The profundal zone is the deepest part of a body of water. Because light does not penetrate into it, photosynthesis does not occur there, so the food web of the profundal zone is based on materials that fall from farther up in the water column. Lack of light is a limiting biotic factor; other important limiting factors include increased pressure, lower oxygen levels, and cold temperatures.
The organisms that live in this region are called benthic organisms, and they are mostly detritivores, organisms which consume and recycle dead organic matter.
Common inhabitants of the profundal zone are leeches and other annelid worms, some species of insect larvae, and a few types of crabs and mollusks. In marine profundal zones echinoderms such as sea urchins, and crustaceans such a a few species of crabs, are also found.
In the profundal zone, there are bacteria present.
Very few invertebrates are able to inhabit the cold, dark and oxygen deprived profundal zone. Those that do live there are often red in colour due to the presence of large amounts of hemogoblin which greatly increases the amount of oxygen carried to the cells. Furthermore, because the concentration of oxygen is low the species that live there usually construct tunnels or burrows in which they can hide and make the minimum movements necessary to circulate water through, drawing oxygen to them without the expending too much energy.
Fish can also be found in the profundal zone mainly due to the fact that they are able to deal with unsuitable abiotic factors in one zone by simply moving to another. For example, a detrital feeder in the profundal zone may find that the oxygen level has dropped to low, and so will move toward the benthic zone.