Deep-sea fish are fish that live in the darkness below the sunlit surface waters, that is below the
epipelagic or
photic zone
The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological pro ...
of the
sea
The sea, connected as the world ocean or simply the ocean, is the body of salty water that covers approximately 71% of the Earth's surface. The word sea is also used to denote second-order sections of the sea, such as the Mediterranean Sea, ...
. The
lanternfish is, by far, the most common deep-sea fish. Other
deep sea fishes include the
flashlight fish,
cookiecutter shark,
bristlemouths,
anglerfish
The anglerfish are fish of the teleost order Lophiiformes (). They are bony fish named for their characteristic mode of predation, in which a modified luminescent fin ray (the esca or illicium) acts as a lure for other fish. The luminescence ...
,
viperfish, and some species of
eelpout.
Only about 2% of known marine species inhabit the
pelagic
The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or w ...
environment. This means that they live in the
water column
A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...
as opposed to the
benthic
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...
organisms that live in or on the sea floor. Deep-sea organisms generally inhabit
bathypelagic
The bathypelagic zone or bathyal zone (from Greek βαθύς (bathýs), deep) is the part of the open ocean that extends from a depth of below the ocean surface. It lies between the mesopelagic above, and the abyssopelagic below. The bathypelagi ...
(1000–4000m deep) and
abyssopelagic (4000–6000m deep) zones. However, characteristics of deep-sea organisms, such as
bioluminescence
Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some b ...
can be seen in the
mesopelagic (200–1000m deep) zone as well. The mesopelagic zone is the disphotic zone, meaning light there is minimal but still measurable. The oxygen minimum layer exists somewhere between a depth of 700m and 1000m deep depending on the place in the ocean. This area is also where nutrients are most abundant. The
bathypelagic
The bathypelagic zone or bathyal zone (from Greek βαθύς (bathýs), deep) is the part of the open ocean that extends from a depth of below the ocean surface. It lies between the mesopelagic above, and the abyssopelagic below. The bathypelagi ...
and
abyssopelagic zones are
aphotic
The aphotic zone (aphotic from Greek prefix + "without light") is the portion of a lake or ocean where there is little or no sunlight. It is formally defined as the depths beyond which less than 1 percent of sunlight penetrates. Above the apho ...
, meaning that no light penetrates this area of the ocean. These zones make up about 75% of the inhabitable ocean space.
The epipelagic zone (0–200m) is the area where light penetrates the water and photosynthesis occurs. This is also known as the photic zone. Because this typically extends only a few hundred meters below the water, the deep sea, about 90% of the ocean volume, is in darkness. The deep sea is also an extremely hostile environment, with temperatures that rarely exceed 3 °C (37.4 °F) and fall as low as −1.8 °C (28.76 °F) (with the exception of hydrothermal vent ecosystems that can exceed 350 °C, or 662 °F), low oxygen levels, and pressures between 20 and 1,000
atmospheres
The standard atmosphere (symbol: atm) is a unit of pressure defined as Pa. It is sometimes used as a ''reference pressure'' or ''standard pressure''. It is approximately equal to Earth's average atmospheric pressure at sea level.
History
The s ...
(between 2 and 100
megapascal
The pascal (symbol: Pa) is the unit of pressure in the International System of Units (SI), and is also used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is defined ...
s).
Environment
In the deep ocean, the waters extend far below the epipelagic zone, and support very different types of pelagic fishes adapted to living in these deeper zones. In deep water,
marine snow is a continuous shower of mostly organic
detritus
In biology, detritus () is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts comm ...
falling from the upper layers of the water column. Its origin lies in activities within the productive
photic zone
The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological pro ...
. Marine snow includes dead or dying
plankton
Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...
,
protist
A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the e ...
s (
diatom
A diatom ( Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising se ...
s), fecal matter, sand, soot and other inorganic dust. The "snowflakes" grow over time and may reach several centimetres in diameter, travelling for weeks before reaching the ocean floor. However, most organic components of marine snow are consumed by
microbe
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s,
zooplankton
Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
and other filter-feeding animals within the first 1,000 metres of their journey, that is, within the epipelagic zone. In this way marine snow may be considered the foundation of deep-sea
mesopelagic and
benthic
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...
ecosystem
An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...
s: As sunlight cannot reach them, deep-sea organisms rely heavily on marine snow as an energy source. Since there is no light in the deep sea (aphotic), there is a lack of primary producers. Therefore, most organisms in the bathypelagic rely on the marine snow from regions higher in the vertical column.
