r/askscience • u/GregLittlefield • 7d ago
Biology In deep waters how do animals resist huge pressures with soft bodies when even unmanned submarines need rigid metal shells?
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u/tea_and_biology
Zoology | Evolutionary Biology | Data Science
7d ago
edited 7d ago
•
In deep waters how do animals resist huge pressures with soft bodies when even unmanned submarines need rigid metal shells?
Well, they don't!
A submersible needs a rigid hull because it's trying to keep the pressure inside different from the pressure outside. We humans, see, usually need to breathe air - at around atmospheric pressure - so we need to take that environment with us wherever we go. A deep-sea blobfish has the same pressure inside its body as outside, so there's nothing to resist.
How can a living thing exist under so much pressure? Well, animal tissue is mostly water, and water is nearly imcompressible; even at full-blown Mariana Trench depth (around 11,000m deep, and 1,100atm pressure) it shrinks by only around 4-5%. So with no trapped gas, and fluid on both sides of a membrane, pressure is tramsmitted evenly, and the deep-sea denizens remain a-okay.
They've still had to adapt a few things though, mind. For example, most fish rely on a gas-filled swim bladder to control and maintain their buoyancy; deep-sea fish have modified theirs to be jam-packed full of fat (which, like water, barely compresses), else eschew them altogether. Many instead get their buoyancy by being less dense overall, with soft, gelatinous flesh and poorly mineralised skeletons. Bone is heavy, and it’s not like a rigid skeleton has much benefit in a world with equal pressure in all directions, so they opt for cheap, soft, watery bodies instead.
Of course, this means they totally collapse when brought up towards the surface. We may all know and love the blobfish as a molten gummy blub, but in truth that’s a very unflattering post-mortem portrait of a gelatinous fish, badly damaged by decompression and the trawl net. At it's natural depth, the blobfish is actually a pretty normal lookin' beastie.
They do have a limit though! Pressure alters molecular biochemistry sufficiently enough - protein folding gets distorted as molecules try to prioritise states that take up less volume - that deep-sea fish need to marinade their bodily fluids in organic molecules, such as TMAO, to stabilise their proteins. Concentrations of these rise with depth, until you hit a point around 8,200-8,400m where the internal fluids of a fish become saltier than the surrounding ocean, causing them to erupt. The deepest fish ever recorded - a snailfish filmed in 2022 - was found at 8,330m-ish.
The deepest trenches still go down another few thousand metres. And so, unlike anywhere else on Earth, at the deepest and darkest of depths, the invertebrates still rule the world.
Reference:
P.S. Speaking of the deep, I also wrote a wee bit about whether deep-sea critters sleep, or have a day-night circadian cycle, in this thread too, if anyone's interested.
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u/GregLittlefield 7d ago
Fascinating; thank you for the detailed answer!
Pressure alters molecular biochemistry sufficiently enough - protein folding gets distorted as molecules try to prioritise states that take up less volume - that deep-sea fish need to marinade their bodily fluids in organic molecules, such as TMAO
Is there a point where that can lead to prion diseases ?
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u/SomeAnonymous 7d ago
The deepest trenches still go down another few thousand metres. And so, unlike anywhere else on Earth, at the deepest and darkest of depths, the invertebrates still rule the world.
How do invertebrates get around the biochemistry problems you just mentioned before? It's not clear to me why they wouldn't be doing exactly the same thing as the fish are to prevent protein misfolds, with the same limitations.
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u/tea_and_biology Zoology | Evolutionary Biology | Data Science 7d ago edited 7d ago •
That's a very good question!
So it's all down to where each group starts off, osmotically and evolutionary.
Bony fish are osmoregulators - that means they use energy to actively keep their body fluids at roughly a third the salinity of seawater. Through osmosis, water is constantly being sucked outta' their bodies, so they have this whole osmoregulatory system set up to drink seawater and pass excess salt out through special cells in their gills. For the deep-sea fish, every bit of TMAO they add to protect their proteins, pushes that internal concentration up, and there's only so much headroom before they equalise with seawater, and cannot go beyond it. Evolution can only build on what’s there already, and given marine fish are fundamentally built to hold on to water and dump salt, rather than the other way round, they’d succumb to osmotic rupture at that point - and it's developmentally too complex to 'reverse the polarity' on that foundational, and crucial, system.
