No light as was already discussed provided two problems; the second problem is the lack of sight. Sight for many animals is a sense that is relied on heavily, animals use it to locate prey, locate other members of their own species and use it to know which direction they are heading or where they are located. Although sight for some animals is not a problem such as sessile corals or other cnidarians that do not need to see where they are going, more complicated organisms such as teleost species that need to copulate or locate specific prey a lack of vision can be a big problem.
Living in the darkness is a constant battle between seeing and being seen, animals have developed various methods to try and keep themselves hidden just as other animals are constantly developing methods to find them. When you descend deeper than 1000 meters there is no visible light at all, the only light that can be found is that of bioluminescence, a process of where animals create their own light, it is through this method that many species rely on for finding their way around or capturing prey.
Bioluminescence is a process of where animals contain luciferin and luciferase. (Hastings 1983) When luciferin comes into contact with oxygen it produces light, and luciferase as an enzyme speeds up the reaction. Many species apply this for intertwining uses of attracting mates and enticing prey, some even use bioluminescence as ‘head lights’ to see what’s in front of them. Large numbers of cnidarians such as Aequorea victoria bioluminescence to entice prey towards them, this works because bioluminescence often acts in bursts, and as many animals are often looking for a mate flashes of light are usually investigated by other animals it could be a trap or a mate but it’s a risk that must be taken. Other types of fish such as Melanocetus johnsonii the humpback angler fish have extensions with bioluminescent lures that are smaller than the rest of their body, allowing them to appear small, this then acts as a lure to other species bringing them in to the predator.
Most predatory species have adaptations that allow them to eat a wide, diverse range of prey of almost any size. Many species such as Anoplogaster brachycera the fangtooth have enlarged mouths and teeth that enable the predator to catch prey easily, a drastic example of this can be seen in the gulper eels of the order Saccopharyngiformes have massive mouths that contribute the majority of their body mass enabling them to eat prey of almost any size, a very helpful adaption.