.

Cold Water Corals

The more well-known tropical reefs are found in water ranging in temperatures from about 68-84 degrees Fahrenheit. Cold-water corals inhabit deep, cold (39-55 degrees F), water.

Less is known about these corals than their tropical counterparts, but the United Nations Environment Programme reports that there are more cold-water coral reefs worldwide than tropical reefs.

There are only about 6 different coral species associated in building with these reefs. The largest cold-water coral reef is the Røst Reef off of Norway.

Marine Life in a Coral Reef

Coral reefs are important habitats. They are thought to support more than 1 million aquatic species. This includes not only several hundred species of coral but thousands of fish and invertebrate species such as sponges, crabs, shrimps, lobsters, sea anemones, bryozoans, worms, sea stars and sea urchins, octopuses, squid, snails, and nudibranchs.

It is estimated that nearly one-quarter of the world’s marine species (Source: Coral Reef Alliance) are sustained by the shelter and food provided by coral reefs.)

Threats to Coral Reefs

Coral reefs are greatly impacted by humans, but they also face natural threats. Types of each of these threats to coral reefs are discussed below.

  • Natural Threats: Natural threats to reefs include damage from weather events such as El Nino or prolonged rain or cold, and predation. The crown-of-thorns starfish is one avid predator that consumes corals and, in the right conditions, can decimate a reef.
  • Water Pollution: Pollutants such as oil, gas or pesticides can contaminate reefs. Human or animal wastes and fertilizers that runoff into the ocean can increase nitrogen levels, causing algae blooms and suffocation of the reef. Trash can cover reefs or affect reef inhabitants who may get entangled in or swallow the trash.
  • Sedimentation: particles from land-based construction and other activities can run off from the shore, cover coral reefs and deprive them of light.
  • Coastal Development and Tourism: As more people move to coastal areas and tourism increases, pollution, and visitation to the reefs will increase. In some areas, reefs have been damaged by construction of piers or other structures on top of them.
  • Fishing: There are fishing techniques that can harm coral, such as cyanide fishing (divers squirt cyanide on the reef to stun fish, which are then caught for the tropical fish trade or for consumption) and blast fishing (fishermen use explosives, which destroy the coral). Overfishing damages fish populations and disrupts the balance of organisms in the environment. Coral reefs can also be damaged by bottom trawling by commercial fishermen, which is a threat to cold-water corals.
  • Mining: Coral is removed to sell as souvenirs or jewelry or to use in road or brick construction.
  • Climate Change: Corals are sensitive to temperature changes. An increase in water temperature can cause coral bleaching, in which the algae living in coral polyps are destroyed, leaving the coral with a white appearance. According to NOAA’s Coral Reef Watch program, incidences of coral bleaching have been on the rise since the late 1990’s, with a record-breaking bleaching event occurring in the Caribbean in 2005.
  • Carbon Dioxide: Increases in carbon dioxide results in coral with weaker skeletons, making them more vulnerable to their environment and human impacts.