Marine habitats can be as different as night and day. The deepest ocean, where hydrothermal vents are located, is perpetually dark. In contrast, the vivid coral reefs nearer the surface are utterly dependent on sunlight. Habitats near shore are different from those in mid-ocean, and the seafloor hosts different communities than the open waters. As in freshwater biomes, the seafloor is known as the benthic realm. In shallow areas, such as the submerged parts of continents, called continental shelves, the photic zone includes both pelagic and benthic regions. In these sunlit areas, photosynthesis by phytoplankton and multicellular algae provides energy for a diverse community of animals.

Sponges, burrowing worms, clams, sea anemones, crabs, and echinoderms inhabit the benthos. The coral reef biome occurs in the photoc zone of warm tropical waters in scattered locations around the globe. A coral reef is built up slowly by successful generations of coral animals a diverse group of cnidarians that secret a hard external skeleton by multicellular algae encrusted with limestone. The photoc zone extends down a maximum of 200 m in the ocean. There is not enough light for photosynthesis between 200 and 1,000 m. Then the twilight zone is dominated by a fascinating variety of small fishes and crustaceans. Some fishes in the twilight zone have enlarged eyes, enabling them to see in the very dim light and light-emitting organs that attract mates and prey. The marine environment also includes distinct biomes where the ocean interfaces with land or with fresh water. In the intertidal zone, where the ocean meets land, the shore is pounded by waves during high tide and exposed to the sun and drying winds during low tide. The rocky intertidal zone is home to many sedentary organisms, such as algae, barnacles, and mussels, which attach to rocks and are thus prevented from being washed away.

Then an estuary is a transition area between a river and the ocean. The saltiness of estuaries ranges from near that of fresh water to that of the ocean. With their waters enriched by nutrients from the river, estuaries, like freshwater wetlands, are among the most productive areas on Earth. Oysters, crabs, and many fishes live in estuaries or reproduce in them. Estuaries are also crucial nesting and feeding areas for waterfowl. For centuries, people viewed the ocean as a limitless resource, harvesting its bounty with increasingly effective and indiscriminate technologies and using it as a dumping ground for wastes. The negative effects of these practices are now becoming clear. Populations of commercial fish species are declining. Then coral reefs are imperiled by ocean acidification and rising sea surface temperatures due to global warming. Finally, in 2010 the census of marine life announced the discovery of more than 6,000 new species.