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By Isabelle GerretsenFeatures correspondent@izzygerretsen

Scientists have recorded the deepest fish ever on camera. How have animals adapted to survive in the dark, crushing depths of our oceans?

Last week, scientists filmed a fish swimming at a depth of more than 8km (27,000ft), setting a new record for the deepest ever fish documented by humans.

The unknown type of snailfish of the genus Pseudoliparis was identified by Alan Jamieson, a marine biologist at the University of Western Australia, and captured by an autonomous camera swimming at a depth of 8,336m (27,349ft) in the Izu-Ogasawara trench, south-east of Japan.

The previous deepest fish recorded was the Mariana snailfish (Pseudoliparis swirei), recorded at a depth of 8,178m (26,831ft) further south between Japan and Papua New Guinea in the Mariana Trench.

Life in the deep

The hadal zone is home to a rich, if somewhat alien, ecosystem of marine life. One of the deepest dwelling creatures discovered to date was a giant amphipod (a type of crustacean closely related to a shrimp) measuring more than 8cm (3in) long, living five miles (8km) beneath the surface in the Peru-Chile Trench. Johanna Weston, a marine biologist from Newcastle University, made the startling discovery in 2018.

The deepest parts of the ocean are known as the hadal zone, named after the Greek god of the underworld, Hades.

The hadal zone, which extends from 6 to 11km (20,000 to 36,000ft), is a forbidding place, characterised by complete darkness, crushing pressure and near-freezing temperatures.

For a long time scientists believed that life in the ocean depths was impossible due to these hostile conditions, but that perception changed dramatically in 1977, when a US research team dropped a remotely operated vehicle 8,000ft (2,440m) into the Pacific Ocean to take images from hydrothermal vents, where seawater meets magma. They were astonished to find these deep-sea vents teeming with life.

Since 1977, marine scientists have discovered up to 600 never-before-seen species around these vents, including a scaly-foot gastropod, (Chrysomallon squamiferum), a type of snail with iron armour, and a new crab named 'The Hoff' (Kiwa tyleri) after US actor David Hasselhoff due to its hairy chest.

Scientists are intrigued by these discoveries and species' ability to survive in the extreme pressures, cold temperatures and pitch black of the hadal zone.

At the bottom of the Mariana Trench there is 1,086 bars of pressure, the equivalent of 100 elephants standing on your head, according to Abbie Chapman, a researcher at University College London who has studied creatures living around hydrothermal vents.

How can creatures thrive in such extreme environments?

Marine animals living in the hadal zone have adapted on a cellular level to enable them to withstand the oppressive conditions.

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Deep ocean discoveries

In 2019, US explorer Victor Vescovo's marine research organisation Caladan Oceanic discovered the deepest shipwreck in history: the USS Johnston, which sank in 1944 after a battle against the world's largest battleship. The wreck was discovered submerged 6km (3.7 miles) below the water off the coast of the Philippines.

Creatures such as giant amphipod crustaceans and the Mariana snailfish have high concentrations of organic molecules called piezolytes (the name comes from the Greek word "piezin" which means pressure), which stop their cellular membranes and proteins from being crushed under extremely high pressure.

These molecules counteract the weight of the surrounding water column by increasing the space that proteins take up inside the organism's cells. According to deep-sea biologist Tim Shank at the Woods Hole Oceanographic Institution in Massachusetts, "it's like putting the stakes up in a tent".

Studies show that the amount of the piezolyte molecule trimethylamine N-oxide (TMAO) increases in ocean organisms in line with the depth of their habitat.

Researchers from the University of Leeds concluded in a 2022 study that TMAO acts like "an anchor point within the water network" by forming strong hydrogen bonds with water molecules. This allows the organism to resist the extreme pressure it is under.

Fish living closer to the ocean surface have a swim bladder, a gas-filled organ which allows them to remain buoyant without sinking or floating to the top. Deep-sea fish such as snailfish don't have swim bladders, as the difference in pressure between the gas-filled cavity and the water pushing in on the outside would rupture them.

Watch the film captured by scientists of a species of snailfish swimming at the deepest ever recorded. The first 15 seconds show the record breaking fish and later fish in the video are at a slightly shallower depth.

In the deep ocean there is no direct sunlight and therefore creatures cannot rely on photosynthesis to convert the Sun's energy into sugars to fuel them. Instead they use chemosynthesis to create sugars using energy released from chemical reactions occurring around the hydrothermal vents in the ocean floor. "They live off chemicals coming out of the sea floor," says Shank.

Deep-sea fish have also adapted to survive in a low-oxygen environment. Mexican cavefish, for example, have larger red blood cells that produce higher concentrations of haemoglobin, the protein that carries oxygen throughout the body, than fish that dwell near the surface, according to a 2022 study.

All these adaptations allow creatures to thrive in the dark abyss of the deep ocean, in one of the harshest environments on our planet.

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