Demersal zone

The demersal zone is the part of the sea or ocean (or deep lake) consisting of the part of the water column near to (and significantly affected by) the seabed and the benthos.[1] The demersal zone is just above the benthic zone and forms a layer of the larger profundal zone.

Being just above the ocean floor, the demersal zone is variable in depth and can be part of the photic zone where light can penetrate and photosynthetic organisms grow, or the aphotic zone, which begins between depths of roughly 200 and 1,000 m (700 and 3,300 ft) and extends to the ocean depths, where no light penetrates.[1]


The distinction between demersal species of fish and pelagic species is not always clear cut. The Atlantic cod (Gadus morhua) is a typical demersal fish, but can also be found in the open water column, and the Atlantic herring (Clupea harengus) is predominantly a pelagic species but forms large aggregations near the seabed when it spawns on banks of gravel.[2]

Two types of fish inhabit the demersal zone, those that are heavier than water and rest on the seabed, and those that have neutral buoyancy and remain just above the substrate. In many species of fish, neutral buoyancy is maintained by a gas-filled swim bladder which can be expanded or contracted as the circumstances require. A disadvantage of this method is that adjustments need to be made constantly as the water pressure varies when the fish swims higher and lower in the water column. An alternative buoyancy aid is the use of lipids, which are less dense than water—squalene, commonly found in shark livers, has a specific gravity of just 0.86. In the velvet belly lanternshark (Etmopterus spinax), a benthopelagic species, 17 % of the bodyweight is liver of which 70 % are lipids. Benthic rays and skates have smaller livers with lower concentrations of lipids; they are therefore denser than water and they do not swim continuously, intermittently resting on the seabed.[3] Some fish have no buoyancy aids but use their pectoral fins which are so angled as to give lift as they swim. The disadvantage of this is that, if they stop swimming, the fish sink, and they cannot hover, or swim backwards.[4]

Demersal fish have various feeding strategies; many feed on zooplankton or organisms or algae on the seabed; some of these feed on epifauna (invertebrates on top of the seafloor), while others specialise on infauna (invertebrates that burrow beneath the seafloor). Others are scavengers, eating the dead remains of plants or animals while still others are predators.[5]


Zooplankton are animals that drift with the current, but many have some limited means of locomotion and have some control over the depths at which they drift. They use gas-filled sacs or accumulations of substances with low densities to provide buoyancy, or they may have structures that slow down any passive descent. Where the adult, benthic organism is limited to life in a certain range of depths, their larvae need to optimise their chances of settling on a suitable substrate.[6]

Cuttlefish are able to adjust their buoyancy using their cuttlebones, lightweight rigid structures with cavities filled with gas, which have a specific gravity of about 0.6. This enables them to swim at varying depths. Another invertebrate that feeds on the seabed and has swimming abilities is the nautilus, which stores gas in its chambers and adjusts its buoyancy by use of osmosis, pumping water in and out.[3]


  1. ^ a b Merrett, N.R. (1997). Deep-Sea Demersal Fish and Fisheries. Springer. p. 296. ISBN 0412394103.
  2. ^ Steele, John H. (2010). Marine Policy & Economics. Elsevier. p. 91. ISBN 978-0-12-378558-9.
  3. ^ a b Schmidt-Nielsen, Knut (1997). Animal Physiology: Adaptation and Environment. Cambridge University Press. pp. 445–450. ISBN 978-0-521-57098-5.
  4. ^ Newman, David. "Buoyancy". Retrieved 5 July 2016.
  5. ^ Sedberry, G.R.; Musick, J.A. (1978) "Feeding strategies of some demersal fishes of the continental slope and rise off the mid-Atlantic coast of the USA" Marine Biology, 44:357–375.
  6. ^ Power, James H. (1989). "Sink or Swim: Growth Dynamics and Zooplankton Hydromechanics". The American Naturalist. 133 (5): 706–721. doi:10.1086/284946. JSTOR 2462076.
Black seabream

The black seabream (Spondyliosoma cantharus) is a species of Sparidae fishes. They are recognisable by their oval compressed body and jaws containing 4-6 rows of slender teeth which are larger at the front. They are silvery in colour with blue and pink tinges and broken longitudinal gold lines. They can reach a maximum size of 60 cm in length.

They live in northern Europe and in the Mediterranean, usually found on the inshore shelf at depths varying from 5 to 300 m. They are usually found in schools feeding on seaweeds and invertebrates. They breed in February to May leaving eggs in the demersal zone.

Black seabream are protogynous meaning females have the ability to change to males.

Cirroctopus hochbergi

Cirroctopus hochbergi is a cirrate octopus living between 800 and 1,070 meters deep off the coast of New Zealand. The species is known from 48 specimens. It is most similar to its sister taxon, Cirroctopus mawsoni; however, C. mawsoni's ventral pigmentation is lighter, and the two species have been found in very different areas (C. mawsoni is only known in waters near Antarctica).This octopus lives near cold seeps and seamounts. It is theorized that it and all other cirrate octopuses live in the demersal zone.

