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Corals, Sponges and Algae
Any sessile organism requires a firm foundation on which to root itself. For stationary corals, sponges and algae, that foundation is the sea floor, marking these creatures as benthic organisms. These benthos live almost exclusively from the intertidal zones to the drop-off of the continental shelf; photosynthesizing algae need sufficient light to create energy, while corals and sponges require the more abundant supply of plankton and krill in shallower waters. Coral reefs eventually raise the sea floor itself, but these reefs start with benthic organisms.
Scavengers
Everything that lives and dies in the oceans produces waste of some sort; these leftovers and remains constitute a rich food supply for benthic scavengers, such as crustaceans, tube worms and echinoderms. These organisms thrive on whole carcasses of large animals or on "marine snow," the tiny particles of detritus that constantly sift down from the upper portions of the water column. Scavengers feed along the sea bottom from the shallow tidal zones to the deepest portions of the abyssal plain.
Benthic Fish
Some species of fish have adapted to life on the sea floor. Flounders, skates, rays and stonefish thrive on the sea floor of continental shelves. While free-swimming fish have swim bladders that keep them neutrally buoyant in their level of the water column, benthic fish have compact bodies without swim bladders so they can move across the sea floor with ease. Flounders, soles and other bony fish that live on the sea floor have eyes only on their upper surface, but that surface is physiologically a side; their spines run along one edge of the flattened fish, not between the creatures' eyes. A flattened shape is another characteristic of benthic marine fish.
Tube Worms
One of the deepest-dwelling macroscopic animals, certain species of tube worms live miles deep near ocean rift valleys. These creatures live in water that is just below boiling heat and receive no light whatsoever. Their nutrition comes from symbiotic bacteria that live on and within them. The bacteria in turn get their nutrients from chemosynthesis, the process of turning inorganic chemicals into usable energy in much the same way that photosynthesis turns sunlight into food. Scientists did not know of the worms' existence until a few decades ago when the first submersible cameras that could reach such depths first found them.
Bacteria
Just as bacteria thrive in terrestrial soil, they also live in and on the benthic marine substrate. Strains of photosynthesizing autotrophic bacteria live in shallow tidal zones where they receive sufficient light; heterotrophic bacteria that cannot make their own food live anywhere from a few feet below the ocean's surface to miles beneath it. The deepest-dwelling species thrive near hydrothermal vents in oceanic rifts, creating their food from the hydrogen sulfide that bubbles up from the Earth's interior.