It is estimated that around 70 percent of the earth's surface is covered by water. The Pacific, Atlantic, Indian and Arctic oceans are the largest bodies of water on earth. Oceans play a key role in maintaining the health of the planet and ensuring the survival of mankind. Not only do they influence weather systems, the oceans also contain life in abundance. Diverse and complex ecosystems house a vast array of ocean organisms including mammals, fish, crustaceans, sessile animals and microorganisms.
Marine Mammals
Marine mammals--such as seals, sea lions, walruses, manatees, dugongs, whales, dolphins and porpoises--are among the largest animals in the oceans. Marine mammals are classified as vertebrates; that's the classification name for animals that have a backbone. Like land mammals, marine mammals are warm-blooded, deliver live young, produce milk and have hair or fur. Marine mammals also obtain their oxygen from the air using lungs. However, while some marine mammals can dive very deep and hold their breath underwater for long periods of time, they must frequently return to the surface to breathe.
Fish
Thousands of species of fish populate the earth's oceans. Fish appear in all shapes, sizes and colors. Some well-known species include clown fish, blow fish and parrot fish. Many species--including tuna, catfish, salmon and trout--serve as a major food source for humans. Like mammals, fish have skeletons that are made of bone. Instead of a bony skeleton, however, some species of fish--such as sharks, rays and skates--have skeletons made of a strong substance known as cartilage. However, unlike mammals that use lungs to breathe, fish obtain oxygen directly from the water using gill slits.
Crustaceans
Crustaceans are a type of arthropod. They are also plentiful in the oceans. Well-known species include crabs, lobsters, krill, barnacles, crayfish and shrimp. Like fish, crustaceans also obtain oxygen using gills; they serve as a major food source for humans. Unlike mammals and fish, crustaceans are invertebrates; that's the term for organisms without a backbone. Instead of an internal skeleton, crustaceans are equipped with a hard external skeleton made of chitin that protects their internal organs. In a process called molting, crustaceans shed and replace their exoskeletons as they grow.
Sessile Animals
Many ocean organisms are motile and have the ability to swim freely through the waters or move along the ocean floor. Sessile organisms, however, cannot move about freely and are confined to a fixed location. Sessile animals include sponges, corals and barnacles. Some of these animals are sessile throughout their entire lives. Others may be sessile in either their larval or adult stage, and they may be motile in the other. While they may appear vulnerable, these organisms have unique adaptations that help them survive. To obtain food, some sessile animals strain particles from the water in a process called filter feeding. To defend themselves against predators, they often contain toxic substances or stinging cells.
Microorganisms
Microorganisms are the most plentiful organisms in the oceans. Some of these microorganisms can be harmful to both humans and marine life. Others are essential to the health of the oceans and their inhabitants. When animals die in the oceans, bacteria help to decompose the bodies and return nutrients back into the environment. Certain species of bacteria also enable the bioluminescence exhibited by some ocean organisms. Plankton are microorganisms that serve as the foundation of the marine food chain. They are a major source of food for many ocean organisms.
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