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Marine biology is the scientific study of organisms in the ocean or other marine or brackish bodies of water. Given that in biology many phyla, families and genera have some species that live in the sea and others that live on land, marine biology classifies species based on the environment rather than on taxonomy. Marine biology differs from marine ecology as marine ecology is focused on how organisms interact with each other and the environment, and biology is the study of the organisms themselves. A large proportion of all life on Earth exists in the oceans. Exactly how large the proportion is unknown, since many ocean species are still to be discovered. Oceans cover about 71% of the Earth's surface, and because of their depth they contain about 300 times the habitable volume of the terrestrial habitats on Earth. The habitats studied in marine biology include everything from the tiny layers of surface water in which organisms and abiotic items may be trapped in surface tension between the ocean and atmosphere, to the depths of the oceanic trenches, sometimes 10,000 meters or more beneath the surface of the ocean. Specific habitats include coral reefs, kelp forests, seagrass meadows, tidepools, muddy, sandy and rocky bottoms, and the open ocean zone, where solid objects are rare and the surface of the water is the only visible boundary. The organisms studied range from microscopic phytoplankton and zooplankton to huge cetaceans 30 meters in length. (via Freebase)