Marine life is the plants, animals, and other organisms that live in the salt water of seas or oceans, or the brackish water of coastal estuaries. In the context of this website, marine life will also include aquatic freshwater life, such as freshwater fishes. At a fundamental level, marine life affects the nature of the planet. Marine organisms, mostly microorganisms, produce oxygen and sequester carbon. Marine life in part shape and protect shorelines, and some marine organisms even help create new land (example, reef building corals).
Most terrestrial life forms evolved initially in marine habitats, and by volume, the oceans provide about 90% of the living space on the planet. The earliest vertebrates appeared in the form of fish, which live exclusively in water. Some of these evolved into amphibians, which spend portions of their lives in water and portions on land. One group of amphibians evolved into reptiles and mammals and a few subsets of each returned to the ocean as sea snakes, sea turtles, seals, manatees, and whales. Plant forms such as kelp and other algae grow in the water and are the basis for some underwater ecosystems. Plankton forms the general foundation of the ocean food chain, particularly phytoplankton which are key primary producers.
Marine invertebrates exhibit a wide range of modifications to survive in poorly oxygenated waters, including breathing tubes as in mollusc siphons. Fish have gills instead of lungs, although some species of fish, such as the lungfish, have both. Marine mammals (e.g. dolphins, whales, otters, and seals) need to surface periodically to breathe air.
As of 2023, more than 242,000 marine species have been documented, and perhaps two million marine species are yet to be documented. An average of 2,332 new species per year are being described.
Marine taxonomic groups
