The fish of the deep sea are among the most elusive and unusual looking creatures on Earth. Since many of these fish live in regions where there is no natural illumination, they cannot rely solely on their eyesight for locating prey and mates and avoiding predators; deep sea fish have evolved appropriately to the extreme sub-photic region in which they live. Many deep sea fish are bioluminescent, with extremely large eyes adapted to the dark, and they can have long feelers to help them locate prey or attract mates in the pitch dark of the deep ocean. The deep sea angler fish in particular has a long fishing-rod-like adaptation protruding from its face, on the end of which is a bioluminescent piece of skin that wriggles like a worm to lure its prey. The lifecycle of deep sea fish can be either permanently deeply submerged or they are sometimes born in shallower water and sink on becoming adults.

Due to the poor level of photosynthetic light reaching deep sea environments, most fish need to rely on organic matter sinking from higher levels, or, in rare cases, hydrothermal vents. This makes the deep sea much poorer in productivity than shallower regions, with most deep sea fish noticeably smaller and have larger mouths and guts than those known to us. It has also been found that the deeper a fish lives, the more jelly-like its flesh and the more minimal its bone structure. This makes them slower and less agile than surface fish.