There are high levels of endemism, and noteworthy examples of gigantism in amphipods, mysids, and isopods and dwarfism in nematodes, copepods, and kinorhynchs. Over 400 species are currently known from hadal ecosystems, many of which possess physiological adaptations to the extreme environmental conditions. Most organisms are scavengers and detrivores. The major sources of nutrients and carbon are fallout from upper layers, drifts of fine sediment, and landslides. They are characterized by complete lack of sunlight, low temperatures, nutrient scarcity, and extremely high hydrostatic pressures. The deepest ocean trenches are considered the least explored and most extreme marine ecosystems. Ecology The hadal zone is the deepest part of the marine environment Similar to other depth ranges, the fauna of the hadal zone can be broadly placed into two groups: the hadobenthic species (compare benthic) living on or at the seabottom/sides of trenches and the hadopelagic species (compare pelagic) living in the open water. Among others, this intermediate limit has been adopted by UNESCO. Although the hadal zone has gained widespread recognition and many continue to use the first proposed limit of 6,000 m (20,000 ft), it has been observed that 6,000–7,000 m (20,000–23,000 ft) represents a gradual transition between the abyssal and hadal zones, leading to the suggestion of placing the limit in the middle, at 6,500 m (21,300 ft).

These shallower trenches lack the distinct shift in lifeforms and are therefore not hadal. ĭepths in excess of 6,000 m (20,000 ft) are generally in ocean trenches, but there are also trenches at shallower depths. About 94% of the hadal zone is found in subduction trenches. The name refers to Hades, the ancient Greek god of the underworld. The term 'hadal' was first proposed in 1956 by Anton Frederik Bruun to describe the parts of the ocean deeper than 6,000 m (20,000 ft), leaving abyssal for the parts at 4,000–6,000 m (13,000–20,000 ft). During the early 1950s, the Danish Galathea II and Soviet Vitjaz expeditions separately discovered a distinct shift in the life at depths of 6,000–7,000 m (20,000–23,000 ft) not recognized by the broad definition of the abyssal zone. Historically, the hadal zone was not recognized as distinct from the abyssal zone, although the deepest sections were sometimes called 'ultra-abyssal'. Most hadal habitat is found in the Pacific Ocean. The cumulative area occupied by the 46 individual hadal habitats worldwide is less than 0.25% of the world's seafloor, yet trenches account for over 40% of the ocean's depth range. The hadal zone ranges from around 6 to 11 km (3.7 to 6.8 mi 20,000 to 36,000 ft) below sea level, and exists in long, narrow, topographic V-shaped depressions. The hadal zone, also known as the hadopelagic zone, is the deepest region of the ocean, lying within oceanic trenches.

Deep water abyssal zone animals