Demersal fish, also known as groundfish, live and feed on or near the bottom of seas or lakes (the demersal zone).Walrond C 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 per cent), whereas pelagic fish can contain up to 30 per cent.
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 of food. Benthic fish which can bury themselves include flatfish, stingrays and dragonets.
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. July 17, 2004. Accessed 2009-06-08. 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 areas 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.
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.Sulak KJ () "The systematics and biology of Bathypterois (Pisces, Chlorophthalmidae) with a revised classification of benthic myctophiform fishes" Ichthyological Research, 32(4)443-446.
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:
Body types of benthic fish Bottom rovers Bottom rovers "have a rover-predator-like body, except that the head tends to be flattened, the back humped, and the pectoral fins enlarged. North American catfish with large mouths at the end of the snout, small armoured catfish with small mouths beneath the snout, and sturgeons, with fleshy protusible lips located well below the snout that are used to suck plant and animal matter off the bottom."Moyle and Cech, 2004, p. 14 Bottom clingers 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." Bottom hiders Bottom hiders "are similar in many ways to bottom clingers. but they lack the clinging devices and tend to have more elongate bodies and smaller heads. These forms usually live under rocks or in crevices or lie quietly on the bottom in still waters. The darters of North American streams are in the category, as are many blennies." Flatfish Flatfish "have the most extreme morphologies of the bottom fish. Flounders are essentially deep-bodied fish which live with one side on the bottom. In these fish, the eye on the downward side migrates during development to the upward side, and the mouth often assumes a peculiar twist to enable bottom feeding. In contrast, skates and rays are flattened dorsoventrally, and mostly move about by flapping their extremely large pectoral fins. Not only is the mouth completely ventral on these fish, the main water intakes for respiration (the spiracles) are located on the top of the head." Rattail-shaped fish Rattail-shaped fish "have bodies that begin with large pointy-snouted heads and large pectoral fins and end in long pointed rat-like tails. These fish are almost all bottom-dwelling (benthic) inhabitants of the deep sea, but exactly why this peculiar morphology is so popular among them is poorly understood. The fish live by scavenging and preying on benthic invertebrates. Examples are the grenadiers, viviparous brotulas (pictured), and chimaeras."
Benthopelagic fish
Benthopelagic fish inhabit the water just above the bottom, feeding on benthos and zooplankton.Mauchline J and Gordon JDM (1986) "Foraging strategies of deep-sea fish"] Mar. Ecol. Prog. Ser. 27: 227-238. Download 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.Koslow JA (1996) Journal of Fish Biology, 49(sA) 54-74. 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.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.
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.Russell, D.J., et al., "Biology, Management and Genetic Stock Structure of Mangrove Jack (Lutjanus argentimaculatus) in Australia," The State of Queensland, Department of Primary Industries and the Fisheries Research Development Corporation, FRDC Project Number 1999/122, 2003.
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."Grady, Denise Venom Runs Thick in Fish Families, Researchers Learn The New York Times 22 August 2006.
Other examples of coastal demersal fish are cod, plaice, monkfish and sole.