The discoverer of the titanic, Dr Robert Ballard famously referred to the deep sea as 'far more alien than going to mars or the moon.' The deep sea is one of the largest virtually unexplored ecosystems on the planet; it is found at a depth of 1000 fathoms  and is subject to adverse changes in temperature, pressure and light penetration amongst other factors. Therefore as expected fish decrease in abundance, and species diversity. This trend is prominent as in order to survive the harsh conditions of the deep sea, fish need a number of specific adaptations. Allowing them to ultimately survive, feed, and reproduce.
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The deep sea is one of the most hostile environments in the world, which a living organism is subjected to. As you progress from the surface (the epipelagic zone) through to the abyssopelagic zone near the basin of the ocean; the environmental characteristics begin to alter dramatically. Light, pressure, oxygen, temperature and food are abiotic factors that have all led to the fascinating adaptations of deep sea life. Pressure alone increases by 1 atmosphere for each 10m in depth which is an astonishing rate. The deep sea temperature remains between 2-4°c, which is just another factor inhabitants must overcome in order to survive, along with a reduced quantity and accessibility of essential factor's like ;oxygen, food and light.
Figure 1From the surface to its deepest depth the ocean is 11km deep, and with this distance comes a vast change in physiological feature of fish as they try to survive the changing conditions.
The bottom of the deep sea exists in darkness as little light penetrates through the surface. Therefore most inhabitants have to rely on their senses to survive. The fish require light to survive; they use it to locate food, during the mating season and to identify prey or predators. It is a vital resource but as light is absent in the deep sea, Many fish have developed special adaptations in order to survive. The viper fish (shown in figure 1) possesses large eyes to catch the little light present. This helps them to locate prey as well as avoid predators.  Light cannot penetrate any deeper than the epipelagic layer, therefore the only light present is that produced from the inhabitants themselves. A percentage of deep sea fish are able to create light through the chemical reaction; bioluminescence. 'Most of the light created by marine organisms is blue-green in colour. As blue light travels best in water and most marine organisms are sensitive to blue light.' Anglerfish produces the chemical luciferin which reacts with oxygen to create light. The light helps species communicate, attract a mate or prey, or deter predators. As the bioluminescent lure is believe to mimic the movements of zooplankton. 
Being the lower layer of the ocean, it is no wonder the pressure is immense for the fish of the deep. The pressure exist between 200-600 atm , and in order to survive this immense pressure deep sea creature have special adapted bodies which have no excess cavities, e.g. Swim bladders that would collapse under intense pressure, instead they have neutral buoyancy where there bladders are filled with lipids. The deep sea dragon fish lives at a depth of 5000 meter, to survive the dragon fish has soft ,flabby, flesh and bones this aid its ability to survive under the extreme pressure.
The deep waters are extremely cold reaching a maximum temperature of 3 degrees, deep sea temperature is relatively stable with the exception of hydrothermal vent where hot water is emitted, but there is little fluctuation in water temperature. The cold water slows the metabolism of the fish, so many of the deep sea animals move very slow and have special enzymes that deal with the unique environment. The sea cucumbers carry high levels of unsaturated fat in their cell walls to maintain membrane fluidity in this cold, high-pressure environment. The cold water is the poorest environment for oxygen content. As oxygen concentration decrease as the temperature does. Fish have evolved by being more effective at removing oxygen from the water by posing a large gill surface area. However in the deep sea oxygen is not replenished as due to the lack of light penetrating, photosynthesis does not occur. In addition the nutrient salt concentration of the deep sea is much higher than found above as the sea floor act of a graveyard for dead biological material. Therefore fish of the deep sea have a low metabolic rate to conserve energy reducing their need for oxygen. Also fish posse's hemocyanin respiratory protein; this protein has a very high affinity for O2 and large Bohr effects.  They also have a short diffuse distance from water to blood allowing the oxygen to be used immediately, satisfying over half of their oxygen and carbon dioxide exchange through the skin, so more oxygen can be collected through the day.
Due to the lack of photosynthesis, plant life exists at its lowest percentage in the deep sea. Zooplanktons and phytoplankton rarely find themselves in the deepest depth of the ocean. So many of the deep sea fish are predators but this means food is minimal in the waters. There are a number of fascinating feeding mechanisms that deep sea fish have employed. Firstly the formation and use of the bioluminescence for hunting, fish use their ability to produce light to mimic patterns that attract prey, the angler fish uses light producing bacteria that help create a special fishing rod like fin that hangs over the head. This light helps attract the prey. The viperfish use light in its mouth to lure the prey into its awaiting stomach.  Some deep sea fish do migrate upwards during the night when less predatory are present to feed but as food is in such low concentration , energy conservation is a high priority to deep sea fish and therefore most of the fish hunt by the lie and wait method. As deep sea fish poses a well developed lateral lines, this is a sense organ that can detect movement and vibrations in the water. This allows the deep sea fish to detect prey passing them so they can dart out to capture them.  With the shortage of food available in the deep sea, the inhabitants must be prepared to eat whatever they find and thus must be equipped for this; they have large mouth and huge jaws. piscivorous is a sit and wait predator, its morphological characteristics are large body size, large gape, long sharp depressible teeth and large eyes. Some have non functioning gill racks so that smaller fish can pass through to its mouth easily ,nearly all deep sea fish have protruding, flexible, huge mouth and extended alimentary canal in order to store more food and prolong the digestive time. Other deep sea fish like lamprey feed on the dead carcasses that fall from above, they are adapted to this mode of feeding as they contain barbells around there mouth which helps them feel for food in the mud. But it is the black dragon fish that is most evolved for predation as it has the ability to produce red light below its eye so can see other fauna where most fish have an inability to see red light. In order to live with the lack of food deep sea inhabitants posses a large lipid rich liver that do not function for buoyancy but as an important energy source. This allows them to use this in times of extreme shortage. The most reliable food source for the deep sea environment is the 'constant rain of organic debris' from the organism above. larvaceans have adapted to produce a mucus that can catch this 'marine snow' so it has a steady food supply.
