Bathydemersal

Demersal fish, also known as groundfish, live and feed on or near the bottom of seas or lakes (the demersal zone).Walrond
Carl . "Coastal fish - Fish of the open sea floor"
Te Ara - the Encyclopedia of New Zealand. Updated 2 March 2009 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.

Types

Demersal fish can be divided into two main types: strictly benthic fish which can rest on the sea floor, and benthopelagic fish which can float in the water column just above the sea floor.

Benthopelagic fish have neutral buoyancy, so they can float at depth without much effort, while strictly benthic fish are denser, with negative buoyancy so they can lie on the bottom without any effort. Most demersal fish are benthopelagic.

As with other bottom feeders, a mechanism to deal with substrate is often necessary. With demersal fish the sand is usually pumped out of the mouth through the gill slit. Most demersal fish exhibit a flat ventral region so as to more easily rest their body on the substrate. The exception may be the flatfish, which are laterally depressed but lie on their sides. Also, many exhibit what is termed an "inferior" mouth, which means that the mouth is pointed downwards; this is beneficial as their food is often below them in the substrate. Those bottom feeders with upward-pointing mouths, such as stargazers, tend to seize swimming prey.

Benthic fish

Benthic fish are denser than water, so they can rest on the sea floor. They either lie-and-wait as ambush predators, at times covering themselves with sand or otherwise camouflaging themselves, or move actively over the bottom in search for food. Benthic fish which can bury themselves include dragonets, flatfish and stingrays.

Flatfish are an order of ray-finned benthic fishes which lie flat on the ocean floor. Examples are flounder, sole, turbot, plaice, and halibut. The adult fish of many species have both eyes on one side of the head. When the fish hatches, one eye is located on each side of its head. But as the fish grows from the larval stage, one eye migrates to the other side of the body as a process of metamorphosis. The flatfish then changes its habits, and camouflages itself by lying on the bottom of the ocean floor with both eyes facing upwards. The side to which one eye migrates depends on the species; with some species both eyes are ultimately on the left side, whereas with other species the eyes are on the right.

File:Pseudopleuronectes americanus.jpg, Flounder have both eyes on one side of their head
File:Flounder camo md.jpg, Some flatfish can camouflage themselves on the ocean floor
File:Taeniura lymma 2.jpg, Bluespotted ribbontail rays migrate in schools onto shallow sands to feed on mollusks, shrimps, crabs and worms.
File:Sphyrna mokarran head.jpg, The great hammerhead detects the electrical signatures of stingrays buried in the sand and pins them with its "hammer".

Flounder ambush their prey, feeding at soft muddy area of the sea bottom, near bridge piles, docks, artificial and coral reefs. Their diet consists mainly of fish spawn, crustaceans, polychaetes and small fish.

The great hammerhead swings its head in broad angles over the sea floor to pick up the electrical signatures of stingrays buried in the sand. It then uses its "hammer" to pin down the stingray.Hammerschlag, Rick
Sandy Plains: Great Hammerhead Shark
''ReefQuest Centre for Shark Research''. Retrieved: 6 June 2022.

File:Pacific hagfish Myxine.jpg, Pacific hagfish resting on bottom. Hagfish coat themselves and any dead fish they find with noxious slime making them inedible to other species.
File:Bathypterois grallator.jpg, The tripodfish (''Bathypterois grallator''), a species of spiderfish, uses its fin extensions to "stand" on the bottom.
File:Caelorinchus mirus (Gargoyle fish).gif, Gargoyle fish
File:Fringe-Lipped Flathead.jpg, The fringe-lipped flathead is found in estuaries

Some fishes do not fit into the above classification. For example, the family of nearly blind spiderfishes, common and widely distributed, feed on benthopelagic zooplankton. Yet they are strictly benthic fish, since they stay in contact with the bottom. Their fins have long rays they use to "stand" on the bottom while they face the current and grab zooplankton as it passes by.

The bodies of benthic fish are adapted for ongoing contact with the sea floor. Swimbladders are usually absent or reduced, and the bodies are usually flattened in one way or another.Moyle and Cech, 2004, p. 13 Following Moyle and Cech (2004) they can be divided into five overlapping body shapes:

Benthopelagic fish

Benthopelagic fish inhabit the water just above the bottom, feeding on benthos. Most demersal fish are benthopelagic.

Deep sea benthopelagic teleosts all have swimbladders. The dominant species, rattails and cusk eels, have considerable biomass. Other species include deep sea cods (morids), deep sea eels, halosaurs and notacanths.Bone 2008, p. 43.

Benthopelagic sharks, like the deep sea squaloid sharks, achieve neutral buoyancy with the use of large oil-filled livers.Bone 2008, p. 42. Sharks adapt well to fairly high pressures. They can often be found on slopes down to about 2000 metres, scavenging on food falls such as dead whales. However, the energy demands of sharks are high, since they need to swim constantly and maintain a large amount of oil for buoyancy. These energy needs cannot be met in the extreme oligotrophic conditions that occur at great depths.

Shallow water stingrays are benthic, and can lie on the bottom because of their negative buoyancy. Deep sea stingrays are benthopelagic, and like the squaloids have very large livers which give them neutral buoyancy.

