Science Questions

How do fish survive the pressure underwater?

Sat, 25th Jul 2015

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Question

Alan Walt asked:

In the deepest ocean where the pressure may be 8 tons per sq inch, why are the creatures which have evolved not simply crushed by the force?

Answer

Ginny Smith dived into Alan's question...

Ginny - Well, the first thing to say is that we ourselves are designed to withstand humpback whalepressure. We have pressure pressing down us all the time from the air above us. And actually, you can see how great that pressure is if you’ve ever seen someone suck the air out of the inside of a can and it immediately crumples because the air pressure is strong enough to actually crush that can. The only reason it doesn’t crush it most of the time is because that there's air inside it as well. Our bodies work the same way. There's air inside us and there's air around us and that kind of balances out. When you go down into the deep sea, there's a huge amount more pressure. In fact, the pressure increases about 1 atmosphere. So, that’s one of the amount of pressure we have on us for every 10 meters, you go down in the sea. So, it could be that deep sea creatures have over a thousand times the pressure on them that we do. But they’ve evolved to live in that pressure. And one of the things they do is that they don’t have air pockets inside them like we do. They use other things. So, their muscles for example have lots of water in them and water isn't compressible. You can't crush it. So, that kind of stops them from being squeezed too much. They have lots of different changes to their physiology. So, some of them have this molecule called a (PISA) light which actually prevents other molecules from being distorted when they're under pressure. We don’t really understand how this works because one of the big challenges of studying these animals is like we couldn’t survive down there, they actually can't survive up here when there isn't as much pressure. If you went down into the deep sea, picked up one of those fish and brought it back to the surface, by the time you got it here, it wouldn’t look a lot like it looked when it was down there, and that makes them really difficult to study.

Chris - Ewen…

Ewen - Yeah, that’s really good Ginny, but how then do whales manage to do it?

Ginny - I was waiting for someone to ask that. So, whales and animals that travel between lots of different pressures actually have the hardest job of all. They have lots of different adaptations that help them. One of them is that their lungs effectively collapse so that when they're diving down, those air-filled pockets basically just disappear so they're not there anymore and they can deal with that kind of physiologically in a way that we couldn’t if our lungs collapsed. That would be really bad news. They also don’t have some of the other air-filled cavities we have like sinuses.

Chris - They also have various behaviours, don’t they? They surface more slowly so that rather than having compressed all this gas that was residual in their lungs down to a miniscule volume, and then effectively made it dissolve in their blood which is what happens when you put it under that pressure, if they were to surface really, really fast, they would get the bends. But they don’t because they come up relatively slowly and this means that there's an opportunity for the gas to come back out of solution in the blood and go into the lung tissue. It does it at a slow enough rate that you don’t get bubbles forming in the bloodstream. But if you scare a whale which can happen with underwater detonations and seismic surveys for oil and that kind of thing, that can make them rush to the surface and then they do get the bends, and people have found whale carcases where they’ve looked at the bones of these whales. Some of which are 100 plus years old and you can see what are called osteonecrotic lesions. This is where a bubble has formed in the blood vessel that supplies that patch of bone and the bubble has lodged in the blood vessel, blocking the supply and you therefore devitalise the bone downstream, and it dies and you get a hole in the bone. This is signs that whales can get the bends probably, in response to being a bit frightened. 

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