me: i want to do a #mermay, one of my characters... not Ghost tho, ive been drawing him for too long now... i'll do Jace
me, 1 hour after with a sketch of Ghost mermaid: GODDAMNIT.. ok but just a sketch. ok?
me, 2 hours later: G A D D A M N I T
A recent study published in PNAS (and summarized in ScienceShot) seems to have figured out why there are no fish in the deepest parts of the oceans. Essentially, they propose that it’s due to a limit imposed on fish biochemistry as result of an evolutionary adaptation not developed with those depths in mind. (It's also a succinct counterpoint to the idea of evolutionary directionality. Ideally, there would be a fish species that could live in the deepest parts of the ocean, but it appears that real fish are limited by a random quirk of a molecule that evolved at shallower depths, without these extreme depths in mind.)
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The deeper under water you go (as with any fluid on earth) the more pressure that water exerts on you. As the paper notes, descending 100 meters (the length of a football field) increases the pressure by 10 atm (for reference, 1 atm is the about the pressure we experience here on the surface of the earth). Seeing as the deepest parts of the ocean are around 11,000 meters deep, that’s an enormous amount of pressure. Living things are sensitive to pressure on a number of structural scales, from organs (which can rupture at excessively high or low pressures) to individual cells (likewise, killing them and impairing tissue functions) to proteins (which can deform, rendering them biologically useless).
To deal with issues of pressure on protein deformation, fish have evolved a molecule, called TMAO (trimethylamine N-oxide) which stabilizes proteins within their cells. TMAO counteracts the effects of water pushing into proteins at high water pressures, allowing the proteins, and therefore the fish, to survive in deep water.
At the same time, fish cells (like all vertebrate cells) are sensitive to how many particles are within them relative to how much water. Those particles (like proteins and nutrients) are critical for a cell’s life, but so is the amount of water (to maintain its structure and biochemical processes). The ratio of the two, the concentration inside a cell, must be maintained on a very particular range or the cell will die. Vertebrates depend on having a concentration within their cells that is a certain ratio lower than the concentration outside their cells. For fish, that outside is the ocean.
Based on some earlier research, the researchers had reason to believe that the amount of TMAO necessary to keep proteins functional is proportional to the amount of pressure: the greater the pressure, the greater the amount of TMAO must be present in cells. Fish, therefore, have two competing necessities. On one hand, they need TMAO at increasing concentrations to survive at lower depths. On the other, they need their overall cellular concentrations to be lower than that of seawater. The researchers of the PNAS paper calculated the theoretical limit imposed by this balance: at 8,200 meters below the surface of the ocean, a fish would need so much TMAO that it’s cellular concentration would match that of the surrounding water. So, fish should not be able to survive below 8,200 meters, since having the same or a higher cellular concentration would kill it.
To test this idea, the researchers measured the concentration of TMAO in related fish species living at different depths, from 900 to 7,000 meters. They found that there really was a proportional relationship between the depth and the concentration of TMAO in the fish. They also found that those concentrations lined up with their predictions, supporting the idea that 8,200 meters is an approximate biochemical limit. These findings, paired with the fact that no living fish has ever been found below 8,370 meters (with that one record-holder standing far ahead of the rest of the pack, which have been above 7,700 meters), support the proposal of a depth limit for fish based on a biochemical balance between TMAO and ocean salinity.
The authors acknowledge that their evidence doesn’t prove this to be the case. Just because no fish have been found beyond that limit so far doesn’t mean there aren’t any, or there are other factors related to depth (such as nutrient availablity) that could be the real cause of the limit. Alternately, the cause could be related to TMAO but not ocean salinity: it could simply be that the concentrations of TMAO required for greater depths are toxic to the fish. Or, everything proposed in this paper might be correct, but there may be deeper-living fish that have evolved an independent mechanism to resolve its pressure and concentration problems. Those gaps being pointed out, it's a fascinating finding in a emerging area of research.
[Image shows two snailfish, the deepwater species sampled in the study, and a bystander brittle star from the ScienceShot article.]
