Marine life can be grouped according to size, mobility, and location within the water column or benthic zone:
Pleuston: Organisms, such as the colonial cnidarians known as the Portuguese man-of-war, that live at and above the surface, half in and half out of the water. These buoyant creatures are wind-blown.
Neuston: Mostly carnivorous animals that cling to the surface film and drift or “walk” on the very top of the water column (e.g., sea skater) or hang just below the surface (e.g., the gastropod lathina)
Plankton: Small organisms that float in the water, unable to propel themselves against tides or currents. Flagella, however, do allow them to move up and down the water column. Phytoplankton are photosythetic and consist mostly of algae and cyanobacteria; they are confined to the euphotic zone. The zooplankton consists of animals such as copepods, salps, and krill. (More on plankton)
Nekton: Active swimmers. Those that live in the upper part of the water column, such as sharks, tuna, and whales, are known as pelagic forms; those that live close to the bottom, such as cod, flounder, and rays, are demersal forms.
Benthos: Organisms restricted to the benthic zone. Sessile forms, such as kelps, seagrasses, and corals attach themselves to the sea bottom; motile forms, such as worms, seastars, mussels and crabs, move through or on top of the sea bottom.