Video: Bright red shrimp cruises by deepest-living fish
No fish has been spotted further than 8000 metres below the ocean surface – now we know why that record may never be beaten
IN 1960, when Jacques Piccard made his epic dive to the deepest part of the ocean, 11 kilometres below the surface, he spotted a flatfish on the sea just as the submersible was about to touch down. “Just beneath us was some type of flatfish, resembling a sole, about 1 foot long and 6 inches across. Even as I saw him, his two round eyes on top of his head spied [us, and] extremely slowly, this flatfish swam away,” Piccard in his book Seven Miles Down. “Moving along the bottom, partly in the ooze and partly in the water, he disappeared into his night.”
Seconds later, the submersible stirred up the sediment as it touched down. Piccard and Don Walsh saw nothing but milky murk for the rest of the 20 minutes they spent at the bottom of the Challenger Deep – the lowest part of the Mariana Trench – so much was made of the flatfish sighting. “Here, in an instant, was the answer that biologists had asked for the decades,” Piccard wrote. “Could life exist in the greatest depths of the ocean? It could!”
Advertisement
Just what did Jacques Piccard see from his submersible? (Image: Superstock/Getty)
To this day, the sighting is still occasionally cited as evidence that fish exist in the deepest parts of the ocean. But , marine biologists suspected that . They knew that no fish had ever been caught much deeper than 7.5 kilometres down.
Explore the depths of life in our interactive ocean: “The Challenger Deep“
More recently, biologists such as have sent cameras to the bottom of the deepest trenches and captured hundreds of hours of film footage. No living fish has ever been spotted deeper than 7700 metres down. “Suddenly they disappear,” Jamieson says. “Their cut-off point seems very distinct across all trenches examined.” The question is why. And he and his colleagues at the University of Aberdeen think they have the answer.
Extreme conditions
Down in the abyss, conditions are extreme. It is dark, cold and, apart from the occasional bonanza like a dead whale, there’s little to eat. Many fish stick to shallow water. Sharks and rays in particular are below 3000 metres. There simply may not be enough for them to eat at extreme depths, says also at the University of Aberdeen. Whereas other fish have an air-filled swim bladder to keep them afloat, sharks depend on fatty food to maintain a huge, oily liver that keeps them buoyant. “Using fat as buoyancy is a massive energy penalty,” says Priede.
The greatest challenge in deep living, though, is the extraordinary pressure. At the bottom of Challenger Deep, it is equivalent to having two elephants standing on your big toe. This has a profound effect on the body. Shallow-living animals cannot simply move deeper at will – at a certain point, their muscles and nerves would stop working.
At high pressures, for instance, the fatty membrane of cells becomes stiffer, impeding normal function. To compensate, deep-living species increase the proportion of viscous fatty molecules to keep the membrane fluid.
High pressure can also alter the shape of proteins, which is a huge problem because the workings of proteins – including the enzymes that catalyse reactions vital for life – depend on their precise shape. One way to adapt is to tweak each individual protein to work at depth. But deep-sea fish have come up with a more general and flexible solution: a chemical called trimethylamine oxide, or TMAO.
TMAO is found in many marine animals. In fact, the characteristic smell of fish comes from the breakdown of TMAO. In shallow-living fish, TMAO helps regulate the concentration of dissolved substances in the fluid inside cells. Crucially, though, it also helps proteins keep their shape. This means fish can keep all their proteins working as they go deeper simply by increasing the concentration of TMAO in their cells.
Levels of TMAO aren’t only higher in deep-living fish species, but also in deeper-living individuals. “They accumulate more TMAO with depth,” says Mackenzie Gerringer of the University of Hawaii, who studies abyssal and trench fish. This may be what allows some fish, such as the abyssal grenadier, to move thousands of metres deeper as they get older.
But there is definitely a limit to how deep fish can go. The cusk eel is often cited as the deepest fish, because one was caught in 1970 during a trawl at a depth of 8370 metres. Since no others have been found so deep, though, biologists now suspect it was caught as the net was being raised or lowered.
The undisputed record for the deepest living fish is held by the snailfish Pseudoliparis amblystomopsis, filmed 7703 metres down in 2008 by an expedition led by Jamieson (the video above is from a different expedition).
The reason fish can’t live much deeper than this, might be to do with TMAO, suggested of Whitman College in Washington a few years ago. Higher levels of TMAO make the fluid inside cells more concentrated. Deep-sea fish, however, have to keep the concentration of the fluid inside their cells at or below that of seawater to survive. At a certain depth, then, cells won’t be able to accumulate any more TMAO.
To test this idea, Yancey and Jamieson looked at TMAO concentrations in several fish species caught at various depths. Some were frozen specimens from other expeditions but the team also caught some Notoliparis kermadecensis snailfish – the second-deepest species ever seen alive. TMAO concentrations increased proportionally with pressure, pointing to a theoretical limit at about 8200 metres, they reported earlier this year ().
That fits in neatly with the actual observations. “All things fell on the magic 8000 metres mark,” Jamieson says.
While the deepest parts of the ocean are devoid of fish, we now know they harbour many other animals, such as woodlice-like amphipods. Among other things, an unknown kind of sea cucumber has been spotted in from James Cameron’s 2012 dive to the bottom of the Mariana Trench – and it has been suggested that Piccard mistook for a flatfish. How these animals survive the immense pressure isn’t known. “They may be doing something completely different,” says Jamieson.
Are there any ultimate limits? Could life exist even at the greatest depths of oceans on other planets? Europa’s ocean is thought to be 800 kilometres deep, for instance. “The biochemistry that we find here on Earth may not apply at all to biochemistry that evolves on another world,” says Yancey. “So I don’t think we have any way to guess how deep animal life could live.”
This article appeared in print under the headline “The deepest limits”
Article amended on 1 January 1970
When this article was first published, the accompanying graphic showed shrimp living at the bottom of the Challenger Deep. This has been corrected to show shrimp-like amphipods.