Some deep-sea pelagic groups, such as the
lanternfish,
ridgehead,
marine hatchetfish, and
lightfish families are sometimes termed ''pseudoceanic'' because, rather than having an even distribution in open water, they occur in significantly higher abundances around structural oases, notably
seamounts and over
continental slopes. The phenomenon is explained by the likewise abundance of prey species which are also attracted to the structures.
Hydrostatic pressure increases by 1 atmosphere for every 10m in depth. Deep-sea organisms have the same pressure within their bodies as is exerted on them from the outside, so they are not crushed by the extreme pressure. Their high internal pressure, however, results in the reduced fluidity of their membranes because molecules are squeezed together. Fluidity in cell membranes increases efficiency of biological functions, most importantly the production of proteins, so organisms have adapted to this circumstance by increasing the proportion of unsaturated fatty acids in the lipids of the cell membranes. In addition to differences in internal pressure, these organisms have developed a different balance between their metabolic reactions from those organisms that live in the epipelagic zone. David Wharton, author of ''Life at the Limits: Organisms in Extreme Environments'', notes "Biochemical reactions are accompanied by changes in volume. If a reaction results in an increase in volume, it will be inhibited by pressure, whereas, if it is associated with a decrease in volume, it will be enhanced". This means that their metabolic processes must ultimately decrease the volume of the organism to some degree.
Most fish that have evolved in this harsh environment are not capable of surviving in laboratory conditions, and attempts to keep them in captivity have led to their deaths. Deep-sea organisms contain gas-filled spaces (vacuoles). Gas is compressed under high pressure and expands under low pressure. Because of this, these organisms have been known to blow up if they come to the surface.
Characteristics
The fish of the deep-sea have evolved various adaptations to survive in this region. Since many of these fish live in regions where there is no natural
illumination, they cannot rely solely on their eyesight for locating prey and mates and avoiding predators; deep-sea fish have
evolved
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variati ...
appropriately to the extreme sub-photic region in which they live. Many of these organisms are blind and rely on their other senses, such as sensitivities to changes in local pressure and smell, to catch their food and avoid being caught. Those that aren't blind have large and sensitive eyes that can use
bioluminescent light. These eyes can be as much as 100 times more sensitive to light than human eyes.
Rhodopsin
Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene and a G-protein-coupled receptor (GPCR). It is the opsin of the rod cells in the retina and a light-sensitive receptor protein that triggers visual phototransductio ...
(Rh1) is a protein found in the eye’s rod cells that helps animals see in dim light. While most vertebrates usually have one Rh1 opsin gene, some deep sea fish have several Rh1 genes, and one species, the
silver spinyfin (''Diretmus argenteus''), has 38.
This proliferation of Rh1 genes may help deep sea fish to see in the depths of the ocean. Also, to avoid predation, many species are dark to blend in with their environment.
Many deep-sea fish are
bioluminescent, with extremely large eyes adapted to the dark. Bioluminescent organisms are capable of producing light biologically through the agitation of molecules of luciferin, which then produce light. This process must be done in the presence of oxygen. These organisms are common in the mesopelagic region and below (200m and below). More than 50% of deep-sea fish, as well as some species of shrimp and squid, are capable of bioluminescence. About 80% of these organisms have photophores – light producing glandular cells that contain luminous bacteria bordered by dark colourings. Some of these photophores contain lenses, much like those in the eyes of humans, which can intensify or lessen the emanation of light. The ability to produce light only requires 1% of the organism's energy and has many purposes: It is used to search for food and attract prey, like the anglerfish; claim territory through patrol; communicate and find a mate, and distract or temporarily blind predators to escape. Also, in the mesopelagic where some light still penetrates, some organisms camouflage themselves from predators below them by illuminating their bellies to match the colour and intensity of light from above so that no shadow is cast. This tactic is known as counter-illumination.
