What is a Coral?

Summary

• Coral polyps are tiny animals that build protective calcium carbonate skeletons.
• Coral polyps create the basic structure of coral reefs with the help of single-celled algae.

Coral reefs can be seen from space, but they are made by one of the world’s simplest animals - some no bigger than the head of a pin.  They are able do this because of their symbiotic relation with single-celled algae.

Tiny reef builders

Every coral, from small free-living individuals to huge colonies, has the same basic body plan: 

• Coral polyp – the living animal, basically a sack with a stomach and a mouth surrounded by retractable, stinging tentacles. These tentacles are used to catch food.
• Corallite - a hard calcium carbonate shell that protects the polyp. 

Reef-building corals are colonial and grow into distinctive shapes formed when many individual polyps growing together. These colonies form the basic structure of a coral reef.

Corallites erode over time; they are broken down and wash into the shore where they create beaches.  A healthy coral reef is one where the loss of material to erosion is balanced by the growth of new coral. The reef grows when the growth rate exceeds the erosion rate. See "Why protect your own reef?" for more information.

Coral's little helpers - zooxanthellae

Reef building coral are successful because they have formed a symbiotic relationship with zooxanthellae algae (Symbiodinium species).  These single-celled algae live inside the coral polyp and producing food by photosynthesis.  The polyp receives some of this food. In return the polyp provides shelter and nutrients to the algae. This gives reef-building coral two sources of food; the food they catch, and food from zooxanthellae.   Zooxanthellae supply up to 95% of a polyp’s food requirements and it is this extra food that enables coral to build reefs.

The bright colors seen in tropical coral are primarily derived from pigments in zooxanthellae algae. Zooxanthellae are susceptible to changes in their environment.  When environmental conditions change, like rising water temperatures, zooxanthellae can die, resulting in a loss of pigment in the coral.  This is called "coral bleaching", because the colony turns white.  Sometimes, after bleaching has occurred, corals can be recolonized by zooxanthellae and recover, but often they die as a result of this bleaching (see "Coral bleaching" for more information).

Coral reefs are found in clear tropical waters that are typically low in nutrients. The relationship between coral and zooxanthellae concentrates what nutrients there are and allows them to be used more efficiently by both organisms.

They work together in the following way to convert nutrients:

1. Coral polyps capture food using their stinging tentacles.
2. The digest their prey and create waste.
3. Zooxanthellae convert the nutrients in the coral's waste products into food.
4. Zooxanthellae give some of this food to the polyp.
5. Polyps consume this food and create more waste.
6. Repeat.

This efficient cycling of nutrients provides the coral with enough food to build the reefs which are the foundation for this productive and diverse ecosystem. Any threats to this relationship ultimately threaten this diversity.
 

References

Anderson G. (2003, June 30). The coral animal. MarineBio.net. Retrieved 29 August 2008 from http://www.marinebio.net/marinescience/04benthon/crani.htm

Coral Reef Alliance. (2008). Coral reef overview. Coral Reef Alliance. Retrieved 29 August 2008 from http://www.coral.org/resources/about_coral_reefs/coral_overview#whatpolyp

Jaap, W. (2007). Coral and coral reefs. Water encyclopedia. Retrieved 29 August 2008 from http://www.waterencyclopedia.com/Ce-Cr/Corals-and-Coral-Reefs.html

NOAA. (2008, March 25). What are corals? NOAA ocean education service. Retrieved 29 August 2008 from http://oceanservice.noaa.gov/education/kits/corals/coral01_intro.html

NOAA. (2008, March 25). Zooxanthellae… What's That? NOAA Ocean Service Education. Retrieved 29 August 2008 from http://oceanservice.noaa.gov/education/kits/corals/coral02_zooxanthellae.html

Romano, S. & Cairns, S. (2002, October 28). Scleractinia. Tree of life web project. Retrieved 8 September 2008from http://www.tolweb.org/Scleractinia

Vernon, J. (2000). Corals of the world, volume 1. Australian Institute of Marine Science: Townsville.