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Asked by Zealousy to Chris, Josh, Rebecca, Rob, Susan on 15 Jun 2015.
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Rebecca Dewey answered on 15 Jun 2015:
We class the animals that live in the deepest sea floors as living in the “sub-photic” zone – this means that they live below the level that light can reach.
To withstand the pressure, they have evolved such that many of the animals are small, so that the pressure on them is easier to withstand. They often have gelatinous (jelly-like) flesh and don’t tend to have a skeleton, so that they are squidgy and not brittle when under pressure. Most of the creatures so not have outside cavities or orifices such as swim bladders (an air filled organ that helps them swim at the same level without needing to use extra energy staying “afloat”) as these would be collapsed at high external pressure.
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Susan Cartwright answered on 15 Jun 2015:
– Health warning: I am not a biologist. Treat this answer with caution. –
The key point is that it’s not pressure that kills, it’s pressure differences. Animals that have evolved to live in the deep oceans have high internal pressures, matching the high external pressures: they are fine in the deep ocean, but if you catch them in a net and bring them to the surface, they die, because their internal pressures are no longer properly balanced by external pressure. Similarly, human beings can survive at depths much greater than those regularly reached by divers – the deepest SCUBA dive on record is 332 m, which is more than 10 times the standard “recreational” SCUBA diving maximum of 30 m – but only if they come back up very slowly, to allow the internal and external pressures to equalise gradually. If you try to come up too fast, gases that have been absorbed into your tissues because of the high internal pressures are released as bubbles, which can cause serious problems if they enter the bloodstream.
The really interesting question isn’t how the deep-water species survive, it’s how some species, particularly sperm whales, manage to dive to extreme depths AND THEN COME UP AGAIN without killing themselves. Sperm whales can dive as deep as 3 km!! (though their normal depth is more like 400 m). This is a major physiological challenge. Sperm whales and other deep divers have many adaptations to solve this problem. First, any air spaces in the bones (sinuses) would be crushed by the pressure at these depths – so sperm whales have fewer sinuses than other mammals, and those sinuses they do have are lined with blood vessels, which swell up when they dive and force out the air, leaving no cavity to collapse. Secondly, their rib cages are made of flexible cartilage, not hard bone, so that when the lung volume is compressed under high pressure, the ribcage can deform to match instead of shattering. The lungs are also lined with special chemicals that make it easier to reinflate them during the ascent.
The sperm whale has other problems besides resisting pressure. How can it adjust its buoyancy so that it sinks on the way down and floats on the way up? (Answer: it adjusts the density of the spermaceti oil that fills much of its head, as well as its layer of blubber.) Also, how does it manage to hold its breath, literally for hours – why doesn’t it run out of oxygen? (Multiple answers: its metabolism slows down, so it doesn’t use oxygen as fast; its blood is much higher in red blood cells, so it can store more oxygen; its muscles are also higher in oxygen-storing tissues; various other technical adaptations too.)
Disclaimer: I cheated. Most of this information came from an online presentation by Stephanie Fafard-Rousseau, who was then (2012) at Wright State University, but whose current whereabouts I don’t know.
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