Editing Deep-sea fish (section)

==Bathypelagic fish==
{{multiple image
|align=right
|direction=vertical
|header=Bathypelagic fish
|width=260
|image1=MelanocetusJohnsoniiFord.jpg
|alt1=
|caption1=The [[humpback anglerfish]] is a bathypelagic ambush predator, which attracts prey with a bioluminescent lure. It can ingest prey larger than itself, which it swallows with an inrush of water when it opens its mouth.<ref name="TeAraBZ"/>
|image2=Gonostoma bathyphilum.jpg
|alt2=
|caption2=Many [[bristlemouth]] species, such as the "spark anglemouth" above,<ref name=Gonostoma/> are also bathypelagic ambush predators which can swallow prey larger than themselves. They are among the most abundant of all vertebrate families.<ref name=Gonostoma>{{FishBase family|family=Gonostoma|month=August|year=2009}}</ref>
|image3=Flabby whalefish.jpg
|alt3=
|caption3=Young, red [[flabby whalefish]] make nightly vertical migrations into the lower mesopelagic zone to feed on [[copepods]]. When males make the transition to adults, they develop a massive liver, and then their jaws fuse shut. They no longer eat, but continue to metabolise the energy stored in their liver.<ref name=royalsociety>{{cite web|url=http://journals.royalsociety.org/content/g06648352k5m1562/|archive-url=https://archive.today/20120709202901/http://journals.royalsociety.org/content/g06648352k5m1562/|url-status=dead|archive-date=2012-07-09|title=Connecting knowledge and people for more than 10 years}}</ref><ref name=aparticle>{{cite web|url=http://www.utsandiego.com/news/2009/jan/22/sci-fish-mystery-012209/|title=Scientists solve mystery: 3 fish are all the same|date=January 22, 2009|access-date=2009-01-22}}</ref>
|image4=Messina Straits Chauliodus sloani.jpg
|alt4=
|caption4=The [[Sloane's viperfish]] can make nightly migrations from bathypelagic depths to near surface waters.<ref name=Chauliodus>{{FishBase|genus=Chauliodus|species=sloani|year=2010|month=April}}</ref>
|image5=Anoplogaster cornuta 2.jpg
|alt5=
|caption5=The widespread [[fangtooth]] has the largest teeth of any fish, proportionate to body size.<ref name=Anoplogaster>{{FishBase|genus=Anoplogaster|species=cornuta|month=August|year=2009}}</ref> Despite their ferocious appearance, bathypelagic fish are usually weakly muscled and too small to represent any threat to humans.
}}

Below the mesopelagic zone it is pitch dark. This is the midnight (or [[bathypelagic zone]]), extending from {{convert|1000|m|0}} to the bottom deep-water [[benthic zone]]. If the water is exceptionally deep, the pelagic zone below {{convert|4000|m|0}} is sometimes called the lower midnight (or [[abyssopelagic zone]]). Temperatures in this zone range from {{convert|1|to|4|C}} and it 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.{{sfn|Moyle|Cech|2004|pp=585}}

Bathypelagic fish have special [[adaptation]]s to cope with these conditions – they have slow [[metabolism]]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.{{sfn|Moyle|Cech|2004|pp=594}}

The dominant bathypelagic fishes are small [[bristlemouth]] and [[anglerfish]]; [[fangtooth]], [[viperfish]], [[daggertooth]] and [[barracudina]] are also common. These fishes are small, many about {{convert|10|cm}} long, and not many longer than {{convert|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.{{sfn|Moyle|Cech|2004|pp=594}} About 20 percent of the food that has its origins in the epipelagic zone falls down to the mesopelagic zone,<ref name="TeAraMZ"/> but only about 5 percent filters down to the bathypelagic zone.<ref name="TeAraBZ">Ryan P [http://www.teara.govt.nz/EarthSeaAndSky/SeaLife/DeepSeaCreatures/3/en "Deep-sea creatures: The bathypelagic zone"] ''Te Ara - the Encyclopedia of New Zealand''. Updated 21 September 2007.</ref>

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 [[wiktionary:elongate|elongated]] with weak, watery muscles and [[skeleton|skeletal]] 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 [[Jaw#The jaw in fish|jaws]] with recurved teeth. They are slimy, without [[Fish scale|scale]]s. The central nervous system is confined to the lateral line and olfactory systems, the eyes are small and may not function, and [[gill]]s, kidneys and hearts, and [[swim bladder]]s are small or missing.<ref name="TeAraBZ"/>{{sfn|Moyle|Cech|2004|pp=587}} These are the same features found in fish [[larvae]], which suggests that during their evolution, bathypelagic fish have acquired these features through [[neoteny]]. As with larvae, these features allow the fish to remain suspended in the water with little expenditure of energy.<ref name="Marshall1984">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.</ref>

