This very low light ‘twilight zone’ (~200-1000m depth) divides the surface epipelagic waters from the deep ocean. Sunlight is too dim for photosynthesis, but in the upper mesopelagic zone there is enough to enable some visual predators to exploit their prey. Often used as a refuge by species that migrate upward at night to feed in more productive epipelagic waters when predation risk is lower, it supports a high but unknown biomass of fish and planktonic detritivores, relying on flux of nutrients from upper oceanic layers. Low oxygen zones can form where patches of high biological activity deplete this limited resource. Bioluminescence is a common trait in mesopelagic organisms.