III.

How Corals Obtain Food

All corals capture at least some of their food with their tentacles. However, because they cannot travel, they do not actively seek prey. For the most part they rely on microscopic particles from the plankton floating or swimming past their tentacles. The nematocysts are then rapidly fired to hold and kill their prey.

Many corals obtain most of their food energy from a close relationship with algae known as zooxanthellae. These tiny algae resemble simple plants. Like all plants they use photosynthesis to transform energy from sunlight into sugars and other compounds. The zooxanthellae live within the cells of the polyps in incredibly large numbers. Typically about 1 million algae live in each square centimeter of coral tissue. Both the algae and the corals benefit considerably from this symbiotic relationship. The algae can thrive without the danger of being eaten, and they make use of the waste products of the coral. The corals, in turn, use the excess sugars and other compounds produced by the algae as a source of food. Some corals get more than 80 percent of the food they need directly from these algae, enabling the corals to grow quickly and to thrive in areas where little food is available in the surrounding waters.