The term the food chain describes the fact that each living creature survives by feeding on plants or other animals. Plants are always the base of the food chain. The animals that eat the plants are one link up the food chain. When the plant-eating animal is killed and eaten by another animal, it is said that the animal who is eating is higher on the food chain than the animal being eaten. In the ocean, the base of the food chain is phytoplankton, or algae, plants that live near the surface of the water (to get maximum sun). The term "plankton" comes from the Greek word "planktos" which means "drifting." Phytoplankton range from microscopic organisms to sea weed. Phytoplankton are eaten by small fish and by zooplankton, a class of plankton-eating microscopic animals that includes single celled animals, larvae of larger animals, and tiny crustaceans. The zooplankton are then eaten by small fish and some whales. The small fish are eaten by larger fish and those are eaten by even larger fish and so on up the food chain. A species is at the top of its food chain if there are no animals who kill and eat it regularly. For example sharks, lions, human beings and elephants are said to be at the top of their food chains. Whales were at the top of their food chain until man started to hunt and kill them.