Editing Deep-sea fish (section)

==Mesopelagic fish==
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|caption1=Most mesopelagic fishes are small filter feeders which ascend at night to feed in the nutrient rich waters of the epipelagic zone. During the day, they return to the dark, cold, oxygen deficient waters of the mesopelagic where they are relatively safe from predators. [[Lanternfish]] account for as much as 65% of all deep-sea fish [[biomass]] and are largely responsible for the [[deep scattering layer]] of the world's oceans.
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|caption2=Most of the rest of the mesopelagic fishes are ambush predators, like this [[sabertooth fish]] which uses its telescopic, upward-pointing eyes to pick out prey silhouetted against the gloom above. Their recurved teeth prevent a captured fish from backing out.
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|caption3=The [[Antarctic toothfish]] have large, upward looking eyes, adapted to detecting the silhouettes of prey fish<ref name=Dissostichus>{{FishBase|genus=Dissostichus|species=mawsoni|year=2009|month=August}}</ref>
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|caption4=The [[barreleye]] has barrel-shaped, tubular [[eye]]s which are generally directed upwards but can be swivelled forward<ref name=sciencedaily>[https://www.sciencedaily.com/releases/2009/02/090223150331.htm Mystery Of Deep-sea Fish With Tubular Eyes And Transparent Head Solved] ''ScienceDaily'', 24 February 2009.</ref>
|image5=gigantura chuni.png
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|caption5=The [[telescopefish]] has large, forward-pointing telescoping eyes with large lenses<ref name=Gigantura>{{FishBase|genus=Gigantura|species=chuni|year=2010|month=October}}</ref>
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Below the epipelagic zone, conditions change rapidly. Between 200&nbsp;m and about 1000&nbsp;m, light continues to fade until there is almost none. Temperatures fall through a [[thermocline]] to temperatures between {{convert|3.9|and|7.8|°C|°F|abbr=on}}. This is the '''twilight''' or [[mesopelagic]] zone. Pressure continues to increase, at the rate of one atm (0.1&nbsp;MPa) every {{convert|10|m|ft|abbr=on}}, while nutrient concentrations fall, along with dissolved oxygen and the rate at which the water circulates.{{sfn|Moyle|Cech|2004|pp=585}}

[[Sonar]] operators, using the newly developed sonar technology during {{nowrap|[[World War II]]}}, were puzzled by what appeared to be a false sea floor {{convert|300–500|m|0}} deep by 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]].<ref name="TeAraMZ">Ryan P., [http://www.teara.govt.nz/EarthSeaAndSky/SeaLife/DeepSeaCreatures/2/en "Deep-sea creatures: The mesopelagic zone"], ''Te Ara – the Encyclopedia of New Zealand''. Updated 21 September 2007.</ref>

Most mesopelagic fish make daily [[Diel vertical migration|vertical migration]]s, moving at night into the epipelagic zone, often following similar migrations of zooplankton, and returning to the depths for safety during the day.{{sfn|Moyle|Cech|2004|pp=585}}<ref name=Bone2008>Bone & Moore 2008, p. 38.</ref> These vertical migrations often occur over large vertical distances, and are undertaken with the assistance of a [[swim bladder]].<ref name="Haedrich1996">Haedrich R. L. (1996) [https://archive.today/20121017043834/http://www3.interscience.wiley.com/journal/119203565/abstract "Deep-water fishes: evolution and adaptation in the earth's largest living spaces"], ''Journal of Fish Biology'', '''49'''(sA):40–53.
</ref> 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.<ref name=Douglas1976>{{cite journal |author1=Douglas E. L. |author2=Friedl W. A. |author3=Pickwell G. V. |year=1976 |title=Fishes in oxygen-minimum zones: blood oxygenation characteristics |url=http://www.sciencemag.org/cgi/content/abstract/191/4230/957 |journal=Science |volume=191 |issue=4230 |pages=957–959 |doi=10.1126/science.1251208 |pmid=1251208 |bibcode=1976Sci...191..957D}}</ref> Some mesopelagic fishes make daily migrations through the [[thermocline]], where the temperature changes between {{convert|50|and|69|°F|°C|abbr=on}}, thus displaying considerable tolerances for temperature change.{{sfn|Moyle|Cech|2004|pp=590}}

These fish have muscular bodies, ossified bones, scales, well developed gills and central nervous systems, and large hearts and kidneys. Mesopelagic [[Filter feeder|plankton feeders]] have small mouths with fine [[gill raker]]s, while the [[piscivore]]s have larger mouths and coarser gill rakers.{{sfn|Moyle|Cech|2004|pp=585}}

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 cell]]s that look upwards. These give binocular vision and great sensitivity to small light signals.{{sfn|Moyle|Cech|2004|pp=585}} This adaptation gives improved terminal vision at the expense of lateral vision, and allows the predator to pick out [[squid]], [[cuttlefish]], and smaller fish that are silhouetted against the gloom above them.

