Hadal
The hadal zone (named after the realm of Hades, the underworld in Greek mythology), also known as the hadopelagic zone and composed of trench zones, is the delineation for the deepest trenches in the ocean. The hadal zone is found from a depth to the bottom of the ocean of around 6,000 to 11,000 metres (20,000 to 36,000 ft) and exists in long but narrow topographic V-shaped depressions.
"In total there are 33 trenches (27 subduction trenches and 6 trench faults) and 13 troughs around the world—46 individual hadal habitats in total." All the trenches together occupy an area less than one-quarter of one percent of the entire seafloor, with 84% of the hadal habitat found in the Pacific Ocean.
Marine life decrease with depth, both in abundance and biomass, but there is a wide range of metazoan organisms in the hadal zone, mostly benthic fauna, including fish, sea cucumber, bristle worms, bivalves, isopods, sea anemones, amphipods, and gastropods. Most of these trench communities probably originate from the abyssal plains but though they indeed have evolved adaptations to high pressure and low temperatures, such as lower metabolism, intra-cellular protein-stabilising osmolytes, and unsaturated fatty acids in cell membrane phospholipids, there is no consistent relationship between pressure and metabolic rate across these communities. Increased pressure can, instead, constrain the ontogenic or larval stages of organisms. Pressure increases 10-fold as an organism moves from sea level to a depth of 90 m (300 ft), whilst pressure only doubles as an organism moves from 6,000 to 11,000 m (20,000 to 36,000 ft). Over geological a time-scale trenches can therefore be accessible as stenobathic (adapted to a limited depth range) evolve into an eurybathic (adapted to a wide range of depths) fauna, such as grenadiers and natantian prawns. Trench communities do, nevertheless, display a contrasting degree of intra-trench endemism and inter-trench similarities at a higher taxonomic level.
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