Benthos refers to species living in or on the ocean bottom and represent the greatest proportion of marine species. In fact 98% of marine species are from the benthic community which comprises a wide range of bacteria, plants and animals, which are classified into three categories epifauna, infauna and demersal. Epifauna are organisms that live in the sediment, infauna are organisms which are either attach to the bottom or substrate (move within the sediment), or that live on the sediment surface and demersal which are fish that feed on the benthic epifauna and infauna. Aside from those three classifications there are even some benthic organisms that are sessile, referring to their incapacity to move so they live attached to the ocean bottom. These benthic organisms have been able to survive in these harsh depths due to the many complexities of the sea bed that have made it easier for these organisms to make a home in a place that for many years was thought of as being abandoned. Many organisms adapted to deep-water pressure cannot survive in the upper parts of the water column. The pressure difference can be very significant (approximately one atmosphere for each 10 meters of water depth).