Oceans and Marine Life
The sea contains countless life forms found nowhere else on Earth. Biologists classify ocean life into three main groups: the plankton (floaters); the nekton (swimmers); and the benthos (creatures on the sea floor). Plankton drift passively with ocean waves and currents. Many organisms of the plankton are microscopic, including a diverse group of tiny algae known as phytoplankton. These microbes carry out photosynthesis, the use of energy from the sun to combine carbon dioxide and water to make nutrients called carbohydrates.
Small drifting or weakly swimming animals called zooplankton are the primary consumers of phytoplankton. Microscopic shrimplike crustaceans (shelled animals with jointed legs), tiny worms, small jellyfish, and various larvae (immature forms) of larger animals, such as clams, are some of the many types of zooplankton.
The nekton consists of strong swimmers that can propel themselves against currents, waves, and tides. Nektonic animals include most of the marine species that people usually associate with the ocean, including fish, whales, and seals. Most animals of the nekton are streamlined and muscular, with bodies adapted for swimming and diving.
The creatures of the benthos burrow into sediments at the bottom of the sea, attach themselves to rocks and reefs, or move about on or near the ocean floor. Sea anemones, barnacles, oysters, lobsters, snails, sole, and halibut are all part of the benthos. Scientists divide the benthic realm into two subdivisions--the littoral (shallow) and deep-sea benthos.
The ocean's diversity of life is particularly rich in littoral waters, such as tide pools. Many animals in these shallow waters, including sea anemones and barnacles, are passive feeders, organisms that take in food particles carried to them by currents.
In most parts of the deep-sea floor, life is scarce, except for some small types of fish and a few simple organisms, including sponges and crinoids (sea lilies). However, there is a variety of deep-sea benthic life near hydrothermal vents, chimneylike structures along mid-ocean ridges from which hot water and minerals flow. At these sites, microorganisms use the ejected minerals to grow and reproduce, while other creatures eat the microbes or feed on nutrients produced by them. These oases of life in the otherwise barren desert of the deep consist of such animals as worms up to 3.7 meters (12 feet) long that live in tubes and clams as large as 30 centimeters (1 foot) across.
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