The Mariana snailfish (Pseudoliparis swirei) – which featured on episode two of the BBC’s Blue Planet II – is officially the deepest fish in the ocean, thriving at depths of up to about 8,000 meters (26,200 feet) along the Mariana Trench near Guam.

Officially described by the research team for the first time in a paper published this week in the journal Zootaxa, the Mariana snailfish doesn’t look like it could survive in harsh conditions thousands of feet below the surface.

Instead of giant teeth and a menacing frame, the fishes that roam in the deepest parts of the ocean are small, translucent, bereft of scales — and highly adept at living where few other organisms can.

Adapted to go deeper

“This is the deepest fish that’s been collected from the ocean floor, and we’re very excited to have an official name,” said lead author Dr Mackenzie Gerringer, a postdoctoral researcher at the University of Washington’s Friday Harbor Laboratories.

“They don’t look very robust or strong for living in such an extreme environment, but they are extremely successful.”

Co-author Dr Thomas Linley, from Newcastle University, UK, added:

“Snail fishes have adapted to go deeper than other fish and can live in the deepest trenches.  Here they are free of predators and the funnel shape of the trench means there’s much more food.

“There are lots of invertebrate prey and the snailfish are the top predator which means they are quite active and look very well-fed.”

In the depths of the Mariana Trench

Snailfish are found at many different depths in marine waters around the world, including off the coast of San Juan Island, where Gerringer is continuing research on the family of fish.

In deep water, they cluster together in groups and feed on tiny crustaceans and shrimp using suction from their mouths to gulp prey. Little is known about how these fish can live under intense water pressure; the pressure at those depths is similar to an elephant standing on your thumb.

This new species appears to dominate parts of the Mariana Trench, the deepest stretch of ocean in the world that is located in the western Pacific Ocean. During research trips in 2014 and 2017, scientists collected 37 specimens of the new species from depths of about 6,900 meters (22,600 feet) to 8,000 meters (26,200 feet) along the trench. DNA analysis and 3-D scanning to analyse skeletal and tissue structures helped researchers determine they had found a new species.

Since then, a research team from Japan has recorded footage of the fish swimming at depths of 8,178 meters (26,830 feet), the deepest sighting so far.

Only a handful of researchers have explored the Mariana Trench, but few comprehensive surveys of the trench and its inhabitants have been completed because of its depth and location.

The footage was captured using lander technology – designed and developed by Newcastle University’s Dr Alan Jamieson, also a co-author on the study.

It can take four hours for a trap to sink to the bottom and after waiting an additional 12 to 24 hours, the researchers send an acoustic signal to the trap, which releases weights and the lander rises to the surface with the help of floatation. This allows the team to catch fish specimens and take video footage of life at the bottom of the ocean.

The Mariana snailfish’s location was its most distinguishing characteristic, but researchers also saw a number of differences in physiology and body structure that made it clear they had found a new species. With the help of a CT scanner at the UW’s Friday Harbor Laboratories, the researchers could look in close digital detail to study elements of the fish.

The authors acknowledge the broad collaboration needed for deep-sea science, particularly in this discovery, and decided the new fish’s scientific name should reflect that collaborative effort. The fish is named after a sailor, Herbert Swire, an officer on the HMS Challenger expedition in the late 1800s that first discovered the Mariana Trench.

The research was funded by the National Science Foundation, Schmidt Ocean Institute, and the Marine Alliance for Science and Technology for Scotland.

Reference: Pseudoliparis swirei sp. nov.: A newly-discovered hadal snailfish (Scorpaeniformes: Liparidae) from the Mariana Trench MACKENZIE E. GERRINGER1 , THOMAS D. LINLEY2 , ALAN J. JAMIESON2 , ERICA GOETZE1 & JEFFREY C. DRAZEN1

Zootaxa 4358 (1): 161–177