The lifecycle of deep-sea fish can be exclusively deep water although some species are born in shallower water and sink upon maturation. Regardless of the depth where eggs and larvae reside, they are typically pelagic. This planktonic — drifting — lifestyle requires neutral buoyancy. In order to maintain this, the eggs and larvae often contain oil droplets in their plasma. When these organisms are in their fully matured state they need other adaptations to maintain their positions in the water column. In general, water's density causes upthrust — the aspect of buoyancy that makes organisms float. To counteract this, the density of an organism must be greater than that of the surrounding water. Most animal tissues are denser than water, so they must find an equilibrium to make them float. Many organisms develop
swim bladder
The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth wit ...
s (gas cavities) to stay afloat, but because of the high pressure of their environment, deep-sea fishes usually do not have this organ. Instead they exhibit structures similar to hydrofoils in order to provide hydrodynamic lift. It has also been found that the deeper a fish lives, the more jelly-like its flesh and the more minimal its bone structure. They reduce their tissue density through high fat content, reduction of skeletal weight — accomplished through reductions of size, thickness and mineral content — and water accumulation makes them slower and less agile than surface fish.
Due to the poor level of
photosynthetic
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...
light reaching deep-sea environments, most fish need to rely on
organic
Organic may refer to:
* Organic, of or relating to an organism, a living entity
* Organic, of or relating to an anatomical organ
Chemistry
* Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...
matter sinking from higher levels, or, in rare cases,
hydrothermal vents
A hydrothermal vent is a fissure on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspo ...
for nutrients. This makes the deep-sea much poorer in
productivity than shallower regions. Also, animals in the pelagic environment are sparse and food doesn't come along frequently. Because of this, organisms need adaptations that allow them to survive. Some have long feelers to help them locate prey or attract mates in the pitch black of the deep ocean. The deep-sea angler fish in particular has a long fishing-rod-like adaptation protruding from its face, on the end of which is a bioluminescent piece of skin that wriggles like a worm to lure its prey. Some must consume other fish that are the same size or larger than them and they need adaptations to help digest them efficiently. Great sharp teeth, hinged jaws, disproportionately large mouths, and expandable bodies are a few of the characteristics that deep-sea fishes have for this purpose. The
gulper eel
The saccopharyngiformes are a derived lineage of unusual eels within the order Anguilliformes, and includes families Cyematidae, Monognathidae, Eurypharyngidae, Saccopharyngidae, and the proposed family Neocyematidae. Most of the fish in this gro ...
is one example of an organism that displays these characteristics.
Fish in the different pelagic and deep water benthic zones are physically structured, and behave in ways, that differ markedly from each other. Groups of coexisting species within each zone all seem to operate in similar ways, such as the small mesopelagic
vertically migrating plankton-feeders, the bathypelagic
anglerfish
The anglerfish are fish of the teleost order Lophiiformes (). They are bony fish named for their characteristic mode of predation, in which a modified luminescent fin ray (the esca or illicium) acts as a lure for other fish. The luminescence ...
es, and the deep water benthic
rattail
Grenadiers or rattails are generally large, brown to black gadiform marine fish of the subfamily Macrourinae, the largest subfamily of the family Macrouridae. Found at great depths from the Arctic to Antarctic, members of this subfamily are amon ...
s.
Ray finned species, with spiny fins, are rare among deep sea fishes, which suggests that deep sea fish are ancient and so well adapted to their environment that invasions by more modern fishes have been unsuccessful.
The few ray fins that do exist are mainly in the
Beryciformes and
Lampriformes, which are also ancient forms. Most deep sea pelagic fishes belong to their own orders, suggesting a long evolution in deep sea environments. In contrast, deep water benthic species, are in orders that include many related shallow water fishes.
Mesopelagic fish
Below the epipelagic zone, conditions change rapidly. Between 200 metres and about 1000 metres, light continues to fade until there is almost none. Temperatures fall through a
thermocline to temperatures between 3.9 °C (39 °F) and 7.8 °C (46 °F). This is the twilight or
mesopelagic zone. Pressure continues to increase, at the rate of one atmosphere every 10 metres, while nutrient concentrations fall, along with dissolved oxygen and the rate at which the water circulates.
