Just came across this
article
(in French) which I thought was quite interesting.
Translation :
I cannot remember who gave me, when I was a child, a small encyclopaedia of animals in South America. But I vividly remember being impressed by a story told on a chapter about piranhas: so their livestock can cross a river infested by these carnivorous fish with sharp teeth, cowherds sacrificed a sick animal by throwing in first the water. While the school of piranhas "cleaned" the animal, the rest of the livestock could walk through. In the dry season, when some streams dry up, the piranhas are trapped in lakes and devour anything that can be eaten in the area, fish, mammals and even unwary birds. Two exceptions: the caiman, for fairly obvious reasons, and a fish called arapaima, for reasons that are much less obvious. How can this huge animal, which can measure up to 2.5 meters and is 200 pounds, with its slightly sweet flesh and constitutes a reserve of food as abundant as it is delicious, manage to escape the dreaded piranha?
The answer is simple: the arapaima is wearing a tooth-proof vest. The proof has just been made in a U.S. study published in January [2012] in the Advanced Engineering Materials magazine. The title - "Battle in the Amazon: arapaima against piranha" - sums up the approach the authors, researchers at the University of California, followed: confront the weapons of the piranha, its teeth, with the armour of the arapaima, its scales. The former are impressive, as can be seen in the picture above. Although the mouth muscles of the piranha are not particularly powerful, the fish compensates with teeth that are as sharp as razor blades. Researchers compared the way they enter into the soft tissue to the way a guillotine blade sliced the heads of the condemned, not horizontally but diagonally, in order cut more easily through the neck muscles.
But it is above all the "armour" of the arapaima which has focused the attention of the researchers. The fish pictured above has huge scales, 10 centimetres long, consisting of two parts: the visible one, dark grey, and the hidden one (covered by the other scales), of lighter colour. The first is hard, rough to the touch, so much so that the Amazonian inhabitants use it as a nail file. The analysis showed that this area of the scale is wavy and the bumps have a high concentration of calcium, which explains the hardness of this bio-mineral material. The hidden part is thinner and softer. The scale is a superposition of these two layers, the first, highly mineralised and the second, made of several layers of collagen fibres. This composite material is both highly resistant in surface and flexible, a essential property to the swimming of the fish. The waviness of the upper layer also facilitates this flexibility.
It remained to be determined if these scales could withstand the bite of the piranha. To find out, researchers have fixed teeth of this fish on a machine and tested their penetration into the Arapaima scales. Reference tests were conducted on synthetic rubber and salmon flesh, which have not survived, unlike the scales. Even though the enamel of piranha teeth is considerably harder than them, the thickness to get through is such that the carnivore cannot reach the flesh of the Amazonian: indeed, the scales overlapping throughout like tiles would require at least 2 to be pierced (four millimetres). By increasing the penetration force, the researchers have only managed one thing: breaking the tooth of the piranha! In fact, a simple calculation showed that the strength a piranha should deploy to bite an Arapaima is well beyond its capabilities. In the evolutionary armament race, the defender took a good step ahead of the attacker. Beyond this observation, the California researchers stress that the scale of Arapaima belongs to this class of amazing biological materials from which the industry could learn (a "plagiarism" called bio-mimicry). In order to make ultralight bullet-proof vests?
Here is a
link
to the full article in the Advanced Engineering Materials magazine.
