At greater depths (~3,000-6,000m) than bathypelagic systems, these very deep open ocean ecosystems receive no light and rely solely on debris from upper layers for nutrients. Other resources such as oxygen are replenished via the ‘global ocean conveyer belt’ (thermohaline circulation) that starts when distant, polar surface waters cool and sink. There is a low diversity and low density of life, largely planktonic detritivores, along with some gelatinous invertebrates and scavenging or predatory fish like the anglerfish. Life histories body structures and physiological traits are adapted to the very high pressure and lack of light (e.g. non-visual sensory organs, specialised metabolic proteins, and low density body structures).