Demersal fish
Demersal fish live on or near the bottom of the sea. Demersal fish are found by the seafloor in coastal areas on the continental shelf, and in the open ocean they are found along the outer continental margin on the continental slope and the continental rise. They are not generally found at abyssopelagic or hadopelagic depths or on the abyssal plain. They occupy a range of seafloors consisting of mud, sand, gravel or rocks. In deep waters, rattails and brotulas are common, and other well established families are eels, eelpouts, hagfishes, greeneyes, batfishes and lumpfishes. The bodies of deep water benthic fishes are muscular with well developed organs. In this way they are closer to mesopleagic fishes than bathopelagic fishes. In other ways, they are more variable. Photophores are usually absent, eyes and swimbladder range from absent to well developed. They vary in size, with larger species greater than one meter not uncommon.
Deep sea benthic 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 benthic fishes appear to have muscles that drum such sounds to attract mates. Smell is also important, as indicated by the rapidity with which benthic fish find traps baited with bait fish.
The main diet of deep sea benthic 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. Deep sea benthic fish can be divided into strictly benthic fish and benthopelagic fish. Usually strictly benthic fish are negatively buoyant while benthopelagic fish are neutrally buoyant. Strictly benthic fish stay in constant contact with the bottom. They either lie-and-wait as ambush predators or move actively over the bottom in search for food.
Benthopelagic fish
Benthopelagic fish inhabit the water just above the bottom, feeding on benthos and benthopelagic zooplankton. Most dermersal fish are benthopelagic. They can be divided into flabby or robust body types. Flabby benthopelagic fishes are like bathopelagic 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. Robust benthopelagic fish are muscular swimmers that actively cruise the bottom searching for prey. They may live around features, such as seamounts, which have strong currents. Examples are the orange roughy and Patagonian toothfish.
Benthic fish
Some fishes don't 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 deepest-living fish known, the strictly benthic Abyssobrotula galatheae, eel-like and blind, feeds on benthic invertebrates.
At great depths, food scarcity and extreme pressure works to limit the survivability fish. The deepest point of the ocean is about 11,000 meters. Bathypelagic fishes are not normally found below 3,000 meters. The greatest depth recorded for a benthic fish is 8,370 m. It may be that extreme pressures interfere with essential enzyme functions.
Benthic fishes are likely to be found, and are more diverse, on the continental slope, where there is habitat diversity and often food supplies. About 40% of the ocean floor consists of abyssal plains, but these flat, featureless regions are covered with sediment and 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.
