With pressure pushing down on it, a deep-sea fish has evolved a catalogue of adaptations to help it survive in the crushing depths of the Pacific Ocean. The fish has extra genes for repairing its DNA and for making a chemical that stabilises essential proteins. It has also lost many of the genes that underpin the sense of smell, perhaps because it has a limited diet.
The fish was collected in 2017, when the Chinese submersible Jiaolong descended into the Yap Trench in the western Pacific Ocean. At a depth of 6903 metres, the submersible caught two fish belonging to a previously unknown species.
Because the new species was found in the hadal zone, which begins at 6000 metres below sea level, it has been tentatively called Yap hadal snailfish. At such depths, there is no sunlight, the water is cold, food is scarce and the pressure is intense.
To find out how the Yap hadal snailfish survives, researchers led by Xinhua Chen at Fujian Agriculture and Forestry University in Fuzhou, China, have now sequenced its genome.
They found that the snailfish genome has extra copies of genes involved in DNA repair, including eight copies of a gene called rad51. Some DNA repair genes also contained mutations that would alter the proteins they coded for – although it is unclear at the moment if or how this is a helpful adaptation for life at depth.
The snailfish also had five copies of a gene called fmo3, which is crucial for the production of trimethylamine N-oxide (TMAO): a chemical that stabilises proteins and may protect them from becoming damaged by the intense pressure. In line with this, the snailfish’s muscle tissue had elevated levels of TMAO, compared with zebrafish that live in shallower waters.
The Yap hadal snailfish had lost many olfactory receptor genes, suggesting a limited sense of smell. Chen says this may be because it has a repetitive diet – the snailfish’s stomach was filled with just one species of crustacean. Curiously, it has extra genes for sour taste receptors, and it isn’t clear why.
Chen and his team also explored the origins of the Yap hadal snailfish. They compared its genome to that of the Mariana hadal snailfish, which lives in the Mariana Trench hundreds of kilometres to the north-east, and was sequenced in 2019.
The two snailfish species turned out to be closely related. Chen says the two trenches they live in formed 8 to 10 million years ago, so it may be that the ancestral snailfish found their way into the two trenches and subsequently evolved independently. “These two trenches are isolated, and they have different depths and environmental conditions,” says Chen.
Journal reference: PLoS Genetics, DOI: 10.1371/journal.pgen.1009530
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