Sonar operators, using the newly developed sonar technology during World War II, were puzzled by what appeared to be a false sea floor 300–500 metres deep at day, and less deep at night. This turned out to be due to millions of marine organisms, most particularly small mesopelagic fish, with swim bladders that reflected the sonar. These organisms migrate up into shallower water at dusk to feed on plankton. The layer is deeper when the moon is out, and can become shallower when clouds pass over the moon. This phenomenon has come to be known as the
deep scattering layer.
[Ryan P.]
"Deep-sea creatures: The mesopelagic zone"
''Te Ara – the Encyclopedia of New Zealand''. Updated 21 September 2007.
Most mesopelagic fish make daily
vertical migrations, moving at night into the epipelagic zone, often following similar migrations of zooplankton, and returning to the depths for safety during the day.
[Bone & Moore 2008, p. 38.] These vertical migrations often occur over large vertical distances, and are undertaken with the assistance of a
swim bladder
The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth wit ...
.
[Haedrich R. L. (1996]
"Deep-water fishes: evolution and adaptation in the earth's largest living spaces"
''Journal of Fish Biology'', 49(sA):40–53.
The swim bladder is inflated when the fish wants to move up, and, given the high pressures in the messoplegic zone, this requires significant energy. As the fish ascends, the pressure in the swim bladder must adjust to prevent it from bursting. When the fish wants to return to the depths, the swim bladder is deflated.
Some mesopelagic fishes make daily migrations through the
thermocline, where the temperature changes between 50 °F (10 °C) and 69 °F (20 °C), thus displaying considerable tolerances for temperature change.
These fish have muscular bodies, ossified bones, scales, well developed gills and central nervous systems, and large hearts and kidneys. Mesopelagic
plankton feeders have small mouths with fine
gill rakers, while the
piscivores have larger mouths and coarser gill rakers.
Mesopelagic fish are adapted for an active life under low light conditions. Most of them are visual predators with large eyes. Some of the deeper water fish have tubular eyes with big lenses and only
rod cells that look upwards. These give binocular vision and great sensitivity to small light signals. This adaptation gives improved terminal vision at the expense of lateral vision, and allows the predator to pick out
squid
True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fittin ...
,
cuttlefish
Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control of ...
, and smaller fish that are silhouetted against the gloom above them.
Mesopelagic fish usually lack defensive spines, and use colour to
camouflage
Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the b ...
themselves from other fish.
Ambush predators are dark, black or red. Since the longer, red, wavelengths of light do not reach the deep sea, red effectively functions the same as black. Migratory forms use
countershaded
Countershading, or Thayer's law, is a method of camouflage in which an animal's coloration is darker on the top or upper side and lighter on the underside of the body. This pattern is found in many species of mammals, reptiles, birds, fish, an ...
silvery colours. On their bellies, they often display
photophores producing low grade light. For a predator from below, looking upwards, this
bioluminescence
Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some b ...
camouflages the silhouette of the fish. However, some of these predators have yellow lenses that filter the (red deficient) ambient light, leaving the bioluminescence visible.
The
brownsnout spookfish
The brownsnout spookfish or brown-snout spookfish (''Dolichopteryx longipes'') is a species of barreleye in the family Opisthoproctidae. It and the glasshead barreleye fish are the only vertebrates known to employ a mirror, in addition to a len ...
, a species of
barreleye, is the only vertebrate known to employ a mirror, as opposed to a lens, to focus an image in its eyes.
[Smith, L. (Jan. 8, 2009)]
"Fish with four eyes can see through the deep sea gloom"
''Times Online''. Times Newspapers Ltd. Retrieved on March 14, 2009.
Sampling by deep
trawling
Trawling is a method of fishing that involves pulling a fishing net through the water behind one or more boats. The net used for trawling is called a trawl. This principle requires netting bags which are towed through water to catch different spec ...
indicates that
lanternfish account for as much as 65% of all deep-sea fish
biomass
Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bio ...
. Indeed, lanternfish are among the most widely distributed, populous, and diverse of all
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
s, playing an important
ecological role as prey for larger organisms. The estimated global biomass of lanternfish is 550–660 million
tonne
The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton ( United State ...
s, several times the entire world fisheries catch. Lanternfish also account for much of the biomass responsible for the
deep scattering layer of the world's oceans.
