The biggest animal in the whole world lives in the ocean. It's the blue whale. In fact, it's the biggest animal that has ever lived. It's bigger than any dinosaur! Some of the world's smallest animals live in the ocean, too. Zooplankton is made up of many kinds of very tiny animals. They drift on the ocean currents. Some kinds of zooplankton are larvae. They will turn into bigger animals like sponges or fish. Some kinds of zooplankton are tiny animals made of only one cell. They will not turn into something bigger. Some kinds of zooplankton are tiny crustaceans. Crustaceans are
Fish and other animals in the ocean breathe oxygen and give off carbon dioxide (CO2), just like land animals. Ocean plants take in the carbon dioxide and give off oxygen, just like land plants. The ocean is great at sucking up CO2 from the air. It absorbs about one-quarter of the CO2 that we humans create when we burn fossil fuels (oil, coal, and natural gas.) If not for the ocean, we'd be in even worse trouble with too much CO2.
Habitats are places where plants and animals live. There are many different habitats in the oceans of the world. Every plant and animal in the ocean prefers a particular habitat. There are a wide variety of plants and animals that live in the different ocean habitats. Most of the life in the ocean is found in the upper 300 feet of water. This is as far as the sun’s light can reach so no plants live below this depth. Without plants or algae to form the base of the food chain, there isn't much to eat way down there. But patches around active volcanoes and vents at the bottom of the ocean have even created deep-water habitats that teem with life.
Nekton are the free swimmers and probably the largest portion of familiar animals found in the ocean belong to this class. Common fishes, the octopus, whales, eels and squid are all examples of nekton. The nekton category includes a number of very diverse creatures. The whale, dolphin and porpoise are certainly very different from codfish or trout because whales represent sea mammals whereas cod are true fishes.