Mesopelagic fish usually lack defensive spines, and use colour to [[camouflage]] themselves from other fish. [[Ambush predator]]s 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 [[Countershading|countershaded]] silvery colours. On their bellies, they often display [[photophore]]s producing low grade light. For a predator from below, looking upwards, this [[bioluminescence]] 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.<ref name=Munz1976>{{cite journal |author=Munz W. R. A. |year=1976 |title=On yellow lenses in mesopelagic animals |journal=Marine Biological Association of the UK |volume=56 |issue=4 |pages=963–976 |doi=10.1017/S0025315400021019|bibcode=1976JMBUK..56..963M |s2cid=86353657 }}</ref>

The [[brownsnout spookfish]], 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.<ref name="wagner et al">{{cite journal |author1=Wagner, H. J. |author2=Douglas, R. H. |author3=Frank, T. M. |author4=Roberts, N. W. |author5=Partridge, J. C. |title=A Novel Vertebrate Eye Using Both Refractive and Reflective Optics |journal=Current Biology |volume=19 |pages=108–114 |year=2009 |pmid=19110427 |doi=10.1016/j.cub.2008.11.061 |issue=2
|s2cid=18680315 |doi-access=free|bibcode=2009CBio...19..108W }}</ref><ref name="smith">Smith, L. (Jan. 8, 2009). [https://web.archive.org/web/20110629123143/http://www.timesonline.co.uk/tol/news/environment/article5469077.ece "Fish with four eyes can see through the deep sea gloom"]. ''Times Online''. Times Newspapers Ltd. Retrieved on March 14, 2009.</ref>

Sampling by deep [[trawling]] indicates that [[lanternfish]] account for as much as 65% of all deep-sea fish [[biomass]].{{sfn|Paxton|Eschmeyer|1998|pp=127-8}} Indeed, lanternfish are among the most widely distributed, populous, and diverse of all [[vertebrate]]s, playing an important [[ecology|ecological]] role as prey for larger organisms. The estimated global biomass of lanternfish is 550–660&nbsp;million [[tonne]]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.<ref name=Cornejo>{{cite web |title=Deep-sea fish diversity and ecology in the benthic boundary layer |author1=R. Cornejo |author2=R. Koppelmann |author3=T. Sutton |url=http://www.agu.org/meetings/os06/os06-sessions/os06_OS45Q.html |access-date=2 March 2013 |archive-date=1 June 2013 |archive-url=https://web.archive.org/web/20130601024839/http://www.agu.org/meetings/os06/os06-sessions/os06_OS45Q.html |url-status=dead }}</ref>

[[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 {{convert|500|m|0}}. These movements are thought to be in response to the vertical migrations of prey organisms in the [[deep scattering layer]].

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File:Malacosteus niger.jpg|The [[stoplight loosejaw]] 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.<ref name="kenaley">{{cite journal |author=Kenaley, C. P. |title=Revision of the Stoplight Loosejaw Genus ''Malacosteus'' (Teleostei: Stomiidae: Malacosteinae), with Description of a New Species from the Temperate Southern Hemisphere and Indian Ocean |journal=Copeia |volume=2007 |issue=4 |pages=886–900 |year=2007 |doi=10.1643/0045-8511(2007)7[886:ROTSLG]2.0.CO;2|s2cid=1038874 }}</ref><ref name="sutton">{{cite journal |author=Sutton, T. T. |title=Trophic ecology of the deep-sea fish ''Malacosteus niger'' (Pisces: Stomiidae): An enigmatic feeding ecology to facilitate a unique visual system? |journal=Deep-Sea Research Part I: Oceanographic Research Papers |volume=52 |issue=11 |year=2005 |pages=2065–2076 |doi=10.1016/j.dsr.2005.06.011 |bibcode=2005DSRI...52.2065S |url=https://nsuworks.nova.edu/cgi/viewcontent.cgi?article=1213&context=occ_facpresentations}}</ref>
File:Malacosteus.JPG|The [[stoplight loosejaw]] is also one of the few fishes that produce red [[bioluminescence]]. As most of their prey cannot perceive red light, this allows it to hunt with an essentially invisible beam of light.<ref name="kenaley"/>
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 {{convert|2|m|ft|abbr=on}}.{{sfn|Moyle|Cech|2004|pp=336}}
File:Daggertooth.PNG|The [[Anotopterus|daggertooth]] paralyses other mesopelagic fish when it bites them with its dagger-like teeth<ref name=Anotopterus>{{FishBase|genus=Anotopterus|species=pharao|year=2010|month=April}}</ref>
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