Bigeye tuna are an epipelagic/mesopelagic species that eats other fish. Satellite tagging has shown that bigeye tuna often spend prolonged periods cruising deep below the surface during the daytime, sometimes making dives as deep as 500 metres. These movements are thought to be in response to the vertical migrations of prey organisms in the
deep scattering layer.
File:Malacosteus niger.jpg, The stoplight loosejaw
The stoplight loosejaws are small, deep-sea dragonfishes of the genus ''Malacosteus'', classified either within the subfamily Malacosteinae of the family Stomiidae, or in the separate family Malacosteidae. They are found worldwide, outside of t ...
has a lower jaw one-quarter as long as its body. The jaw has no floor and is attached only by a hinge and a modified tongue bone. Large fang-like teeth in the front are followed by many small barbed teeth.
File:Malacosteus.JPG, The stoplight loosejaw
The stoplight loosejaws are small, deep-sea dragonfishes of the genus ''Malacosteus'', classified either within the subfamily Malacosteinae of the family Stomiidae, or in the separate family Malacosteidae. They are found worldwide, outside of t ...
is also one of the few fishes that produce red bioluminescence
Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some b ...
. As most of their prey cannot perceive red light, this allows it to hunt with an essentially invisible beam of light.
File:Longnoselancetfish.jpg, Long-snouted lancetfish. Lancetfish are ambush predators which spend all their time in the mesopelagic zone. They are among the largest mesopelagic fishes (up to 2 metres).
File:Daggertooth.PNG, The daggertooth
The daggertooths (genus ''Anotopterus'') are a genus of marine mesopelagic fish in the order Aulopiformes, the sole genus of the family Anotopteridae. They are found in oceans worldwide, but prefer cooler waters.
Description
Daggertooths are not ...
paralyses other mesopelagic fish when it bites them with its dagger-like teeth
Bathypelagic fish
Below the mesopelagic zone it is pitch dark. This is the midnight (or
bathypelagic zone
The bathypelagic zone or bathyal zone (from Greek βαθύς (bathýs), deep) is the part of the open ocean that extends from a depth of below the ocean surface. It lies between the mesopelagic above, and the abyssopelagic below. The bathypelagi ...
), extending from 1000 metres to the bottom deep water
benthic zone
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...
. If the water is exceptionally deep, the pelagic zone below 4000 metres is sometimes called the lower midnight (or
abyssopelagic zone). Temperatures in this zone range from 1 to 4 degrees celsius and is completely aphotic.
Conditions are somewhat uniform throughout these zones; the darkness is complete, the pressure is crushing, and temperatures, nutrients and dissolved oxygen levels are all low.
Bathypelagic fish have special
adaptation
In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...
s to cope with these conditions – they have slow
metabolism
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run ...
s and unspecialized diets, being willing to eat anything that comes along. They prefer to sit and wait for food rather than waste energy searching for it. The behaviour of bathypelagic fish can be contrasted with the behaviour of mesopelagic fish. Mesopelagic fish are often highly mobile, whereas bathypelagic fish are almost all lie-in-wait predators, normally expending little energy in movement.
The dominant bathypelagic fishes are small
bristlemouth and
anglerfish
The anglerfish are fish of the teleost order Lophiiformes (). They are bony fish named for their characteristic mode of predation, in which a modified luminescent fin ray (the esca or illicium) acts as a lure for other fish. The luminescence ...
;
fangtooth,
viperfish,
daggertooth
The daggertooths (genus ''Anotopterus'') are a genus of marine mesopelagic fish in the order Aulopiformes, the sole genus of the family Anotopteridae. They are found in oceans worldwide, but prefer cooler waters.
Description
Daggertooths are not ...
and
barracudina are also common. These fishes are small, many about 10 centimetres long, and not many longer than 25 cm. They spend most of their time waiting patiently in the water column for prey to appear or to be lured by their phosphors. What little energy is available in the bathypelagic zone filters from above in the form of detritus, faecal material, and the occasional invertebrate or mesopelagic fish. About 20 percent of the food that has its origins in the epipelagic zone falls down to the mesopelagic zone,
but only about 5 percent filters down to the bathypelagic zone.
