Ecology: Biology of Interaction. 5.27. (supplement) Pressure at Depth: Enduring and Overcoming
Sometimes environmental impacts are so severe that resisting them is nearly impossible. Water pressure at great depth is one such case. Deep-sea organisms usually do not resist pressure directly but endure it via pressure equilibration, while some large vertebrates evolved active biomechanical strategies for deep diving.
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5.26. (supplement) Factors affecting organism development
D. Shabanov, M. Kravchenko. Ecology: Biology of Interaction Chapter 5. Autecology and Foundations of Environmental Science
6.01. The modern ecological crisis
6.01. Contemporary ecological crisis 5.27. (supplement) Pressure at depth: enduring and overcoming Sometimes environmental impacts are so severe that resisting them is almost impossible. This applies, for example, to water pressure at great depth. We live under a thick atmospheric layer and are used to ignoring its pressure, about 1 kg per cm2 of surface (approximately equivalent to 10 m of water column). But during descent, pressure rises rapidly. At 10 m depth it is already 2 atmospheres (one from air pressure, one from ten-meter water column). Maximum ocean depth exceeds 11 km, and down to these extreme depths the World Ocean is inhabited by animals, including fishes, crustaceans, mollusks, and echinoderms. As you can calculate, pressure there exceeds 1,100 atmospheres, i.e., more than 11 tons per square centimeter of body surface. Can a living creature withstand such pressure? Imagine lowering an empty (air-filled) metal can to that depth. External pressure of 1,100 atmospheres will inevitably crush this can, reducing its volume 1,100-fold. Construction of bathyscaphes capable of withstanding such pressure is an exceptional engineering challenge requiring great wall and viewport thickness. But what happens if an identical can is lowered open, or at least with a hole? Nothing special: water under pressure fills the can without changing its shape, since external wall pressure is balanced by internal pressure. Deep-sea organisms withstand pressure by the same mechanism as open cans — by not resisting it directly. Tissues of organisms on our planet are water-based. Water is practically incompressible under pressure. External pressure on deep-sea tissues is balanced internally, so organisms do not “feel” it as crushing force. Can we consider conditions at great depth equivalent to those at surface? No. At high pressure, chemical reactions are accelerated and gas solubility in water increases. “Boiling” (bubble formation) during pressure decrease explains not only fizz in an opened carbonated bottle but also swelling of deep-sea fishes brought up by nets and decompression sickness in divers. Can one actively counter enormous depth pressure? Surprisingly, to some extent yes. The sperm whale — largest toothed whale, adapted for hunting deep-sea squid — can do this. The most notable feature of sperm whale appearance is its huge head. It occupies up to one-third of body length and appears rectangular in profile due to a cavity above upper jaw containing spermaceti, a wax-like substance. Its name comes from old belief that this material was whale sperm (and that sperm whale stored huge reserves in its head). Incidentally, spermaceti drove intensive whale hunting because it was an excellent base for perfumery, improving fixation (persistence) of expensive fragrances. Its real function was understood only recently. In terrestrial tetrapods (including humans), diving difficulties are related to air in lungs. Before diving, we inhale deeply. Air-filled lungs provide positive buoyancy that must be overcome during descent. But after descending deeper, situation changes: since pressure of 10 m water column corresponds roughly to 1 atmosphere, at that depth external pressure doubles and lung volume halves. Buoyancy becomes negative, pulling body deeper — just when one needs to ascend. Sperm whales routinely dive deeper than one kilometer (recorded maximum about 2,200 m, and likely not absolute limit) to hunt giant squid. Naturally, this requires a thorax capable of withstanding over 200-fold volume reduction (under much smaller compression, in humans either ribs fracture or lungs separate from pleural structures). But even with such “folding” thorax, a sperm whale would have to both dive and ascend while fighting unfavorable buoyancy forces — unless it used its spermaceti organ. Spermaceti is liquid at body temperature and solidifies, substantially increasing volume, with slight cooling. Before diving, sperm whale increases blood flow to spermaceti organ. Spermaceti melts, head volume decreases, and whale is pulled downward. When ascent time comes, it cools spermaceti (either by reducing blood flow or taking in cold external water through nasal passages). Spermaceti expands, increasing head volume and overcoming tremendous external pressure. Head-first, sperm whale rises to surface, holding weakening squid in jaws. For bathyscaphes and submarines, buoyancy change requires expenditure of substances: dropping ballast, releasing kerosene from external tanks, spending compressed air to blow tanks. A scuba diver wears excess weight belt (which can be dropped if needed) and balances buoyancy with compensator bladder by adding air from tanks or venting it. A sperm whale, changing buoyancy, spends only energy obtained by oxidizing prey captured at depth with atmospheric oxygen taken at surface. How can one not admire perfection of this adaptation? 5.26. (supplement) Factors affecting organism development
D. Shabanov, M. Kravchenko. Ecology: Biology of Interaction Chapter 5. Autecology and Foundations of Environmental Science
6.01. Contemporary ecological crisis
6.01. The modern ecological crisis