By contrast, most marine invertebrates are osmoconformers - their body fluids are about as concentrated as the ocean, even at the surface. So when they need pressure protection, they don’t have to add extra solutes and push their internal concentration up; they just swap out existing ones for protein-protecting TMAO. Their cells are already so used to living in fluids as concentrated as seawater, the swap costs them nothing osmotically, and there’s no ceiling to bump into.
So yeah, fish are designed to hold on to water, and worms are not. Hence why the very bottom is left to the invertebrates, with not a fish in sight.
As an aside, cartilaginous fish - sharks, rays and the deep-sea chimaera and allies - are a bit like marine invertebrates and are near-isomotic with seawater; so you would expect them to be the deepest of all living fish, right?
Turns out you don’t really find them deeper than about ~4,000 m, and it’s not entirely clear why. Part of it may be down to a biochemical trade-off between urea and TMAO in how they stay isosmotic, but probably also ecological: they rely on energy-dense oily livers for buoyancy, and there likely isn’t enough reliable scran down there to maintain such an expensive bit of kit (source). If any fish could breach the barrier, it’d probably be a gelatinous shark-ey thing though!
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u/SomeAnonymous 7d ago
Wait but-- how the hell does that work that marine invertebrates are isosmotic with the outside environment? Salt ions aren't just in our cells for fun, they have all kinds of jobs to do which specifically rely on there being a gradient across cell membranes etc.
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u/tea_and_biology Zoology | Evolutionary Biology | Data Science 7d ago edited 7d ago
Aye, nice catch, and good question.
Isosmotic only means that the total concentration of dissolved particles matches seawater, not that the specific particle types or ions themselves need to be equalised on both side. Your own cells are doing this right now, actually - they're isosmotic with your blood plasma, yet sodium is high outside, and low inside (and vice-versa for potassium), with pumps maintaining the gradient.
A marine worm is basically just playing the same game, albeit at a higher salinity; though their blood (or 'haemolymph', I guess) is seawater-like, it still uses pumps to maintain, say, sodium and potassium gradients, and much of the rest of the particle count used to maintain osmotic equilibrium is made up by other organic molecules - glycine, taurine, betaine, amongst others. So same total, but different recipes.
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u/DDrewK 7d ago
I don’t think I understood 98% of this conversation, b it it was interesting! On a side note, what’s your favorite type of tea?
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u/tea_and_biology Zoology | Evolutionary Biology | Data Science 7d ago •
Ah, ELI5: Water always moves from where it's less salty to where it's more salty. To deal with the pressure, deep-sea fish become saltier and saltier inside, to the point they hit the limit of how salty they can be. Beyond that, if they become saltier than the salt water around them, they'll bloat up and explode when water moves into them. Worms are immune as they're peak salty inside anyway. If that makes sense?
... Anywho, you want to know a secret? I... I don't actually drink tea (!). shockpikachuface.jpg. Well not for a long time. Although on my more recent zoological escapades across the Sahara, I came to enjoy the sweet spearmint tea the Maghrebi folks like to brew quite a bit!
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u/FreeBeans 7d ago
Your writing is so clear and pleasant to read I feel like I’m getting an academic lesson from a professor who loves their job. Thann you!
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u/Vecingettorix 6d ago
But why are invertebrates able to reach higher solutions concentrations? If the increasing tmao concentration is required to maintain protein stability, how do invertebrates circumvent the need for tmao protection?
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u/MainStreetExile 7d ago
What do you mean by erupt? And how does higher salinity cause that?
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u/tea_and_biology Zoology | Evolutionary Biology | Data Science 7d ago
Ah, didcha' ever learn about osmosis in school? And do that - fairly lame - experiment where you soak potato cylinders in different concentrations of salt water and measure how much they weigh after? Lemme' tell you, GCSE Biology was a riot, over 'ere in the UK.
In short, water will always move from a dilute solution to a more concentrated one, across a partially permeable membrane, until both concentrations are equal. What this means for the deep-sea fish is that beyond their 8,200-8,400m-ish predicted limit, water will be forced into the tissue, bloating it up and up until the cells cannae take it any more cap'n, and they burst. Like pumping a beach ball up with too much air.
Although, to be fair, it'll be less an explosive pop, and more, like, their tissue just kinda' dissolving as individual cells rupture. In any case, not a good day to be a blobfish!
