What living organisms are found in any one place in the ocean is determined by the environment at that place: temperature, pressure, and the amounts of light, salt, and turbulence in the water. Marine biologists have classified ocean-dwelling organisms into three main types: plankton, those that do not swim, or swim weakly; nekton, those that swim; and benthos, those that are bottom-dwelling.
Plankton and nekton live in the pelagic zone; that is, away from the ocean bottom and shore. Most pelagic organisms are found over the continental shelves, where food is plentiful. The pelagic zone is also the habitat of certain birds, such as the albatross and storm petrel, and the small group of marine insects that includes the marine water strider.
Benthos live on the bottom in zones determined by depth; in the intertidal, or littoral, zone (from the high-tide line to the low-tide line); in the sublittoral zone (from the low-tide line to about 650 feet [200 m] deep); in the bathyal zone (from about 650 feet deep to about 9,840 feet [3,000 m] deep); in the abyssal zone (from about 9,840 feet deep to about 19,680 feet [6,000 m] deep); and in the hadal zone (below 19,680 feet).
Organisms living in the intertidal zone are adapted to being repeatedly exposed to air and to the action of waves. Most marine life is found in the sublittoral zone on the continental shelves, where there is enough light for photosynthesis. In deeper zones there is little or no light. Animals in these zones have such adaptations as light-producing organs, high sensitivity to pressure and vibrations (especially in blind forms), and very sensitive eyes.
These organisms are drifters, carried from place to place by wind, waves, and current. Phytoplankton consists mainly of single-celled algae, such as diatoms, peridinians, and coccolithophores. These organisms make their food by photosynthesis.
Zooplankton includes a wide variety of microscopic organisms—protozoans, such as foraminifers and radiolarians; crustaceans, such as copepods, cyclops, and water fleas; and rotifers. Zooplankton also includes such larger organisms as argonauts.
Organisms that are zooplankton their entire life are called holoplankton. Those that are zooplankton for only a part of their life are called meroplankton. The eggs and larval forms (young) of many animals, including sponges, tunicates, and fish, live as plankton for the first few weeks or months of their lives and then develop into nekton or benthos.
Most zooplanktonic organisms obtain their food by filtering other plankton or organic debris from the water.
Among the nekton are large, complex animals. Fish of all varieties are a major part of this group. Other members are reptiles, such as sea turtles and sea snakes; and mammals, such as seals, whales, and porpoises. Invertebrates, such as jellyfish, sea worms, squids, shrimps, and scallops, complete the group. Animals of the nekton feed on other animals and on seaweeds, plankton, and floating organic matter.
Bottom-dwelling organisms form the most varied group in the ocean. Because areas of rocky bottom afford a greater variety of living places than do flat, sandy areas of the ocean floor, there are many more species living among rocks than in flat, sandy areas.
Many types of seaweeds are found in the benthos. They are most numerous in shallow water, where light and nutrients are readily available. Most seaweeds grow attached to rocks. Seaweeds found floating in the ocean or washed up on beaches generally have been torn from rocks by waves.
There are four main groups of benthic animals: burrowing animals, sessile (attached) animals, crawling animals, and swimming animals.
Burrowing Animals live in the sand or mud. Many types of sea worms and a few mollusks are burrowers. These animals feed chiefly by creating currents of water through their bodies. They filter small organisms and organic debris from the water as it passes through.
Sessile Animals form a varied group. Among the more common sessile animals are sponges and corals. Other sessile animals are bryozoans (the moss animals), mollusks, barnacles, and tunicates. They are largely dependent upon currents and waves to bring them food. The sessile life is concentrated in shallow water, on the upper portions of submerged reefs, and in places where currents constantly renew the food supply.
Crawling Animals include snails, chitons, and sea anemones, all of which slide along on a foot. Starfish and sea urchins walk on tube feet. Lobsters and crabs walk on legs. Other crawling animals, such as clams, pull themselves along through the sand with a foot. Animals of this group catch other animals eat algae or debris, or filter their food from the water.
Swimming Animals, or Nektobenthos, live on, but are not confined to, the bottom of oceans. In this group are fish and octopuses, which live among seaweeds, coral, and other sessile organisms. Many of these swimmers have bright coloration that blends in with the surroundings. These animals catch other animals or eat algae or organic debris. Demersal zooplankton are animals, such as opossum shrimp, that are bottom-dwellers part of the day and drifters or swimmers the rest.
The survival of any organism in the ocean depends on its interrelationships with other organisms in regard to food. Such interrelationships form what is known as the food web. There are four primary parts in a food web:
- Nonliving Components, including sunlight and the oxygen, nitrogen, and organic compounds suspended in the water.
- Producers, consisting of phytoplankton and seaweeds. They produce their own food from the nonliving components through the process of photosynthesis.
- Consumers, the animals that feed on the producers.
- Decomposers, the organisms, such as bacteria and fungi, that break down the dead producers and consumers and return the raw materials to the nonliving parts of the food web.
The relationships of ocean organisms to each other can also be likened to a pyramid. At the bottom of the pyramid are the multitudes of microscopic organisms that provide food for a smaller number of larger organisms, which in turn provide food for a stillmaller number of yet larger organisms. At the top of the pyramid are the large fish, such as sharks, and other large sea-dwelling animals, such as whales.
At the beginning of the Paleozoic Era, about 570,000,000 years ago, all of the phyla of the invertebrates were present. Fossil evidence shows that they had developed extensively in saltwater before this time. The earliest vertebrates, which were types of jawless fish, lived in saltwater about 500,000,000 years ago.
The most widely distributed group of fishes, the bony skeletoned fishes, is primarily marine. Among this group are the main food and game fishes, such as trout, mackerel, herring, and perch; and many oddshaped fish, such as marine sunfishes, globefishes, flying fishes, sea horses, and eels.
The fish ancestors of marine reptiles and mammals were freshwater fish that became adapted to drought conditions. These adaptations eventually led to the development of land animals. Turtles and snakes that lived on land are the ancestors of sea turtles and sea snakes. The remote ancestors of whales and porpoises are ancient land herbivores. Seals and walruses have evolved more recently from land carnivores. Sea cows (dugongs and manatees) share a common ancestor with the modern elephant.
As a source of food the ocean has been invaluable to humans for thousands of years. The animals that constitute the main sources of food are the ones that are most readily available: those that live on or near the shore and over the continental shelves.
In recent centuries, fishing for pelagic animals has become increasingly important. The bony fishes, such as cod, salmon, tuna, and flounder, are the most important food fishes. Other food animals are mollusks, such as clams and oysters; and crustaceans, such as crabs, lobsters, and shrimps. Food and food additives are made from seaweeds such as kelp, agarweed, laver, Irish moss, and dulse.