Partners for Life

February 1, 2007
By Chris Shank

Corals are the fundamental building blocks for spectacular reefs that decorate tropical seas throughout the world.A single coral head is actually a colony of individual organisms, called polyps, and each polyp has its own stony skeleton which is joined to its neighbor’s skeleton. Together, the colony forms a massive outcropping on the sea floor that functions as a single animal.


Coral colonies can grow in the tropics or in the deep, cold oceans. However, only in shallow, sunlit waters do corals share their life with another organism in a unique relationship known as symbiosis. In every symbiotic relationship the livelihood of the two species is intricately connected.

Shallow-water corals share their lives with tiny plant-like organisms called zooxanthellae which live inside the polyps. Just like plants on land, these zooxanthellae use light and carbon dioxide to produce energy. Carbon dioxide is a waste product that polyps produce during respiration, so the zooxanthellae recycle this waste to produce energy. In return, they release nutrients which the polyps consume.

Stressful environmental conditions, such as high water temperature or exposure to excessive ultraviolet light, can disrupt this symbiotic give-and-take causing the polyps to expel the colorful zooxanthellae. This exposes the coral’s stony skeleton making the reef look white — a process called “coral bleaching.” If the polyps cannot reacquire zooxanthellae, bleaching can kill the entire colony.

As the Earth’s climate changes and the oceans warm, it is important to understand how corals will respond, not just for their own sake, but because healthy coral reefs provide essential habitat for an incredibly diverse array of marine life.

copyright 2007, The University of Texas Marine Science Institute