Despite their ferocious appearance, these forms are mostly miniature fish with weak muscles, and are too small to represent any threat to humans. An exception to the generally small size of bathypelagic fish is the [[Yokozuna slickhead]] (''Narcetes shonanmaruae''), described in 2021, which is the largest known entirely bathypelagic [[Osteichthyes|bony fish]] at over {{convert|2.5|m|ft}} in length.<ref name=":2">{{Cite journal |last1=Fujiwara |first1=Yoshihiro |last2=Tsuchida |first2=Shinji |last3=Kawato |first3=Masaru |last4=Masuda |first4=Kotohiro |last5=Sakaguchi |first5=Sakiko Orui |last6=Sado |first6=Tetsuya |last7=Miya |first7=Masaki |last8=Yoshida |first8=Takao |date=2022-07-01 |title=Detection of the Largest Deep-Sea-Endemic Teleost Fish at Depths of Over 2,000 m Through a Combination of eDNA Metabarcoding and Baited Camera Observations |journal=Frontiers in Marine Science |language=English |volume=9 |doi=10.3389/fmars.2022.945758 |doi-access=free |issn=2296-7745}}</ref>

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.<ref name=Horn1970>{{cite journal|author=Horn MH|year=1970|title=The swim bladder as a juvenile organ in stromateoid fishes|url=https://archive.org/stream/breviora351379harv/breviora351379harv_djvu.txt|journal=Breviora|volume=359|pages=1–9}}</ref>

A couple of known exceptions are the cusk-eel (''[[Holcomycteronus profundissimus]]''), retrieved from 7160 meters deep, and the rough abyssal grenadier (''[[Coryphaenoides yaquinae]]''), found at 7259 meters depth. These species still have functional swim bladders, which allows them to maintain high bone density and strong jaws.<ref>[https://journals.biologists.com/jeb/article/227/3/jeb246522/342685 In situ observation of a macrourid fish at 7259 m in the Japan Trench: swimbladder buoyancy at extreme depth]</ref>

The most important sensory systems are usually the [[inner ear]], which responds to sound, and the [[lateral line]], which responds to changes in water pressure. The [[olfactory]] system can also be important for males who find females by smell.<ref name=Jumper1991>{{cite journal|vauthors=Jumper GY, Bair RC|year=1991|title=Location by olfaction: a model and application to the mating problem in the deep-sea Hatchetfish ''Argyropelecus hemigymnus''|journal=The American Naturalist|volume=138|issue=6|pages=1431–58|jstor=2462555|doi=10.1086/285295|s2cid=84386858}}</ref>
Bathypelagic fish are black, or sometimes red, with few [[photophore]]s. 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 raker]]s which prevent small prey that have been swallowed from escaping.{{sfn|Moyle|Cech|2004|pp=587}}

It is not easy finding a mate in this zone. Some species depend on [[bioluminescence]], where bioluminescent patterns are unique to specific species. Others are [[hermaphrodite]]s, which doubles their chances of producing both eggs and sperm when an encounter occurs.<ref name="TeAraBZ"/> The female anglerfish releases [[pheromone]]s 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]] 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]] ensures that, when the female is ready to spawn, she has a mate immediately available.<ref name="doran">{{cite journal|url=http://www.nature.com/nature/journal/v256/n5512/abs/256038a0.html|author=Theodore W. Pietsch|title=Precocious sexual parasitism in the deep sea ceratioid anglerfish, Cryptopsaras couesi Gill|volume=256|issue=5512|doi=10.1038/256038a0|journal=Nature|pages=38–40|year=1975|bibcode=1975Natur.256...38P|s2cid=4226567}}</ref>

Many forms other than fish live in the bathypelagic zone, such as squid, large whales, octopuses, sponges, [[brachiopod]]s, sea stars, and [[echinoid]]s, but this zone is difficult for fish to live in.

<gallery>
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]], with its distensible stomach, is notable for its ability to swallow, whole, [[bony fish]]es ten times its mass.<ref name="jordan">{{cite book|title=A Guide to the Study of Fishes|url=https://archive.org/details/aguidetostudyfi02jordgoog|author=Jordan, D.S.|publisher=H. Holt and Company|year=1905}}</ref><ref name=Chiasmodon>{{fishBase|genus=Chiasmodon|species=niger|year=2009|month=August}}</ref>
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.
File:Narcetes shonanmaruae a frozen specimen.jpg|The [[Narcetes shonanmaruae|Yokozuna slickhead]] is the largest known bathypelagic bony fish, at over {{convert|250|cm|ft|sp=us}} in length.<ref name=":2" />
</gallery>