The Biology of Coral Reefs

Thousands of coral species inhabit the world's oceans, however, only those belonging to the phylum 'Cnidaria' build reefs. Reef building corals, called hermatypic corals, live in large colonies. The individual coral is a tiny animal with a tubular body and an oral gap fringed with tentacles. The tentacles of the individual coral 'polyp' are equipped with nematocysts, poisonous cells that sting its prey. Most corals feed at night on small marine organisms ranging in size from zooplankton to small fish. (Barnes & Hughes, An Introduction to Marine Ecology vol III, 1999). However, the majority of the coral's energy requirements are provided by photosynthetic organisms that live in its tissue, called zooxanthellae. Zooxanthellae supply the coral polyps with oxygen and food, and are responsible for the colour of the corals. In return, the corals provide a protected living area for the zooxanthallae. The coral 'reef' slowly builds itself from calcium carbonate, which is the by-product of hermatypic coral growth. Text courtesy of IOC

The symbiotic relationship that exists between the coral polyps and the zooxanthellae in their tissue results in coral colonies having characteristics of both plants and animals. The majority of reef building corals are sessile and flourish in clear, warm, nutrient poor (oligotrophic), saline water. Access to light is necessary for the zooxanthellae to photosynthesize, and therefore, fundamental to healthy coral reefs. The incredible efficiency of nutrient cycling among hermatypic corals allow them to grow in nutrient poor waters (which are relatively free from suspended particles that could block necessary light), yet remain one of the Earth's most highly productive ecosystems. The density and distribution of corals on the sea floor has an inverse relationship with the depth of the water, and is directly proportional to the rate of light extinction. Hermatypic corals also engage in inter-species competitive behavior for territory that can be either passive or aggressive. Text courtesy of IOC
For more information visit http://www.coris.noaa.gov/about/biology/biology.html