Benthopelagic fish can be divided into flabby or robust body types. Flabby benthopelagic fishes are like bathypelagic fishes; they have a reduced body mass, and low metabolic rates, expending minimal energy as they lie and wait to ambush prey. An example of a flabby fish is the cusk-eel ''Acanthonus armatus'', a predator with a huge head and a body that is 90 percent water. This fish has the largest ears (otoliths) and the smallest brain in relation to its body size of all known vertebrates.Fine ML, Horn MH and Cox B (1987
"''Acanthonus armatus'', a Deep-Sea Teleost Fish with a Minute Brain and Large Ears"
''Proceedings of the Royal Society B'', 230(1259)257-265.

Deepwater benthopelagic fish are robust, muscular swimmers that actively cruise the bottom searching for prey. They often live around features, such as seamounts, which have strong currents. Commercial examples are the orange roughy and Patagonian toothfish.

Habitats

The edge of the continental shelf marks the boundary where the shelf gives way to, and then gradually drops into abyssal depths. This edge marks the boundary between coastal, relatively shallow, benthic habitats, and the deep benthic habitats. Coastal demersal fishes live on the bottom of inshore waters, such as bays and estuaries, and further out, on the floor of the continental shelf. Deep water demersal fish live beyond this edge, mostly down the continental slopes and along the continental rises which drop to the abyssal plains. This is the continental margin, constituting about 28% of the total oceanic area.P. J. Cook, Chris Carleton (2000) "Continental Shelf Limits: The Scientific and Legal Interface", Other deep sea demersal fish can also be found around seamounts and islands.

The term ''bathydemersal fish'' is sometimes used instead of "deep water demersal fish". ''Bathydemersal'' refers to demersal fish which live at depths greater than 200 metres.

The term ''epibenthic'' is also used to refer to organism that live on top of the ocean floor, as opposed to those that burrow into the seafloor substrate. However the terms ''mesodemersal'', ''epidemersal'', ''mesobenthic'' and ''bathybenthic'' are not used.

Coastal

Coastal demersal fish are found on or near the seabed of coastal waters between the shoreline and the edge of the continental shelf, where the shelf drops into the deep ocean. Since the continental shelf is generally less than 200 metres deep, this means that coastal waters are generally epipelagic. The term includes demersal reef fish and demersal fish that inhabit estuaries, inlets and bays.

File:Mangrovejack.jpg, The mangrove jack eats crustaceans
File:Puffer Fish DSC01257.JPG, Many puffer fish species crush the shells of molluscs
File:Opsanus beta 1.jpg, The venomous toadfish, a benthic ambush predator, blends into sandy or muddy bottoms.
File:Titan Triggerfish.jpg, Triggerfish use a jet of water to uncover sand dollars buried in sand

Young mangrove jacks, a sought after eating and sport fish, dwell in estuaries around mangrove roots, fallen trees, rock walls, and any other snag areas where smaller prey reside for protection. When they mature, they migrate into open waters, sometimes hundreds of kilometres from the coast to spawn.

Stargazers are found worldwide in shallow waters. They have eyes on top of their heads and a large upward-facing mouth. They bury themselves in sand, and leap upwards to ambush benthopelagic fish and invertebrates that pass overhead. Some species have a worm-shaped lure growing out of the floor of the mouth, which they wiggle to attract prey. Stargazers are venomous and can deliver electric shocks. They have been called "the meanest things in creation."

Other examples of coastal demersal fish are cod, plaice, monkfish and sole.

Deep water

Deep water demersal fish occupy the benthic regions beyond the continental margins.

On the continental slope, demersal fishes are common. They are more diverse than coastal demersal fish, since there is more habitat diversity. Further out are the abyssal plains. These flat, featureless regions occupy about 40 percent of the ocean floor. They are covered with sediment but largely devoid of benthic life (benthoss) can intercept deep sea currents, and cause productive upwellings which support benthic fish. Undersea mountain ranges can separate underwater regions into different ecosystems.Moyle and Cech, 2004, p. 587

Rattails and brotulas are common, and other well-established families are eels, eelpouts, hagfishes, greeneyes, batfishes and lumpfishes.

The bodies of deep water demersal fishes are muscular with well developed organs. In this way they are closer to mesopelagic fishes than bathypelagic fishes. In other ways, they are more variable. Photophores are usually absent, eyes and swimbladders range from absent to well developed. They vary in size, and larger species, greater than one metre, are not uncommon.

Deep sea demersal fish are usually long and narrow. Many are eels or shaped like eels. This may be because long bodies have long lateral lines. Lateral lines detect low-frequency sounds, and some demersal fishes have muscles that drum such sounds to attract mates.Haedrich RL (1996
"Deep-water fishes: evolution and adaptation in the earth's largest living spaces"
''Journal of Fish Biology'' 49(sA):40-53.
Smell is also important, as indicated by the rapidity with which demersal fish find traps baited with bait fish.

The main diet of deep sea demersal fish is invertebrates of the deep sea benthos. Smell, touch and lateral line sensitivities seem to be the main sensory devices for locating these.Moyle and Cech, 2004, p. 588

Like coastal demersal fish, deep sea demersal fish can be divided into benthic fish and benthopelagic fish, where the benthic fish are negatively buoyant and benthopelagic fish are neutrally buoyant.

The availability of plankton for food diminishes rapidly with depth. At , the biomass of plankton is typically about 1 percent of that at the surface, and at about 0.01 percent. Given there is no sunlight, energy enters deep water zones as organic matter. There are three main ways this happens. Firstly, organic matter can move into the zone from the continental landmass, for example, through currents that carry the matter down rivers, then plume along the continental shelf and finally spill down the continental slope. Other matter enters as particulate matter raining down from the overhead water column in the form of marine snow, or as sinking overhead plant material such as eelgrass