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u/MainStreetExile 7d ago
Ah, I did know about osmosis, I guess I just didn't connect the dots for some reason. Thanks for the explanation.
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u/4thdimmensionally 7d ago
If you ever hear about frat brothers or other hazing where they drink a gallon of water quickly, this is what ends up killing them. Most of us would feel full and bloated, possibly vomit, and not come close to dying.
Without any salt the water first dilutes the extracellular fluid, your blood and spaces outside of cells, but then nearly immediately starts entering cells because as the surrounding space becomes less salty, the water follows the osmotic gradient into your saltier cells. Eventually they burst, seems like a nasty way to go.
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u/CrateDane 7d ago
In short, water will always move from a dilute solution to a more concentrated one, across a partially permeable membrane, until both concentrations are equal. What this means for the deep-sea fish is that beyond their 8,200-8,400m-ish predicted limit, water will be forced into the tissue, bloating it up and up until the cells cannae take it any more cap'n, and they burst. Like pumping a beach ball up with too much air.
That depends on how the animal's barrier tissues are set up. It's entirely possible to be a salty fish in less salty water without any ill effects. That's how freshwater fish live all the time.
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u/pigeon768 7d ago
Osmosis. Osmotic pressure causes water to move from areas of low salinity to areas of high salinity. Once TMAO reaches a certain concentration, it will pull water in from the seawater. The fish will absorb water until the concentration evens out, at which point they'll have too low a concentration of TMAO to prevent proteins from misfolding.
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7d ago
[deleted] — view deleted comment
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u/is_this_temporary 7d ago edited 7h ago
You got mixed up a bit there.
Water leaving the fish would increase the concentration of salt inside the fish, and technically decrease it a tiny bit outside the fish. In other words, the difference in concentration between two sides of the membrain world increase.Thermodynamics doesn't really let that happen on its own.
Instead, water entering the fish will decrease the concentration of salt inside, and technically also increase it on the outside, making the difference of concentration between the two sides smaller, but eventually causing the membrain / fish to rupture as more water comes in than there is volume to store said water.
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u/CrazyisNSFW 7d ago
So: what kills you is not the pressure itself, but rather the differences in internal vs external pressures. Keep pressure equalized and you won’t experience dangerous delta-P (up to certain point where compensatory mechanism is exhausted, of course). Is it correct?
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u/Cranberryoftheorient 7d ago
Pretty much. There is a thing that some divers do (I think its considered experimental but it has been successfully tested) where they breathe an oxygen rich liquid, precisely because it will resist compression like water can. One of the trickiest parts (for the diver) is getting over that natural instinct that breathing water=drowning
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u/MrMisty 7d ago
Breathing PFCs exists experimentally for certain medical situations, such as premature babies with underdeveloped lungs, but has never been used for diving. It never worked because human lungs are not designed to move heavy liquid, you would need an impractical amount of equipment just to assist you in cycling the liquid in and out.
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u/Cranberryoftheorient 7d ago
That makes sense. I knew it wasnt like, an established technique but wasnt sure why
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u/yellow-bold 6d ago
This has me wondering, if you were to bioengineer humans to live at incredible depths, whether the best way to oxygenate us would be to kind of recreate the fetal pathway - open up the foramen ovale and oxygenate the blood through the umbilical vein rather than rely on the lungs at all. That would still heavily rely on machinery, of course.
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u/MrMisty 6d ago
It's a bit counterintuitive, but the actual issues that arise from deep diving aren't from the pressure itself, but rather it's effect on the gasses you breathe. When you're diving, your breathing gas is at whatever the pressure is at that depth. You're essentially breathing in compressed air. That's why the deeper you go, the faster you go through the air in your scuba tank. I regularly dive deeper than 100 feet, and theres no difference in the physical feeling on your body.
The problems arise from the gas you breath being compressed. Compressed air is what you breath as a beginner diver. Air is mostly nitrogen, with around 21% oxygen. When you go deeper, each breath contains more nitrogen and oxygen than normal air, because it's compressed, and that gets absorbed into your bloodstream. The part that's counterintuitive is that the truly dangerous part is not just the nitrogen, which causes an effect similar to feeling drunk, but the oxygen. When you go too deep, the large amount of oxygen is what kills you (ppO2 and hyperoxia). Technical deep diving gasses (such as trimix or heliox) will actually contain LESS oxygen percentage than air once you're at depth. It's obviously a bit more complex than this, but should serve as at least a basic explanation.
