<P> The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth . The word "pelagic" is derived from Greek πέλαγος (pélagos), meaning' open sea' . The pelagic zone can be thought of in terms of an imaginary cylinder or water column that goes from the surface of the sea almost to the bottom . Conditions differ deeper in the water column such that as pressure increases with depth, the temperature drops and less light penetrates . Depending on the depth, the water column, rather like the Earth's atmosphere, may be divided into different layers . </P> <P> The pelagic zone occupies 1,330 million km (320 million mi) with a mean depth of 3.68 km (2.29 mi) and maximum depth of 11 km (6.8 mi). Fish that live in the pelagic zone are called pelagic fish . Pelagic life decreases with increasing depth . It is affected by light intensity, pressure, temperature, salinity, the supply of dissolved oxygen and nutrients, and the submarine topography, which is called bathymetry . In deep water, the pelagic zone is sometimes called the open - ocean zone and can be contrasted with water that is near the coast or on the continental shelf . In other contexts, coastal water not near the bottom is still said to be in the pelagic zone . </P> <P> The pelagic zone can be contrasted with the benthic and demersal zones at the bottom of the sea . The benthic zone is the ecological region at the very bottom of the sea . It includes the sediment surface and some subsurface layers . Marine organisms living in this zone, such as clams and crabs, are called benthos . The demersal zone is just above the benthic zone . It can be significantly affected by the seabed and the life that lives there . Fish that live in the demersal zone are called demersal fish, and can be divided into benthic fish, which are denser than water so they can rest on the bottom, and benthopelagic fish, which swim in the water column just above the bottom . Demersal fish are also known as bottom feeders and groundfish . </P> <P> Depending on how deep the sea is, the pelagic zone can extend over up to five horizontal regions in the ocean . From the top down, these are: </P>

The bottom environments of the oceans are referred to as the