The deepest-dwelling fish in the sea is small, pink and delicate

Thanks to movies and nature videos, many people know that bizarre creatures live in the ocean's deepest, darkest regions. They include viperfish with huge mouths and big teeth, and anglerfish, which have bioluminescent lures that make their own light in a dark world.

However, the world's deepest-dwelling fish – known as a hadal snailfish – is small, pink and completely scaleless. Its skin is so transparent that you can see right through to its liver. Nonetheless, hadal snailfish are some of the most successful animals found in the ocean's deepest places.

Our research team, which includes scientists from the United States, United Kingdom and New Zealand, found a new species of hadal snailfish in 2014 in the Mariana Trench. It has been seen living at depths of almost 27,000 feet (8,200 meters). We recently published its scientific description and officially christened it Pseudoliparis swirei. Studying its adaptations for living at such great depths has provided new insights about what kinds of life can survive in the deep ocean.

We discovered this fish during a survey of the Mariana Trench in the western Pacific Ocean. Deep-sea trenches form at subduction zones, where one of the tectonic plates that form the Earth's crust slides beneath another plate. They extend 20,000 to 36,000 feet deep below the ocean's surface. The Mariana Trench is deeper than Mount Everest is tall.

Ocean waters in these trenches are known as the hadal zone. Our team set out to explore the Mariana Trench from top to bottom in an effort to understand what lives in the hadal zone; how organisms there interact; how they survive under enormous pressure created by six to seven miles of water above them; and what role hadal trenches play in the global ocean ecosystem.

Sending instruments to the ocean floor is pretty straightforward. Bringing them back up is not. Researchers studying the deep sea often use nets, cameras or robots connected to ships by cables. But a 7-mile-long cable, even if it is very strong, can break under its own weight.

We used free-falling landers – mechanical platforms that carry instruments and steel weights and are not connected to the ship. When we deploy landers, it takes about four hours for them to sink to the bottom. To call them back, we use an acoustic signal that causes them to release their ballast and float to the surface. Then we search for them in the water (each carries an orange flag), retrieve them and collect their data.

Hadal trenches are named after Hades, the Greek god of the underworld. To humans, they are harsh, extreme environments. Pressure is as high as 15,000 pounds per square inch – equivalent to a large elephant standing on your thumb, and 1,100 times greater than atmospheric pressure at sea level. Water temperatures are as low as 33 degrees Fahrenheit (1 degree Celsius). Yet, a host of animals thrive under these conditions.

Our team put down cameras baited with mackerel to attract mobile animals in the trench. At shallower depths, from approximately 16,000 to 21,000 feet (5,000-6,500 meters) on the abyssal plain, we saw large fish such as rattails, cusk eels and eel pouts. At the upper edges of the trench, below 21,000 feet, we found decapod shrimp, supergiant amphipods (swimming crustaceans), and small pink snailfish. This newly discovered species of snailfish that lives to near 27,000 feet (8,200 meters), is now the world's deepest living fish.

At the trench's greatest depths, near 36,000 feet (11,000 meters), we saw only large swarms of small scavenging amphipods, which are somewhat similar to garden pill bugs. Amphipods live all over the ocean but are highly abundant in trenches. The Mariana snailfish that we filmed were eating these amphipods, which make up most of their diet.

The Mariana Trench houses the ocean's deepest point, at Challenger Deep, named for the HMS Challenger expedition, which discovered the trench in 1875. Their deepest sounding, at nearly 27,000 feet (8,184 meters), was the greatest known ocean depth at that time. The site was named Swire Deep, after Herbert Swire, an officer on the voyage.

We named the Mariana snailfish Pseudoliparis swirei in his honor, to acknowledge and thank crew members who have supported oceanographic research throughout history.

Hadal snailfish have several adaptations to help them live under high pressure. Their bodies do not contain any air spaces, such as the swim bladders that bony fish use to ascend and descend in the water. Instead, hadal snailfish have a layer of gelatinous goo under their skins that aids buoyancy and also makes them more streamlined.

Hadal animals have also adapted to pressure on a molecular level. We've even found that some enzymes in the muscles of hadal fish are adapted to function better under high pressure.

Whitman College biologist Paul Yancey, a member of our team, has found that deep-sea fish use a molecule called trimethyl-amine oxide (TMAO) to help stabilize their proteins under pressure.

However, to survive at the highest water pressures in the ocean, fish would need so much TMAO in their systems that their cells would reach higher concentrations than seawater. At that high concentration, water would tend to flow into the cells due to a process called osmosis, in which water flows from areas of high concentration to low concentration to equalize. To keep these highly concentrated cells from rupturing, fish would have to continually pump water out of their cells to survive.

The evidence suggests that fish don't actually live all the way to the deepest ocean depths because they are not able to keep enough TMAO in their cells to combat the high pressure at that depth. This means that around 27,000 feet (8,200 meters) may be a physiological depth limit for fish.

Mackenzie Gerringer is a postdoctoral researcher at the University of Washington. This article was originally published in February 2018 issue of The Conversation.

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