With the shortage of food and the increase of carnivores in the deep sea, one of the most important aspects is protection against predators. Deep sea fish have developed many ways to survive the jaws of another inhabitant. Firstly coloration plays a massive role, as fish can produce a variety of colours and some fish like the flat fish can change it coloration to match it surroundings at the time. fish have distinct marking on their body to disguise themselves, the jacknife high hat have dark lines that run through its eyes to hide them, so you cannot tell where the fish or looking or if it is in fact a fish. Butterfly fish trick there predators with the presence of spots on their bodies that resembles eyes. Other fish exhibit collation known as counter shading, this means they are dark on top against the dark background and light on the bottom, so if a fish look up its disguised by the little light showing. Some fish have mirror like scales that reflect the light so the fish will mirror the background. Most fish that inhabit the deep sea are usually red or transparent as this means they will not be detected by predators.
Other forms of protect is the ability to produce venom, sting rays have poison glands that can help catch prey and ecsacpe predators. Others use electric organs that can be used as a defence mechanism or simply to feed . deep sea shrimp has an antennae that sense chemical in the water to help avoid predators whereas giant squid have two eyes , a larger one that looks to the surface catching light , this light in then passed onto the smaller one which can now look down with the possession of light to see any prey or predatory below.
Successful reproduction in lightless habitats in based mainly on chemical communication, as the fish must provide effective fertilisation in the absence of any visual orientation . In the deep sea many of the fish are simultaneous hermaphrodites; they can spawn with any individual encounter . Overall reproduction is very similar in the deep ocean to that of the fish at the surface, although fish in the deep sea are known to produce large quantities of sperm and eggs to increase their chances of successful reproduction.
The darkness of the habitat makes locating a mate is difficult as they are few and far between, so deep sea fish have evolved to be able to detect the smell of other fish in water, Latern fish have developed small body organs on their sides called photophores, these pigments can produce light that are arranged in specific pattern, these can be seen at long distances and are recognised by potential mates. Viper fish have a unique method of fertilization. Once there eggs are fertilized they float to the surface and become part of plankton, but as larva develop into juveniles they tend descend to the bottom, which mean food is not scarce during their essential development stage. This method increases the survival of viper fish by over 50 %. The angler fish is yet another fish who has a unique method of reproduction , the angler fish method ensure successful reproduction as the male anglerfish has a over developed olfactory organ which allows them to locate the female , the male is small and is therefore more active than the female . Once the female is located the male bites down and becomes a permanent attachment and begins to receive his nourishment from the female while he provides the sperm. The female can have more than one male attach which mean productivity is high and survival is more likely.
All fish characteristics their body to suit their environment, deep sea fish are no different those near coral reef are vertically compressed like angel fish; it allows them to move about in the crevices of the reef. As their highest priority is manoeuvrability  Sting rays that inhabit the benthic zone of the sea have a depressed shape. They have extended pectoral fins giving them a heart shaped disc. Fish that live on the floor of the ocean rely on their shape and camouflage to catch prey and to survive against predators. The benthic fish bury themselves in the floor of the ocean to hide from their predator; their anterior down turned mouth is useful for bottom feeders. There pectoral fins have sensory receptors and taste buds to locate food. The most amazing adaption of bottom dwelling fish is there negative buoyancy which allows them to lay on the bottom of the ocean with ease. This is just another adaption deep sea inhabitant have secured in order to live.
Deep sea fish are often described as bizarre looking as they have adapted their bodies to suit their habitat. These adaptions include large eyes, adjusted body shape, self-luminous cells and sensory organs. The deep sea is virtually unexplored compare to any other environment, meaning knowledge is minimal the only reason for the lack of understanding is our inability to research in the extreme conditions, the fish of this fierce habitat are extremely adapted, removal of the inhabitant alive is rare. proving that deep sea fish have evolved massively. They no longer resemble inhabitant from the layers above. You can go as far as to say they are almost a species of their own.
But ultimately adaptations are highly important in surviving any environment but in the deep sea the fish have had to evolve immensely just to survive.
 Deep sea and extreme shallow water habitats: affinities and adaptions by Franz Uiblein, Jorg Ott and Michael Stacowitsh 1996
 Adapted from http://www.mbari.org/earth/mar_tech/EITS/ob_deep/od_back.html
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