The snorkelling elephant
Jacques Cousteau, the inventor of the aqualung, dreamed of creating underwater living space for humans. So far that has remained a dream for humans, but it is reality for many animals.
Living under water has its advantages: One can’t be seen easily by aerial and terrestrial predators, waste removal is unproblematic, temperature fluctuations are less frequent and less abrupt, and most of all (at least in freshwater) there is no danger of drying out. However, there is one problem: animals need oxygen. Those with gills like crustaceans, some aquatic insects, fish and larval amphibians obtain their oxygen directly from that which is dissolved in water, but those that do not have gills either have to come to the surface periodically (like some water beetles, diving birds, seals, and whales) or they need a siphon, i.e., a breathing tube like the snorkel we humans use to keep our heads under water when observing underwater life. Mosquito larvae use short siphons of a few mm length projecting from the rear ends of their bodies to breathe. Longer siphons, up to 5 cm, are developed in water scorpions, which are not actually scorpions, but predatory hemipteran bugs that hunt under water. The longest breathing tubes in insects are found in larval syrphid flies. These larvae are known as ‘rat tails’, because with body lengths of maximally 2 cm, they possess three-segmented telescopic siphons that can be extended to a full 15 cm! With them, accessing atmospheric air, they can survive in an oxygen-deficient environment that few other animals can tolerate, namely sewage.
While the rat tail siphon is at the end of its body, the siphon of a much larger animal is at its front: I am talking of the elephant. Elephants are good swimmers and can cross rivers several metres deep simply by holding up their trunks into the air above the water surface. But as every diver knows, for humans a snorkel must not be longer than 30 cm, because the increasing air-pressure at depths greater than that will prevent the lungs from achieving gas exchange through the snorkel. But elephants can when they are in water 2 m deep. How do they do that? Well, of all mammals, elephants possess unique lungs. This has nothing to do with their size, for rhinoceroses have lungs like any other mammal (only the elephant is different). During snorkelling the elephant’s neck, chest, abdomen, and limbs are exposed to the increased pressure of the water, but the lungs are connected to air via the trunk. Consequently, a huge pressure differential develops between inside and outside the lungs. If the tissue surrounding the lungs of the elephant were as delicate as that of other mammals, the snorkelling elephant would face massive pulmonary malfunction through transudation. In the lungs’ surroundings of the elephant, the normally delicate tissue has been replaced by layers of dense and loose connective tissue that contain only a few well protected blood vessels. These do not rupture despite the pressure differential acting on them. Quite likely the elephant’s ancestors were aquatic grazers like the dugong (actually a close relative of the elephant).
But unlike the latter elephants use their trunks to breathe under water. For submerged mammals, this is a unique way to obtain oxygen, but even more amazing is the method that larvae of the mosquito Mansonia spp. have perfected: they don’t surface and stick their siphons into the air: they use their siphons to puncture air-stores in the stems of aquatic weeds and get their gaseous life support from them!
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2015.
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