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.
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.
From 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...