Living at the bottom of the Mariana Trench would feel like 50 jumbo jets were piled on top of you. Surveys of our planet’s watery depths reveal that life can thrive even at these outrageously high pressures. But fish, it seems, do not.
Sharks, eels, rock cod, and other fish are absent from the oceans trenches (8,400-11,000 meters). Some snailfish and cusk-eels have been found below 6,000 meters, but the apparent limit for fish life seems to be no greater than about 8,000 meters. beneath that there is little to be seen. This limit doesn’t line up with a cutoff in food, oxygen, salinity, temperature or predators. What it does correlate with is pressure; at 8,000 meters, pressures are near 800 atmospheres (800 times sea-level pressure).
Writing in Proceedings of the National Academy of Sciences, Paul Yancey of Whitman College and colleagues propose that fish hit an osmoregulatory barrier at about 8,400 meters.
To cope with high pressures, which can wreak havoc on protein function, deep sea fish have altered their protein structures to increase resilience. Likely, this is only part of the story. Small organic solutes known as “piezolytes” also seem to counteract the effects of pressure. The chemical stabilizer (or “chemical chaperone”) trimethylamine oxide (TMAO) is found in many marine animals and has been found to increase in concentration in deeper living bony fish and invertebrates.
Using a free-falling fish trap, the authors caught the world’s second-deepest known fish, the hadal snailfish (Notoliparis kermadecensis) at a depth of 7,000 meters in Kermadec Trench off the coast of New Zealand. It was the first time it had been caught in 59 years. Sampling the fish’s muscle, the authors found “the highest recorded TMAO contents.” They then used this concentration to extrapolate a theoretical maximum for fish depth.
The body fluids of most marine organisms conform to the osmotic pressure of their environment. Most vertebrates, however, regulate their osmotic pressure. At around 8,000-8,500 meters, the authors estimate, fish would become so concentrated in TMAO to compensate for pressure, they’d be isosmotic with the seawater around them. Living deeper than this would call for significant physiological rearranging because the fish would be hyperosmotic. Some species can switch between hyper- and hypoosmotic environments, such as salmon during migration. But, the researchers say, these fish are rare and usually acclimatization takes a long time. “We propose that it is unlikely that a hadal fish could evolve the ability to migrate down a trench slope and then wait for hours or days at 8,400 m while altering physiological systems for hyperosmotic regulation to migrate deeper,” they write.
If true, this finally explains why ocean trenches are typically poor places to go fishing.