To help prevent that sinking feeling
Whether we indulge in hang-gliding (not that I would ever have done that let alone attempt to do that) or relish the feeling of near weightlessness during a dive (which I did have experience with), nobody can really escape gravity on Earth. Staying afloat (in many ways) is staying alive. This holds true even for the smallest forms of plankton: without certain specializations to stay afloat they would all gradually sink to the bottom of the sea. Dead fish only float at the surface as long as their swim bladder is still intact – once ruptured they, too, sink.
So, what can aquatic organisms do to defy gravity? They can, like the fish just mentioned, charge up certain structures with gas or lipids to compensate for body weight. The gas need not have the same composition as air and in deep-sea fishes it often approaches pure oxygen, while -strangely- in certain siphonophore jellyfish species it consists of high amounts of carbon monoxide (a poison to most animals and very dangerous to humans, because it is odourless). The livers of some species of fish are so enormous, because they store huge amounts of oil, which in addition to the other functions it has, provides buoyancy. Diatoms, planktonic algae with shells of silica, but also eggs of many marine fishes, reduce their sinking speed by incorporating lipid droplets in their cytoplasm. Numerous planktonic crustaceans, worms, molluscs, sea urchin-, starfish- and fish-larvae possess long, feathery body appendages that increase drag and thus help these animals to stay in the water column. Interestingly, such appendages are very much more elaborate in the tropics than in polar waters. If you remember your physics classes, you would know why, because physics has the answer: in cold water sinking is only half as rapid as in the warm tropical water, because of the greater viscosity (i.e., higher density) of cold water.
An interesting method of becoming lighter is employed by certain squids and Noctiluca, the main producer of the sea’s so-called “phosphorescence”. These organisms exchange the heavier ions in their body fluids for lighter ones; sulphate, for example, is replaced by the lighter chloride and sodium by ammonium. This reduction in body weight, even if it is small, represents an energy save, as the organisms have to make fewer “correcting swims” upward.
More sophisticated still than simply increasing lift or reducing weight, are methods that allow an animal control over its buoyancy. Expanding and retracting branched appendages represents one way to vary sinking speeds; another is to change the amount of gas in the body by a valve as in the shelled squid Nautilus or by burping. All fishes of the carp family have a connection from the swim bladder to the gut. As long as the fish is neutrally buoyant, everything is fine (the fish is seemingly totally balanced in the water and can remain motionless), but as soon as the atmospheric air-pressure is, for example, reduced the fish would be too light and end up struggling to dive even a few centimetres down from the water surface. Burping air, however, releases some air from the swim bladder and takes care of the problem and neutral buoyancy is re-established. The opposite, namely to replace lost gas in the swim bladder, takes a little longer than burping, but can be achieved with the help of the fish’s circulatory system. But burping in humans has a different function and should not even have been mentioned in this essay on Keeping Afloat -or should it?
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2020.
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