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photic zone marine snow includes dead or dying plankton protists diatoms 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 microbes zooplankton 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 can be considered the foundation of deep sea mesopelagic and benthic ecosystems as sunlight cannot reach them deep sea organisms rely heavily on marine snow as an energy source 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 that also are attracted to the structures the 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 anglerfishes and the deep water benthic rattails 17 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 18 the few ray fins that do exist are mainly in the beryciformes and lampriformes which also are 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 19 many species move daily between zones in vertical migrations in the following table they are listed in the middle or deeper zone where they regularly are found species by pelagic zone zone species and species groups include epipelagic 6 mackerel requiem and whale sharks clupeiforms herring anchovy salmonidae salmon atheriniforms flyingfishes halfbeaks sauries perciforms jacks dolphinfish pomfrets barracudas tunas billfish mesopelagic lanternfish opah longnose lancetfish barreleye ridgehead sabretooth stoplight loosejaw marine hatchetfish 20 bathypelagic principally bristlemouth and anglerfish also fangtooth viperfish black swallower telescopefish hammerjaw daggertooth barracudina black scabbardfish bobtail snipe eel unicorn crestfish gulper eel flabby whalefish benthopelagic 6 rattail and brotula are particularly abundant benthic flatfish hagfish eelpout greeneye eel stingray lumpfish and batfish 6 comparative structure of pelagic fishes epipelagic mesopelagic bathypelagic deep sea benthic muscles muscular bodies poorly developed flabby skeleton strong ossified bones weak minimal ossification scales yes none nervous systems well developed lateral line and olfactory only eyes large and sensitive small and may not function variable well developed to absent photophores absent common common usually absent gills well developed kidneys large small heart large small swimbladder vertically migratory fish have swimbladders reduced or absent variable well developed to absent size usually under 25 cm variable species greater than one metre are not uncommon mesopelagic fish edit most mesopelagic fishes are small filter feeders that 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 most of the rest of the mesopelagic fishes are ambush predators such as this sabertooth fish the sabertooth 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 below the epipelagic zone conditions change rapidly between 200 metres and approximately 1000 metres light continues to fade until darkness is nearly complete temperatures fall through a thermocline to temperatures between 4 c 39 f and 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 2 21 sonar operators using the sonar technology developed 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 swimbladders that reflected the sonar mesopelagic organisms migrate into shallower water at dusk to feed on plankton the layer is deeper when the moon is out and may move higher when the sky is dark this phenomenon has come to be known as the deep scattering layer 22 most mesopelagic fish make daily vertical migrations moving each night into the epipelagic zone often following similar migrations of zooplankton and returning to the depths for safety during the day 21 2 23 these vertical migrations occur over hundreds of meters 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 21 2 vertically migratory fish have swimbladders 18 the fish inflates its swimbladder to move up given the high pressures in the mesopelagic zone this requires significant energy as the fish ascends the air in the swimbladder must decrease to prevent the swimbladder from bursting to return to the depths the swimbladder is deflated 24 the migration takes them through the thermocline where the temperature changes between 10 and 20 c thus displaying considerable temperature tolerance 25 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 such as the telescopefish have tubular eyes with big lenses and only rod cells that look upward these give binocular vision and great sensitivity to small light signals 2 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 above them 21 mesopelagic fish usually lack defensive spines and use colour for camouflage 21 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 silvery colours on their bellies they often display photophores producing low grade light for a predator from below looking upward 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 26 the antarctic toothfish have large upward looking eyes adapted to detecting the silhouettes of prey fish 27 the barreleye has barrel shaped tubular eyes that generally are directed upward but may be swivelled forward 28 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 29 30 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 29 the brownsnout spookfish is a species of barreleye and is the only vertebrate known to employ a mirror as opposed to a lens to focus an image in its eyes 31 32 sampling via deep trawling indicates that lanternfish account for as much as 65 of all deep sea fish biomass 33 indeed lanternfish are among the most widely distributed populous and diverse of all vertebrates playing an important ecological role as prey for larger organisms the estimated global biomass of lanternfish is 550 660 million tonnes 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 sonar reflects off the millions of lanternfish swim bladders giving the appearance of a false bottom 34 the 2010 malaspina circumnavigation expedition traveled 60 000 km undertaking acoustic observations it reported that mesopelagic biomass was 10 billion tonnes or more 10x prior estimates comprising about 90 percent of all ocean fish biomass 35 estimates of how much carbon these fish sequester remained highly uncertain as of 2024 36 mesopelagic fish do not constitute a major fishery as of 2024 initial efforts in iceland norway and the soviet union did not create a commercial industry the european union funded the meeso project to study abundance and fishing technologies for key mesopelagic species to date fish that appeal to the human palate have not been identified leading harvesters to focus on animal feed markets instead 36 bigeye tuna are an epipelagic mesopelagic species that is carnivorous eating 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 1 600 feet these movements are thought to be in response to the vertical migrations of prey organisms in the deep scattering layer longnose lancetfish lancetfish are ambush predators that frequent the mesopelagic they are among the largest mesopelagic fishes up to 2 metres 37 the telescopefish has large forward pointing telescoping eyes with large lenses 38 the daggertooth slashes other mesopelagic fish when it bites them with its dagger like teeth 39 bigeye tuna cruise the epipelagic zone at night and the mesopelagic zone during the day a collection of mesopelagic forage fishes trawled from the gulf of mexico that includes myctophids larval anglerfishes bristlemouths and a barracudina bathypelagic fish edit 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 40 many bristlemouth species such as the spark anglemouth above 41 are also bathypelagic ambush predators that can swallow prey larger than themselves they are among the most abundant of all vertebrate families 42 young red flabby whalefish make nightly vertical migrations into the lower mesopelagic zone to feed on copepods when males mature into 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 43 below the mesopelagic zone it is pitch dark this is the midnight or bathypelagic zone extending from 1000 m to the bottom deep water benthic zone if the water is exceptionally deep the pelagic zone below 4 000 metres 2 5 miles sometimes is called the lower midnight or abyssopelagic zone 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 2 bathypelagic fish have special adaptations to cope with these conditions they have slow metabolisms 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 are often highly mobile whereas bathypelagic fish are almost all lie in wait predators normally expending little energy in movement 44 the dominant bathypelagic fishes are small bristlemouth and anglerfish fangtooth viperfish daggertooth and barracudina are also common these fishes are small many about 10 centimetres 3 9 inches long and not many longer than 25 cm 9 8 in 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 44 about 20 of the food that has its origins in the epipelagic zone falls down to the mesopelagic zone 22 but only about 5 filters down to the bathypelagic zone 40 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 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 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 gills kidneys and hearts and swimbladders are small or missing 40 45 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 46 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 swimbladders 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 swimbladders that 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 47 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 also can be important for males who find females by smell 48 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 that prevent small prey that have been swallowed from escaping...
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