Flycatchers in the Mariana Trench reveal clues to life under pressure

A soft, translucent creature called the hadal snail (Pseudoliparis swirei) is the first animal from the extreme depths of the ocean to have its genome sequenced. Its genetic roadmap reveals the basis of adaptations allowing animals to live in hostile environments such as the Mariana Trench, the deepest place in the ocean.

Deep-sea creatures face a cold, dark environment and extreme pressures. But without the genome of any animal living below 6,000 meters – a region known as the hadal zone – scientists were unsure of how creatures had adapted to survive. Results published on April 15 in Nature Ecology & Evolution¹ report the skeletal and cellular changes upon which the hadal fish-furs support to support these difficult conditions.

Paul Yancey, marine biologist at Whitman College in Walla Walla, Washington State, has never done this study so far. "It's exciting."

Live under pressure

The study authors captured a hadal snail fish at about 7,000 meters altitude in the Marianas Trench, in the western Pacific Ocean. After sequencing the fish genome, the team looked for clues that could explain the characteristics of the hadal snail fish, such as a cartilage skeleton and cell membranes that continue to operate under immense pressure. The pressure in the Mariana Trench is similar to that felt by someone if the full weight of the Eiffel Tower rested on his big toe.

The researchers compared the DNA of the fish with that of a close relative, the Tanaka snail (Liparis tanakae), who lives in the tide pools. Both species separated from a common ancestor about 20 million years ago.

Shunping He, ichthyologist of the Wuhan Chinese Academy of Sciences and co-lead author of the study, found several genetic changes in the genome of the hadal snail fish, related to rapid adaptation to the deep sea.

The high pressures in the Mariana Trench would crush normal bones. But a gene that is an integral part of bone hardening is inactive in the Hadal fly fish – which is consistent with the idea that a cartilage skeleton is more resistant to pressure, write the authors.

Hadal fly fish also lost several genes involved in light detection. However, he and his colleagues found five of these genes that are still active, suggesting that the fish might have a residual capacity to see.

Some groups of genes, many of which are involved in fatty acid metabolism, have developed in hadal fly fish. The authors explain that the presence of certain fatty acids helps cell membranes to remain flexible at great depths. Otherwise, high pressure can make these membranes rigid and impenetrable. Other genes prevent proteins from folding incorrectly under extreme pressure.

Unique in it's genre?

Santiago Herrera, a molecular ecologist at Lehigh University in Bethlehem, Pennsylvania, says, "It's exciting to see this genome sequence, as well as ideas about how vertebrates adapt to extreme depths. "Understanding how this kind of life is possible in this environment is really innovative."

This study also provides a basis for future work on how fish have evolved to survive in extreme environments, says Natalya Gallo, an oceanographer at the Scripps Institution of Oceanography in La Jolla, California. Researchers can now conduct laboratory experiments with tools such as CRISPR, edit genes to explore the traits they control, she adds.

It will be interesting to see if the genetics at the origin of hadal fly fish adaptations is representative of the animals living in the deep sea, or if each species has its own set of genetic strategies to survive in this extreme environment, says Herrera.