Demersal fish

Fish that live and feed on or near the bottom of seas or lakes

Demersal fish, also known as groundfish, 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. 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. 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. 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. Following Moyle and Cech (2004) they can be divided into five overlapping body shapes:
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! Bottom clingers
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| Bottom clingers "are mainly small fish with flattened heads, large pectoral fins, and structures (usually modified pelvic fins) that enable them to adhere to the bottom. Such structures are handy in swift streams, or intertidal areas with strong currents. The simplest arrangement is possessed by sculpins, which use their small, closely spaced pelvic fins, as antiskid devices. However, other families of fishes, such as gobies, and clingfishes have evolved suction cups."]] Benthopelagic fish inhabit the water just above the bottom, feeding on benthos and zooplankton. Most demersal fish are benthopelagic. Benthopelagic sharks, like the deep sea squaloid sharks, achieve neutral buoyancy with the use of large oil filled livers. 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. An example of a flabby fish is the cusk eel Acanthonus armatus, a predator with a huge head and a body that is 90 per cent water. This fish has the largest ears (otoliths) and the smallest brain in relation to its body size of all known vertebrates. 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. 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 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. 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 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 per cent of the ocean floor. They are covered with sediment but largely devoid of benthic life (benthos). Deep sea benthic fishes are more likely to associate with canyons or rock outcroppings among the plains, where invertebrate communities are established. Undersea mountains (seamounts) can intercept deep sea currents, and cause productive upwellings which support benthic fish. Undersea mountain ranges can separate underwater regions into different ecosystems. Rattails and brotulas are common, and other well established families are eels, eelpouts, hagfishes, greeneyes, batfishes and lumpfishes. 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 and carrion. Smell, touch and lateral line sensitivities seem to be the main sensory devices for locating these. 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. Although deep water demersal fish species are not generally picky about what they eat, there is still some degree of specialisation. For example, different fish have different mouth sizes, which determines the size of the prey they can handle. Some feed mostly on benthopelagic organisms. Others fed mostly on epifauna (invertebrates on top of the seafloor surface, also called epibenthos), or alternatively on infauna (invertebrates that burrow into the seafloor substrate). Infauna feeders can have considerable sediment in their stomachs. Scavengers, such as snubnosed eels and hagfish, also eat infauna as a secondary food source. Some feed on carrion. Cameras show that when a dead fish is placed on the bottom, vertebrate and invertebrate scavengers appear very quickly. If the fish is large, some scavengers burrow in and eat it from the inside out. Some fish, such as grenadiers, also appear and start feeding on the scavenging invertebrates and amphipods. Other specialization is based on depth distribution. Some of the more abundant upper continental slope fish species, such as cutthroat eel and longfinned hake, mainly feed on epipelagic fish. But generally, the most abundant deep water demersal fish species feed on invertebrates. At great depths, food scarcity and extreme pressure limits the ability of fish to survive. The deepest point of the ocean is about 11,000 metres. Bathypelagic fishes are not normally found below 3,000 metres. It may be that extreme pressures interfere with essential enzyme functions. The deepest living fish known, the strictly benthic Abyssobrotula galatheae, eel like and blind, feeds on benthic invertebrates. A living example was trawled from the bottom of the Puerto Rico Trench in 1970 from a depth of 8,370 metres (27,453 ft). In 2008, a shoal of 17 hadal snailfish, a species of deep water snailfish, was filmed by a UK Japan team using remote operated landers at depths of in the Japan Trench in the Pacific. The fish were 30 centimetres long (12 in), and were darting about, using vibration sensors on their nose to catch shrimps. The team also reported that the appearance of the fish, unlike that of most deep sea fish, was surprisingly "cute", and that they were surprised by how active the fish were at these depths.