[Ryan ]
"Deep-sea creatures: The bathypelagic zone"
''Te Ara - the Encyclopedia of New Zealand''. Updated 21 September 2007.
Bathypelagic fish are sedentary, adapted to outputting minimum energy in a habitat with very little food or available energy, not even sunlight, only bioluminescence. Their bodies are
elongated with weak, watery muscles and
skeletal
A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...
structures. Since so much of the fish is water, they are not compressed by the great pressures at these depths. They often have extensible, hinged
jaws
Jaws or Jaw may refer to:
Anatomy
* Jaw, an opposable articulated structure at the entrance of the mouth
** Mandible, the lower jaw
Arts, entertainment, and media
* Jaws (James Bond), a character in ''The Spy Who Loved Me'' and ''Moonraker''
* ...
with recurved teeth. They are slimy, without
scales. The central nervous system is confined to the lateral line and olfactory systems, the eyes are small and may not function, and
gill
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...
s, kidneys and hearts, and
swim bladder
The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth wit ...
s are small or missing.
These are the same features found in fish
larvae
A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.
T ...
, which suggests that during their evolution, bathypelagic fish have acquired these features through
neoteny
Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compare ...
. As with larvae, these features allow the fish to remain suspended in the water with little expenditure of energy.
[Marshall (1984) "Progenetic tendencies in deep-sea fishes", pp. 91-101 in Potts GW and Wootton RJ (eds.) (1984) ''Fish reproduction: strategies and tactics'' Fisheries Society of the British Isles.]
Despite their ferocious appearance, these beasts of the deep are mostly miniature fish with weak muscles, and are too small to represent any threat to humans.
The swim bladders of deep sea fish are either absent or scarcely operational, and bathypelagic fish do not normally undertake vertical migrations. Filling bladders at such great pressures incurs huge energy costs. Some deep sea fishes have swim bladders which function while they are young and inhabit the upper epipelagic zone, but they wither or fill with fat when the fish move down to their adult habitat.
The most important sensory systems are usually the
inner ear
The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in th ...
, which responds to sound, and the
lateral line
The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...
, which responds to changes in water pressure. The
olfactory
The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste.
In humans, ...
system can also be important for males who find females by smell.
Bathypelagic fish are black, or sometimes red, with few
photophores. When photophores are used, it is usually to entice prey or attract a mate. Because food is so scarce, bathypelagic predators are not selective in their feeding habits, but grab whatever comes close enough. They accomplish this by having a large mouth with sharp teeth for grabbing large prey and overlapping
gill rakers which prevent small prey that have been swallowed from escaping.
It is not easy finding a mate in this zone. Some species depend on
bioluminescence
Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some b ...
, where bioluminescent patterns are unique to specific species. Others are
hermaphrodite
In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes.
Many taxonomic groups of animals (mostly invertebrates) do not have ...
s, which doubles their chances of producing both eggs and sperm when an encounter occurs.
The female anglerfish releases
pheromones to attract tiny males. When a male finds her, he bites on to her and never lets go. When a male of the anglerfish species ''
Haplophryne mollis'' bites into the skin of a female, he releases an
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
that digests the skin of his mouth and her body, fusing the pair to the point where the two circulatory systems join up. The male then atrophies into nothing more than a pair of
gonads. This extreme
sexual dimorphism
Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most an ...
ensures that, when the female is ready to spawn, she has a mate immediately available.
Many forms other than fish live in the bathypelagic zone, such as squid, large whales, octopuses, sponges,
brachiopods, sea stars, and
echinoids, but this zone is difficult for fish to live in.
File:Eurypharynx pelecanoides.jpg, The pelican eel uses its large mouth like a net by opening its jaws and swimming towards prey. It has a luminescent organ at the tip of its tail to attract prey.
File:Chiasmodon niger.jpg, The black swallower
The black swallower, ''Chiasmodon niger'', is a species of deep sea fish in the family Chiasmodontidae. It is known for its ability to swallow fish larger than itself.
It has a worldwide distribution in tropical and subtropical waters, in the m ...
, with its distensible stomach, is notable for its ability to swallow, whole, bony fish
Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartil ...
es ten times its mass.
