|MadSci Network: Zoology|
You're right, in the deep oceans, the pressure of the water is immense! However, it's not necessarily the pressure that causes problems for animals, but rather pressure differences. You may know that many fish have something called a swim bladder, which is basically a bag of gas inside the fish. By inflating and deflating that bag of gas, the fish are able to change their density (which allows them to move up and down in the water). For more on that, I'll refer you to a question on that I answered a while ago: Re: Do fish ascend and descend the same way as submarines?.
Now then, imagine if you were to take an inflated balloon and lower it into the ocean: the pressure of the water would squish the balloon down to nothing, right? The same thing would happen to a fish with a swim bladder- the difference in pressure between the gas bag and the water pushing in on the outside of the fish would squish it (if it were unlucky to go deep enough!) So, in order to avoid being squished like our balloon, deep sea fish do not have a swim bladder, or any type of air cavity at all. To illustrate this, let's go back to our balloon: imagine if you took the same balloon, but didn't inflate it. This time, if you lowered it into the water, it can't be squished by the water pressure, because it's already as small as it can get.
As well, deep sea fish also tend to have very gelatinous muscles, with a high water content, so that the pressure that is being exerted on them is exerted over their entire body.
For some really neat pictures of deep sea fish, check out the NOVA Online site on the Deep Sea Beastiary.
Hope that helps!
Rob Campbell, MAD Scientist
Try the links in the MadSci Library for more information on Zoology.