It's cute, almost pink, and about twice as long as a cigar, with flesh so translucent you can see its liver from the outside.
Scientists on Tuesday formally documented the world's newest fish, Pseudoliparis swirei, an odd little snailfish caught at 7,966 metres in the Mariana Trench—nearly twice as far below the sea's surface as Wyoming's Grand Teton towers above it.
This creature of the dark, frigid ocean region known as the hadal zone was first caught in 2014 and again early in 2017 but is only now being described. And this species, the deepest fish ever collected from the sea, is probably not what most people think of when they imagine the creatures far below.
"The public normally thinks about anglerfish or viperfish," the black, monster-jawed fish with the dangling lanterns usually found a few thousand metres down, says Mackenzie Gerringer, a postdoctoral fellow at the University of Washington's Friday Harbor Laboratories. "By the time you get this deep, fish take a really different form. They have no scales, no big teeth, and they're not bioluminescent—that we know of." (See more photos of deep-sea creatures.)
That lack of certainty, of course, comes with the territory. This is just one of the two species of snailfish—there are more than 350 snailfish species known globally—caught on film during recent expeditions to the trench. Scientists on multiple expeditions have hauled up 37 individuals of Pseudoliparis swirei. They've also filmed one at 8,178 metres.
But they still haven't caught a single individual of the other fish, which was filmed at about the same depth. That one makes an appearance in the BBC's new Blue Planet 2. It has a body so delicate a scientist once compared it to "tissue paper being dragged through water." It remains an as-yet undescribed species with no formal name. Scientists have taken to calling it the "ethereal snailfish."
Yet despite these recent discoveries—and even though the deepest part of the ocean extends almost another 2 miles down to just shy of 11,000 meters—scientists suspect they are unlikely to ever find a fish that lives deeper than these two.
"Not that we haven't looked," Gerringer says.
LIFE UNDER EXTREME PRESSURE
Pseudoliparis swirei is named for an officer on the HMS Challenger, the 1870s British expedition that discovered thousands of new ocean species and led to the initial discovery of the trench. Challenger officer Herbert Swire, a navigational sub-lieutenant, published journals from the journey. "We named this fish after him in acknowledgment of the crews that serve on oceanographic research vessels," Gerringer says. "It takes a lot of people to keep a ship running and we wanted to sincerely thank them."
This snailfish is about twice as long as a cigar, yet it can withstand more water pressure than 1,600 elephants standing on its head.
PHOTOGRAPH BY MACKENZIE GERRINGER
The species is almost certainly endemic to the trench, and appears to be abundant—scientists saw several on cameras carried by deep-diving autonomous vehicles in 2014. Their eggs are unusually large—almost a centimetre wide—and based on dissections of the individuals scientists hauled up, these snailfish don't lack for food. Inside their bellies Gerringer found hundreds of tiny crustaceans shaped like the roly-poly pill bugs one might find in a garden.
There are a whole host of animals that thrive down deep—foraminifera, odd decapod shrimp, sea cucumbers, microbes. But it's a hard life for fish.
"There are real limitations to life in these trenches," Gerringer says. Snailfish are thought to be able to handle pressures equal to the weight of 1,600 elephants. "They have evolved adaptations to that pressure to keep their enzymes functioning and membranes moving."
This pressure is also why, scientists suspect, fish have not been found in the deepest quarter of the ocean. Fish may be chemically unable to withstand the destabilising effects of pressure on proteins below about 8,200 metres.
But, of course, we don't yet know for sure.
CONNECTIONS TO THE SURFACE
"There are trenches all around the Ring of Fire in the Pacific, but we don't know how similar they are or how connected they all are," Gerringer says. "We don't know how closely tied they are to the environment above.
"We're still at the 'discovering-who-is-down-there phase,'" Gerringer says.
Even so, it's not as if we haven't left our mark. Even these poorly understood trenches are not immune from our influence. Scientists recently discovered staggeringly high amounts of persistent organic pollutants (POPs) in crustaceans taken from the deepest part of the trench. They are almost certainly byproducts from degrading pieces of plastic.
Lead Image: This CT imagery reveals the skeleton of a snailfish that is now the deepest living species of fish with a formal name. PHOTOGRAPH BY ADAM SUMMERS, FRIDAY HARBOR LAB, UNIVERSITY OF WASHINGTON