File:Hamol u0.gif, Female '' Haplophryne mollis'' anglerfish trailing attached males which have atrophied into a pair of gonads, for use when the female is ready to spawn.
Adaptation to high pressure
As a fish moves deeper into the sea, the weight of the water overhead exerts increasing
hydrostatic pressure
Fluid statics or hydrostatics is the branch of fluid mechanics that studies the condition of the equilibrium of a floating body and submerged body " fluids at hydrostatic equilibrium and the pressure in a fluid, or exerted by a fluid, on an i ...
on the fish. This increased pressure amounts to about one
standard atmosphere for every 10 meters in depth (one atmosphere is the pressure exerted at the surface of the sea by the atmosphere above). For a fish at the bottom of the
bathypelagic zone
The bathypelagic zone or bathyal zone (from Greek βαθύς (bathýs), deep) is the part of the open ocean that extends from a depth of below the ocean surface. It lies between the mesopelagic above, and the abyssopelagic below. The bathypelagi ...
, this pressure amounts to about 400 atmospheres (nearly 6000 pounds per square inch).
Deep sea organisms possess adaptations at cellular and physiological levels that allow them to survive in environments of great pressure. Not having these adaptations limits the depths at which shallow-water species can operate. High levels of external pressure affects how metabolic processes and biochemical reactions proceed. The equilibrium of many chemical reactions is disturbed by pressure, and pressure can inhibit processes which result in an increase in volume. Water, a key component in many biological processes, is very susceptible to volume changes, mainly because constituents of cellular fluid have an effect on water structure. Thus, enzymatic reactions that induce changes in water organization effectively change the system's volume. Proteins responsible for catalyzing reactions are typically held together by weak
bonds and the reactions usually involve volume increases.
To adapt to this change, the protein structure and reaction criteria of deep sea fish have been adapted to withstand pressure in order to perform reactions in these conditions. In high pressure environments, bilayer
cellular membranes experience a loss of fluidity. Deep-sea cellular membranes favor
phospholipid bilayers with a higher proportion of
unsaturated fatty acid
In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, f ...
s, which induce a higher fluidity than their sea-level counterparts.
Deep sea species exhibit lower changes of
entropy
Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodyna ...
and
enthalpy
Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...
compared to surface level organisms, since a high pressure and low temperature environment favors negative enthalpy changes and reduced dependence on entropy-driven reactions. From a structural standpoint,
globular protein
In biochemistry, globular proteins or spheroproteins are spherical ("globe-like") proteins and are one of the common protein types (the others being fibrous, disordered and membrane proteins). Globular proteins are somewhat water-soluble (for ...
s of deep sea fish due to the tertiary structure of
G-actin are relatively rigid compared to those of surface level fish. The fact that proteins in deep sea fish are structurally different from surface fish is apparent from the observation that actin from the muscle fibers of deep sea fish are extremely heat resistant; an observation similar to what is found in lizards. These proteins are structurally strengthened by modification of the bonds in the tertiary structure of the protein which also happens to induce high levels of thermal stability. Proteins are structurally strengthened to resist pressure by modification of bonds in the tertiary structure. Therefore, high levels of hydrostatic pressure, similar to high body temperatures of thermophilic desert reptiles, favor rigid protein structures.
Na+/K+ -ATPase is a
lipoprotein
A lipoprotein is a biochemical assembly whose primary function is to transport hydrophobic lipid (also known as fat) molecules in water, as in blood plasma or other extracellular fluids. They consist of a triglyceride and cholesterol center, ...
enzyme that plays a prominent role in
osmoregulation and is heavily influenced by hydrostatic pressure. The inhibition of Na+/K+ -ATPase is due to increased compression due to pressure. The rate-limiting step of the Na+/K+ -ATPase reaction induces an expansion in the bilayer surrounding the protein, and therefore an increase in volume. An increase in volume makes Na+/K+ -ATPase reactivity susceptible to higher pressures. Even though the Na+/K+ -ATPase activity per gram of
gill tissue is lower for deep sea fishes, the Na+/K+ -ATPases of deep sea fishes exhibit a much higher tolerance of hydrostatic pressure compared to their shallow-water counterparts. This is exemplified between the species ''C. acrolepis'' (around 2000m deep) and its
hadalpelagic counterpart ''
C. armatus'' (around 4000m deep), where the Na+/K+ -ATPases of ''C. armatus'' are much less sensitive to pressure. This resistance to pressure can be explained by adaptations in the protein and lipid
moieties of Na+/K+ -ATPase.
