seadwelling

Sea-dwelling, or marine, refers to organisms that live in saltwater environments, from shallow coastal zones to the deep ocean. Marine life is categorized by habitat: pelagic (open water) and benthic (sea floor); and by zones: photic (well-lit) and aphotic (dark). Common habitats include coral reefs, kelp forests, seagrass beds, abyssal plains, and hydrothermal vent communities.

Organisms range from microscopic phytoplankton and zooplankton to large vertebrates such as whales and sharks, and to invertebrates like corals, mollusks, crustaceans, and sea urchins. Plants and algae, including seagrasses, macroalgae (kelp), and phytoplankton, form the basis of many marine food webs, converting sunlight into chemical energy and supporting higher trophic levels.

Adaptations vary: gills in fish, air-breathing lungs in marine mammals, salt regulation mechanisms, streamlined bodies and fins for efficient swimming, buoyancy control via swim bladders or lipids, and in deep-sea species, bioluminescence and slow metabolisms.

Ecologically, sea-dwelling organisms sustain global biodiversity, coastal protection, nutrient cycling, and fisheries. Human activities such as overfishing, habitat destruction, plastic pollution, and climate change threaten marine ecosystems through warming, acidification, deoxygenation, and loss of habitats like coral reefs and mangroves.

The term emphasizes organisms tied to the ocean’s salty milieu, spanning a wide range of life forms and ecological roles beyond freshwater or terrestrial ecosystems.