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More than 7000 metres beneath the ocean鈥檚 surface in a trench off the western coast of South America, a bright blue fish swims in endless darkness.

The species has now been described by scientists for the first time, and its discovery may yield clues on how fish living in the deepest part of the planet have evolved a lifestyle to fit their extreme environment.

In 2018, at Newcastle University in the UK and his colleagues participated in a scientific research cruise that aimed to study the Atacama Trench, a dramatic gash in the Pacific Ocean floor running parallel with Chile and Peru.

Linley and his team deployed platforms outfitted with lights and cameras that sat on the seafloor, automatically snapping photos and recording video at regular time intervals. Some of these landers were plopped more than 6000 metres down in the deepest ocean realm 鈥� the hadal zone.

Thanks to baited traps, the team鈥檚 cameras spotted three different species of snailfish in these hadal depths. Snailfish 鈥� slowly swimming, gelatinous, tadpole-shaped creatures 鈥� are the deepest dwelling fishes on the planet, with some living in waters deeper than 8100 metres.

Two of the Atacama snailfish species were typical denizens of the extreme deep, says Linley: ghostly white with tiny eyes. But the third species looked very different. It was an intense, azure blue in colour, with large eyes. It also lacked the sucking disc on its underside found in many other snailfishes.

When the team鈥檚 traps retrieved a specimen and the researchers analysed the fish鈥檚 genes and physical features, they found it was more closely related to snailfish found near Antarctica, which mostly live within 3000 metres of the surface, than to its neighbours in the Atacama Trench. This may mean that these bright blue snailfish evolved from cold-water Antarctic ancestors that somehow migrated north to the trench. The fish was also distinct enough to warrant classification as a new species, Paraliparis selti.

The finding is exciting, says Linley, because 鈥渢his new species is from a very different branch of the [snailfish] family tree, and so it has adapted to living super deep independently of its relatives鈥�.

Until now, there was just one known lineage of hadal snailfishes.

Evolution for life in these incredible depths happening more than once, and independently, 鈥渟hows that the snailfish family are somehow more able to adapt to and make a living in the deep ocean trenches鈥�, says Linley. 鈥淟ighting struck twice.鈥�

It is still unclear what P. selti鈥檚 intense blue colour may be for, as most deep-sea fish are white, jet black, or bright red, says Linley. The colour may be an evolutionary holdover from its shallow water ancestors, or the brilliant blue could assist the fish in its deep-sea life in a manner yet to be discovered.

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