Lanternfish
Sampling via deep trawling indicates that
lanternfish account for as much as 65% of all deep-sea fish
biomass
Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bio ...
. Indeed, lanternfish are among the most widely distributed, populous, and diverse of all
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
s, playing an important ecological role as prey for larger organisms. With an estimated global biomass of 550–660 million
metric ton
The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton (United States ...
s, several times the entire world fisheries catch, lanternfish also account for much of the biomass responsible for the
deep scattering layer of the world's oceans. In the
Southern Ocean
The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of , it is regarded as the second-smal ...
, Myctophids provide an alternative food resource to
krill for predators such as
squid
True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fittin ...
and the
king penguin
The king penguin (''Aptenodytes patagonicus'') is the second largest species of penguin, smaller, but somewhat similar in appearance to the emperor penguin. There are two subspecies: ''A. p. patagonicus'' and ''A. p. halli''; ''patagonicus'' ...
. Although these fish are plentiful and prolific, currently only a few commercial lanternfish fisheries exist: these include limited operations off
South Africa
South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring coun ...
, in the
sub-Antarctic, and in the
Gulf of Oman
The Gulf of Oman or Sea of Oman ( ar, خليج عمان ''khalīj ʿumān''; fa, دریای عمان ''daryâ-ye omân''), also known as Gulf of Makran or Sea of Makran ( ar, خلیج مکران ''khalīj makrān''; fa, دریای مکرا� ...
.
Endangered species
A 2006 study by Canadian scientists has found five species of deep-sea fish –
blue hake,
spiny eel – to be on the verge of extinction due to the shift of
commercial fishing
Commercial fishing is the activity of catching fish and other seafood for commercial profit, mostly from wild fisheries. It provides a large quantity of food to many countries around the world, but those who practice it as an industry must oft ...
from
continental shelves
A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...
to the slopes of the
continental shelves
A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...
, down to depths of 1600 meters. The slow reproduction of these fish – they reach
sexual maturity at about the same age as human beings – is one of the main reasons that they cannot recover from the
excessive fishing.
See also
*
Census of Marine Life
*
Deep ocean water
*
Deep sea
*
Deep sea communities
A deep sea community is any community of organisms associated by a shared habitat in the deep sea. Deep sea communities remain largely unexplored, due to the technological and logistical challenges and expense involved in visiting this remote b ...
*
Deep water fish
*
Demersal fish
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 oc ...
*
Pelagic fish
Pelagic fish live in the pelagic zone of ocean or lake waters—being neither close to the bottom nor near the shore—in contrast with demersal fish that do live on or near the bottom, and reef fish that are associated with coral re ...
Citations
References
*
*
*
*
*
*
*
Further reading
* Gordon J. D. M. (2001
"Deep-sea fishes"In: John H. Steele, Steve A. Thorpe, Karl K. Turekian (eds.) ''Elements of Physical Oceanography'', pages 227–233, Academic Press. .
* Hoar W. S., Randall D. J. and Farrell A. P. (eds.) (1997
''Deep-Sea Fishes'' Academic Press. .
* Shotton, Ross (1995
In: ''Review of the state of world marine fishery resources'', FAO Fisheries technical paper 457, FAO, Rome. .
* Tandstad M., Shotton R., Sanders J. and Carocci F. (2011
"Deep-sea Fisheries"In: ''Review of the state of world marine fishery resources'', pages 265–278, FAO Fisheries technical paper 569, FAO, Rome. .
External links
* https://www.pbs.org/wgbh/nova/abyss/life/bestiary.html
* http://ocean.nationalgeographic.com/ocean/photos/deep-sea-creatures/
Deep Sea Creatures- Articles, facts and images of deep sea animals
{{DEFAULTSORT:Deep Sea Fish
Deep sea fish
Articles containing video clips
Oceanographical terminology
Ichthyology