Why would a fish that lives in the bathypelagic zone lack a swim bladder and what adaptations would in maintain neutral bouyancy

Answer: The bathypelagic zone is a layer of the ocean that lies between 1000 and 4000 meters or 3,280-13,120 feet. This region has a considerable water pressure and receives no sunlight with temperatures ranging generally between  5° and 15° C or 41° and 59° F.

Due to the high pressure, fishes living here require specific adaptations. Neutral buoyancy is attained mostly by having reduced heavy tissues such as muscles or bones. This is a common strategy for deep water fishes that have a sparse food supply. Thus, such an energy expensive tissues that also includes a swim bladder or compounds such as lipids are either reduced or completely absent in bathypelagic fishes.

A swim bladder and other energy expensive tissues appear once more in bethopelagic fishes as they hover or swim slowly above the deep sea floor. These have access to an increased food supply compared with bathypelagic fishes.