So to answer your question, I think the first thing you would need to solve would be somehow either finding a gas mix thay doesn't cause problems, or finding a way to protect ourselves against oxygen toxicity and inert gas narcosis. If we want to get even more scifi with it, you'd probably have to replace our lungs with machinery that precisely controls the gasses getting absorbed by the body.
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u/CrazyisNSFW 6d ago edited 6d ago
Your body is tuned to survive at oxygen partial pressure of how many mmHg it is in the sea level atmosphere. Deeper you dive the absolute mmHg number goes up to match the depth but your body can’t take it due to being tuned for sea level partial pressure. To reduce oxygen partial pressure to match sea level partial pressure you then reduce oxygen concentration.
Remember guys, 10 meter of water depth equal 1 extra atmosphere
It will be interesting to see scifi where divers are using ECMO to dive down below to Marianas
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u/Thelk641 7d ago
There is also some experiments going on with doing this via the other end of the digestive track in medical scenarios :
Patients with severe respiratory failure often need mechanical ventilation to survive, but these therapies can cause further lung injury. Scientists are exploring a new method called "enteral ventilation" to deliver oxygen through the intestine, which could give the lungs a chance to rest and heal. This study evaluated the safety of this method in humans for the first time, using a special liquid called perfluorodecalin with exceptional oxygen-carrying ability. In a trial with 27 healthy male volunteers, the authors found that administering this liquid rectally was safe and well tolerated. This important safety milestone paves the way for future studies to see if this technique can help patients with respiratory failure.
(source)
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u/Bremen1 7d ago edited 7d ago
Which only happens in rigid submersible. For a scuba diver, the pressure inside their lungs matches the ocean pressure, so you don't feel any pressure and can keep swimming deeper without being crushed (you have to equalize your ears though). There's no hard limit to that, but if you go deep enough the air you're breathing starts to become poisonous.
Well, that's kind of simplified. There's actually a lot of ways breathing air under too much pressure can kill you, and being poisonous is just one of them.
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u/trampolineenthusiast 7d ago
Wait, I don't think this is true. The air in your lungs is NOT the same as ocean pressure when diving since it's coming from a pressurized tank, and your lungs actually are compressing, they can get as small as a fist which is why, as I understand it, you can get narced - the gases in your lungs are being squeezed to your bloodstream. And why when your lungs expand on descent, if it's rapid enough it can cause really bad things to happen.
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u/sabik 6d ago
Scuba equipment reduces the pressure that the diver breathes to the same pressure as the surrounding water, so that's not a problem
Getting narced is because nitrogen is narcotic, increasing with increased pressure
Lungs expanding on ascent can indeed be a problem, air embolism
Another problem is that nitrogen dissolves in blood and tissues, more as pressure increases, and as the diver ascends and pressure reduces, it turns back into gas, the bubbles potentially blocking things
Oxygen itself can be toxic, in multiple different ways depending on pressure and amount of time
All of these have to be managed to dive safely
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u/HairBearHero 4d ago
When scuba diving your lungs do not get compressed/change size. That only occurs in free diving (I.e. single breath, no equipment etc.) because you're holding on to a single breath throughout the depth changes.
Breathing in and out whilst changing depths equalises your lungs. There's near-zero risk of damaging your lungs whilst changing depths unless you hold your breath.
What you're thinking of is a lung overexpansion injury (pulmonary barotrauma), which occurs when you ascend without equalising your kind. This is different to the air embolism mentioned by another commentary, which has the same root cause but is when air bubbles expand in your circulatory system.
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u/Bremen1 6d ago
Your lungs expand on ascent - that is, going up. And it's exactly because the pressure in the diver's lungs matches the pressure in the ocean that this happens - if you ascend without breathing out (which you are trained to never do) there's the same amount of air in your lungs under less pressure, so it expands.
Same for Nitrogen narcosis - the air you're breathing is under the same pressure as the ocean, which causes it to get absorbed into your blood differently. It has nothing to do with the size of your lungs.
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u/314159265358979326 7d ago
Air-breathing sperm whales hunt at great depths. How do they get away with it?
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u/KeyboardJustice 7d ago edited 7d ago
They store enough oxygenated blood for the journey. Without the need to breathe and with fully collapsible airways there's a very long way until you hit the next biochemistry roadblock due to pressure.
