Thanks to remote-controlled traps that took four days to sink to their target on the bottom of the seven-mile deep Mariana Trench, scientists have at last captured, formally identified and described the world's deepest-known fish: Pseudoliparis swirei, the Mariana snailfish. Appearing like a cross between an anemic tadpole and Flounder from The Little Mermaid, it’s not much to look at when unceremoniously yanked to the surface:
However, in their home in the deep they are, if not quite attractive, certainly less off-putting. Here is some video collected by the Japan Agency for Marine-Earth Science and Technology back in May, at two depths including the current record for the species of 8,178 m. They appear more white than transparent here.
Note the skeletonized remains of the bait still strapped to the probe’s arm at right, and do click the full screen button in the lower right as the video is high definition.
The depth in the second video is noteworthy. The calculated theoretical depth limit for bony fish is around 8,200 meters. More on that in a minute.
Scientists were at last able to describe the fish -- first sighted back in 2014 -- thanks to ingeniously-designed traps that drop their weights and float to the surface by remote control when scientists want to retrieve the contents. In this way, they captured 37 of the fish.
They had suspected based on video recordings that they were snailfish, a family of 400 species with the widest depth range of any ocean fish family whose members are particularly good at living in trenches. Real specimens confirmed this suspicion. Scientists described the animals as pinkish-white and transparent with the internal organs and muscles clearly visible through their abdomen. They lacked scales. They also appeared fat and healthy in spite of the extreme depth; the small crustaceans they eat are abundant in the trench, which funnels nutrients falling from above, and predators are scare or non-existent at such crushing depths. The scientists likened the pressure there to the force of an elephant standing on your thumb.
Why a limit of 8,200 meters (30,000 feet)? The Mariana itself is nearly 11,000 meters (36,000 feet) deep. Why would a bony fish not be capable of reaching the bottom if 8,000 meters is no prob?
It’s actually an interesting story. Here’s how I told it, in part, back in 2015:
This molecule regulates the osmotic balance of cells but also acts as a "chemical chaperone" by supporting proper protein folding and preventing water under high pressure from forcing its way into proteins and deforming their shape, which is critical to their function. The deeper a bony fish lives, scientists have discovered, the more TMAO it makes (intriguingly, TMAO degrades to trimethylamine (TMA), which is the chief chemical responsible for the "fishy" smell of aging seafood).
Ocean fish are hypoosmotic to their environment; that is, the ocean is salty and the inside of fish is less so. In such a situation, the forces of osmosis (remember high school biology?) will tend to cause water to seep from their bodies and into the ocean, producing lethal dehydration. To combat this effect, evolution has endowed fish with a number of systems to fight the escape of water. But each of these systems is predicated on the sea being saltier than the fish.
TMAO, in addition to protecting against increasing pressure, is a solute and raises the osmolarity of fish internally. According to Yancey, the reason no fish is believed to be able to live deeper than about 8,400 m (27,600 ft) is that is the pressure at which the fish becomes as salty as the sea, so to speak (TMAO is actually not a salt but a solute, of which salts are one kind. Solutes are what matter for osmosis). If TMAO concentrations increased further, the fish would become hyperosmotic.
For a fish to swim deeper, it would have to, in effect, reverse its osmolarity regulation systems, from trying to prevent water from leaving, to trying to shove it out the door. This is not unheard of; salmon do it every time they cross from freshwater to saltwater or vice versa. But they must pause and wait hours or days at the boundary for their gills and kidneys to transmogrify. Yancey considers it unlikely that any deep sea fish would swim down to a certain depth, hang out for hours or days for their bodies to reconfigure, and them swim lower.
There's a lot more to this story, including why deep-sea fish have the same problems as meat-loving humans and the accident of evolution that made fish vulnerable to this effect in the first place. You can find out about both at my old post.
GERRINGER, MACKENZIE E., THOMAS D. LINLEY, ALAN J. JAMIESON, ERICA GOETZE, and JEFFREY C. DRAZEN. "Pseudoliparis swirei sp. nov.: A newly-discovered hadal snailfish (Scorpaeniformes: Liparidae) from the Mariana Trench." Zootaxa 4358, no. 1 (2017): 161-177.
Yancey P.H., M.E. Gerringer, J. C. Drazen, A. A. Rowden & A. Jamieson (2014). Marine fish may be biochemically constrained from inhabiting the deepest ocean depths, Proceedings of the National Academy of Sciences, 111 (12) 4461-4465. DOI: http://dx.doi.org/10.1073/pnas.1322003111