Plankton, nekton and benthos are the three groups into which aquatic living beings may be divided. The plankton is formed by the algae and small animals that float near the water surface carried by the stream. The nekton is composed of animals that actively swim and dive in water, like fishes, turtles, whales, sharks, etc. The benthos comprises the animals ecologically linked to the bottom, including many echinoderms, benthonic fishes, crustaceans, molluscs, poriferans and annelids.