Hadal zone

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.^[1][2]

"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." ^[3] 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.^[4]

Fauna

The hadal zone is the deepest part of the marine environment

Marine life decreases 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 a geological time-scale trenches can therefore be accessible as stenobathic (adapted to a limited depth range) evolve into a 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.^[5]

Conditions

Most life at this depth is sustained by marine snow or the chemical reactions around thermal vents. The low nutrient level, extreme pressure and lack of sunlight create hostile living conditions in which few species are able to exist. As no sunlight reaches this layer of the ocean, deep sea creatures have reduced eyesight, with very large eyes for receiving only bioluminescent flashes.

The most common organisms include jellyfish, viperfish, tube worms and sea cucumbers.^[6] The hadal zone can reach far below 6,000 meters (20,000 feet) deep; the deepest known extends to 10,911 meters (35,814 ft).^[7] At such depths the pressure in the hadal zone exceeds 1,100 standard atmospheres (110 MPa; 16,000 psi). Lack of light and extreme pressure makes this part of the ocean difficult to explore.

Exploration

In 1960, Jacques Piccard and Don Walsh reached the bottom of the Mariana Trench, the deepest known trench on Earth, and observed life.^[8] James Cameron also reached the bottom in 2012 using the Deepsea Challenger.^[9]

See also

• Abyssal plain
• Deep sea

References

Notes

Sources

Further reading

• Alan Jamieson: The hadal zone - life in the deepest oceans. Cambridge Univ. Press, Cambridge 2015, ISBN 978-1-10-701674-3.

External links

• Forscher filmen lebende Fische in Rekordtiefe (In German) from Spiegel 10/09/2008 about an expedition filming fish at a depth of more than 7,000 m