The term "coral reef" generally refers to a marine ecosystem in which the main organisms are corals that house algal symbionts, called zooxanthelae, within their tissues. The algae photosynthesize and share this energy with the corals in exchange for protection and access to light. Coral reefs are some of the most diverse ecosystems in the world and are therefore often refered to as the "rainforests of the ocean".  Because coral reefs are sensitive to such factors as water temperature, salinity and other chemical and physical factors, they can serve as indicators of environmental changes.

Image: Nick Hobgood