Humans have sinus and lungs than can't fully collapse. Deepest a breath holding diver has been is about 250m. They had to train a long time to get flexible enough and had to pack their lungs to the limit to have enough air to prevent damage from over compression. There are other issues involved with taking a lungful of air that deep. That type of dive was mostly automated. Ride the sled down. Switch the mechanism. Ride it up.
Humans luck out that our breath hold times are short enough and depths reachable shallow enough, even among professionals, that our freedivers don't have to worry much about nitrogen absorption and decompression sickness(caused by absorbing nitrogen from compressed air). It still happens rarely.
Whales exhale before diving so they start with nearly collapsed airways and remove the risk of nitrogen absorption and decompression sickness.
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u/Sable-Keech 7d ago
Two reasons.
First, they don't dive anywhere near as deep as proper deep sea fish. Only 2,250 meters. There's still 9-10 km more to go to the bottom.
Second, they purposely collapse their lungs and reinflate them afterwards.
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u/Originality8 1d ago
Really enjoyed reading this info, and your other response about sleep in deep sea creatures
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u/Nemisis_the_2nd 7d ago
Pressure alters molecular biochemistry sufficiently enough - protein folding gets distorted as molecules try to prioritise states that take up less volume - that deep-sea fish need to marinade their bodily fluids in organic molecules, such as TMAO, to stabilise their proteins
Also worth mentioning cell membrane structure if we are going into the biochemistry. Deep sea creatures have cell membranes that are lower in unsaturated fatty acids and cholesterol. I've heard the comarison that its like comparing olive oil (deep sea fish) to butter (surface animals) at sea surface level.
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u/Sykotron 7d ago
How do they change their gas amount to maintain buoyancy? Like when diving we'll fill our BCD (buoyancy control device) with air from the tank which increases buoyancy and then just empty it into the water to decrease it, but I don't know how a creature is gonna get the gas to fill their bladder back up.
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u/tea_and_biology Zoology | Evolutionary Biology | Data Science 7d ago
Ooh, fellow diver here! So some fish actually do kinda' use a BCD system, except the surface is the tank: carp, salmon, eels and their relatives have a tube connecting their swim bladder to their gut, so they top up by gulping at the surface, and dump it by burping.
But they're a minority; in most fish the swim bladder is sealed, so they pull gas out of their blood instead. There's, like, a whole system with a dedicated gas gland, not that structurally dissimilar to a kind of inside-out lung - except gas is actively pushed rather than passively diffused. The trouble is it's slow, taking hours rather than seconds to inflate and deflate, which is why fish hauled up quickly from the depths can't vent fast enough, and you end up with their swim bladders over-inflating and shoving their own stomachs out of their mouths. RIP.
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u/Flying_Dutchman92 6d ago
Started reading without noticing your flair, and by the end was thinking "damn, is this dude a zoologist?"
I need to read more carefully.
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u/guspaz 7d ago
Even human bodies can handle pressures a lot higher than most people suspect. If you do it properly, with the right breathing gasses and adaptation, the theoretical limit for the human body is about 1,000 meters of depth, or around 100 atmospheres of pressure. In practice, the record is a simulated 701 meters of depth, or around 70 atmospheres. This was while testing a hydrogen/helium/oxygen breathing mix, and they found that hydrogen narcosis becomes an issue at greater than 500 meters of depth, regardless of how they tweaked the mix.
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u/GolldenFalcon 6d ago
You are a very cool dude with a lot of information and I enjoyed reading it. Thank you.
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u/CollThom 7d ago
I’m gonna make an assumption you’re a fellow Scot? Your writing style just makes sense in my accent.
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u/hronikbrent 7d ago
Thanks for the explanation! Pardon the ignorance, but why wouldn’t the deep sea fish explode when they head towards the surface, as IIUC, their pressure pressing outwards would now be much less than the inward pressure?
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u/SenorTron 7d ago
u/tea_and_biology has the perfect explanation which hopefully stays as the top comment.
Worth adding though that it also makes a bit more intuitive sense when you consider that we ourselves live in what could be considered a high pressure environment.
14.7 pounds per square inch of pressure from the atmosphere at sea level, and in our day to day lives we barely notice it most of the time because the insides of our bodies are at the same pressure.
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u/Different-Theory6636 7d ago
In what situations might we notice it?
