Few fish can survive the immense pressure of living in the deep sea. The Mariana Trench can reach around 36,200 feet deep with pressures of over 1,000 atmosphere. Scientists have placed landers at various depths ranging from 16,400-34,750 feet along the walls of the trench and have discovered what appears to be the "World's Deepest Fish". The fish was found at 26,872 feet which is estimated to be near the lower limit for bony fishes (of 27,600 feet). This newly discovered fish has what look like wings and tentacles. The video below was captured by the Hadal Ecosystem Studies expedition (HADES) sponsored by the National Science Foundation. Without a live specimen, however, it remains nameless.

Fish living in the deep ocean have large amounts of a chemical called TMAO (trimethylamine N-oxide), which not only helps to regulate the cellular osmotic balance but also ensure proteins stay folded properly. Turns out the deeper a fish lives, the more TMAO it has.

Not surprisingly, ocean fish are not as salty as their environment. Keeping in mind the mantra that "water follows salt", this protein (among other mechanisms) helps prevent the fish from losing water to the environment and hence prevents dehydration. TMAO does this by helping to increase the osmolarity of fish.

Sources

Yancey PH, Gerringer ME, Drazen JC, Rowden AA, Jamieson A. (2014). Marine fish may be biochemically constrained from inhabiting the deepest ocean depths, Proceedings of the National Academy of Sciences, 111 (12) 4461-4465. DOI:http://dx.doi.org/10.1073/pnas.1322003111

Scientific American