Clavus clavata

Clavus clavata is a species of sea snail, a marine gastropod mollusk in the family Drilliidae.

Demersal fish

Demersal fish live and feed on or near the bottom of seas or lakes (the demersal zone). They occupy the sea floors and lake beds, which usually consist of mud, sand, gravel or rocks. In coastal waters they are found on or near the continental shelf, and in deep waters they are found on or near the continental slope or along the continental rise. They are not generally found in the deepest waters, such as abyssal depths or on the abyssal plain, but they can be found around seamounts and islands. The word demersal comes from the Latin demergere, which means to sink.

Demersal fish are bottom feeders. They can be contrasted with pelagic fish which live and feed away from the bottom in the open water column.

Demersal fish fillets contain little fish oil (one to four percent), whereas pelagic fish can contain up to 30 percent.

Drillia monodi

Drillia monodi is a species of sea snail, a marine gastropod mollusk in the family Drilliidae.

Globidrillia micans

Globidrillia micans is a species of sea snail, a marine gastropod mollusk in the family Drilliidae.

Leptadrillia quisqualis

Leptadrillia quisqualis is a species of sea snail, a marine gastropod mollusk in the family Drilliidae.

Liparis antarcticus

Liparis antarcticus is a fish from the genus Liparis. It is a marine fish that lives in the demersal zone. Liparis antarcticus can be found in the Southeast Pacific Ocean by Chile, and it is the only known species from its genus to live in the Southern Hemisphere.

Liparis brashnikovi

Liparis brashnikovi is a fish from the genus Liparis. The fish grows to a maximum of 15 cm (in total length). It is a marine fish that lives in the demersal zone. Distribution includes the Sea of Japan in the Northwest Pacific Ocean.

Liparis bristolensis

Liparis bristolensis is a marine fish from the genus Liparis. It lives in the demersal zone at a depth between thirty-one to seventy-seven meters. The species may be found in the Northwest Pacific Ocean, specifically in the South Bering Sea and the western Gulf of Alaska.

Liparis burkei

Liparis burkei is a fish from the genus Liparis. It lives in shallow waters in marine environments in the demersal zone. Liparis burkei grows to a maximum length of 8.3 cm (in standard length) and is found in the Northwestern Pacific Ocean by Japan.

Marine botany

Marine botany is the study of aquatic plants and algae that live in seawater of the open ocean and the littoral zone, along shorelines of the intertidal zone, and in brackish water of estuaries.

It is a branch of marine biology and botany.

Neritic zone

The neritic zone is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately 200 meters (660 ft) in depth.

From the point of view of marine biology it forms a relatively stable and well-illuminated environment for marine life, from plankton up to large fish and corals, while physical oceanography sees it as where the oceanic system interacts with the coast.

Ocean bank

An ocean bank, sometimes referred to as a fishing bank or simply bank, is a part of the seabed which is shallow compared to its surrounding area, such as a shoal or the top of an underwater hill. Somewhat like continental slopes, ocean banks slopes can upwell as tidal and other flows intercept them, resulting sometimes in nutrient rich currents. Because of this, some large banks, such as Dogger Bank and the Grand Banks of Newfoundland, are among the richest fishing grounds in the world.

There are some banks that were reported in the 19th century by navigators, such as Wachusett Reef, whose existence is doubtful.

Oceanic zone

The oceanic zone is typically defined as the area of the ocean lying beyond the continental shelf, but operationally is often referred to as beginning where the water depths drop to below 100 meters (328 feet), seaward from the coast to the open ocean.

It is the region of open sea beyond the edge of the continental shelf and includes 65% of the ocean’s completely open water. The oceanic zone has a wide array of undersea terrain, including crevices that are often deeper than Mt. Everest is tall, as well as deep-sea volcanoes and ocean basins. While it is often difficult for life to sustain itself in this type of environment, some species do thrive in the oceanic zone.

There are four ocean zones. The sunlight zone, twilight zone, midnight zone, and abyssal zone.

Paracanthocobitis maekhlongensis

Acanthocobitis (Paracanthocobitis) maekhlongensis also known as the Maekhlong zipper loach is a species of ray-finned fish in the genus, or subgenus, Paracanthocobitis. This species is known from the Maeklong River basin, Kanchanaburi Province, Thailand.This fish is distinguished from others of its type due to its suborbital groove, pelvic fin, large dorsal fin, irregular light and dark spots, and thin dorsal saddles. It is found in a freshwater environment, demersal zone, and tropical range.

Pelagic zone

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 Ancient 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.

The pelagic zone occupies 1,330 million km3 (320 million mi3) 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.

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.

Splendrillia albicans

Splendrillia albicans is a species of sea snail, a marine gastropod mollusk in the family Drilliidae.


A strait is a naturally formed, narrow, typically navigable waterway that connects two larger bodies of water. Most commonly it is a channel of water that lies between two land masses. Some straits are not navigable, for example because they are too shallow, or because of an unnavigable reef or archipelago.

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