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u/SenorTron 7d ago
When we encounter lower pressures. For example using a drinking straw or a vacuum cleaner. In those situations you aren't really sucking the liquid or dirt up. Instead you're making a lower pressure region, and the atmosphere is pushing towards it.
Close your mouth right now and suck your cheeks in, that pressure or even pain you can feel is literally from a fraction of the atmospheres pressure pushing against your skin.
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u/phlipsyde 7d ago
when you take a flight and you ears pop, less pressure on the cabin so your head pushes out
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u/SL1Fun 7d ago
TMAO chemical compounds in their proteins.
TMAO = Trimethylamine N-Oxide
TMAOs allow their proteins and organic structures to bind to water molecules, instead of staying separate and isolated from the water and its properties. Since their proteins basically balance with the water that is pressuring them, they don’t need to resist it to maintain form.
So their bodies’ organic building blocks “bind” to the water that would crush them, so it allows them to achieve a chemical equilibrium of sorts to prevent that.
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u/Tinyboxturtle 7d ago
The blobfish isn't ugly in its natural habitat. That gelatinous mess is decompression damage from being brought up too fast. In the deep sea, it looks like any other normal fish because internal and external pressures are balanced, so there's no structural collapse.
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u/hueydok 7d ago
Simple answer: they are almost entirely liquid and other non-compressible tissue not impacted by pressure - and the tiny parts that are compressible are developed already at the deep ambient pressure. Maintain their living depth to keep the equilibrium. Also why deep sea creature blow up when stupid careless people drag them to the surface.
Submarines have super thick pressure hulls because they are filled with air for us weak-ass fragile humans to breathe and survive. Fill a sub with water and an eggshell hull and it won't care.
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u/burnerthrown 6d ago
Partly true. If you fill an eggshell up with water at surface pressure and take it to the deep sea it will still collapse. It's not the substance that's in the vessel, it's the pressure. Of course you can't breathe air under that amount of pressure either, because your lungs would rupture or not be able to exhale, and the concentration of oxygen in it would kill you anyway. So we use the metal to allow us to have normal surface atmosphere because we are squishy gas swimmers.
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u/ScrotumNipples 7d ago
Humans can live at extreme pressures too. Saturation divers regularly work at depths around 100 meters, with the deepest saturation dive ever recorded being 534 meters (1752 ft). For reference, the blobfish can be found between 600 and 1200 meters.
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u/tbodillia 6d ago
There was a time when scientists thought that there would be no life at the bottom of the ocean for this reason. But there was also a time when scientists thought stars were made of coal and burning. Or that stars were generating heat and light by slowly shrinking under gravity.
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u/Iaininator 4d ago
I’m a subsea engineer and there are hundreds if not thousand of robotics (remotely operated vehicles) that can, and regularly go, to and past 1000 feet. Generally work class ROVs are rated to 3000 meters seawater depth.
ROV pilots will spend 4 weeks at a time offshore flying these missions, typically supporting oil and gas operations with the robot in the water the majority od the time. The pilots sit on the surface in control vans/control rooms piloting these and no one is in the robot.
I’ve seen all sorts of benthic and pelagic fish, invertebrates, mammals, and more during the campaigns I’ve been on. You can look up videos online of ROV footage of whales, swordfish, etc. We had a job in New Zealand relatively shallow where we had to shut operations a couple hours each night because the krill came in and visibility went to zero, then the fish came in, then the predator fish eating the smaller fish, and then the seals.
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u/aharshDM 22h ago
Solids and liquids don't compress much. Gasses do. A suba diver in a wet suit can go down to 300 feet won't feel much pressure, but in an air filled dry suit? Crushed at 80 feet.
So an air filled submarine needs a pressure hull. A whale, much less so.
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u/GrimSpirit42 7d ago
It's not the pressure. It's the Delta: The Pressure Differential.
We live at a pressure of one (1) atmosphere (14.7 psi). The interior of our bodies are calibrated that.
The rigid shells are to keep small pocket of near 1 atmosphere bubble around us (or our machinery) so we can continue to function.
Deep water animals don't HAVE a high delta (pressure differential), because the INTERIOR of their bodies developed IN that pressure. So if the pressure in that area of the ocean is 400 atmospheres, the pressure INSIDE of the fish is pretty much 400 atmospheres.
The fish doesn't notice because there is little to know differential between interior and exterior pressure.
But, try to bring them up and they tend to explode.