Coral and Coral Reef

Adult corals are benthic (bottom-dwelling), sessile (attached) animals usually found in single-species colonies. These colonies may house hundreds or thousands of polyps. The polyps are joined to one another by a thin tissue layer called the coenosarc (pronounced SEE-na-sark). The coenosarc connects the entire coral colony and covers the underlying coral skeleton. Reproduction through an asexual budding process results in development of duplicate daughter polyps and allows for growth of the colony. A single polyp can develop into a massive coral head through multiple budding episodes. Corals also reproduce sexually, producing multitudes of planktonic larvae that ocean currents disperse widely. This allows colonization of suitable habitats, resulting in development of new colonies and new reefs.

Single-celled dinoflagellate algae known as zooxanthellae live symbiotically within coral polyps. Chemical exchanges occur between the coral polyps and zooxanthellae, and both thrive in a mutually beneficial relationship (mutualism). The zooxanthellae, which are essentially tiny green plants that can produce food from sunlight, water, and dissolved minerals, supply some coral species with more than 90% of their nutrition on sunny days. In exchange for nutrients, the coral polyps supply a habitat and essential minerals to the algae. Another result of this relationship is more rapid development of coral reefs. During photosynthesis, the zooxanthellae remove carbon dioxide from the water, which promotes calcium carbonate production, in turn allowing the coral to more easily secrete its home.

In addition to the food provided by their zooxanthellae, corals prey on tiny planktonic organisms. Some corals paralyze their prey using stinging cells, or nematocysts, located on their tentacles. Other corals feed by creating weak water currents with cilia to draw food into their mouth, or by producing sticky mucus with which to trap tiny planktonic animals. Most species feed at night; during the day, they retract into their corallites for protection. The members of the colony share nutrients by passing them to their neighbors through the coenosarc.