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Pirarucu scales form a special armor

The composition and structure of pirarucu scales make them highly resistant to punctures and tension

Robert Schlappal / Wikimedia Commons

The scales of the pirarucu (Arapaima gigas), which can reach up to 7 centimeters long, give the fish an almost impenetrable armor. They are light, flexible, and at the same time, highly resistant, all of which help protect one of the largest freshwater fish in the world from sharp-toothed predators like piranhas. Native to the Amazon, the pirarucu breeds in and spends much of the year in temporary lakes that are often also inhabited by shoals of piranhas. A team of material engineers led by Marc Meyers of the University of California, San Diego, and Robert Ritchie of Berkeley, both in the USA, decided to test just how resistant pirarucu scales are to bites by subjecting scale specimens to a number of experiments. The group has previously observed that the structure of the scales—formed by a mineralized outer layer and an inner layer filled with collagen fibers—were highly resistant to penetration by sharp objects, such as the teeth of a piranha. In the most recent tests, using pre-fractured scales, the researchers found that the collagen fiber deposits—which in the pirarucu form one of the thickest layers ever seen in fish scales—slow down propagation of the fracture. The collagen, which is a very elastic material, is arranged in parallel fibers inside the scales that form overlapping layers. The direction of the fibers differs from one layer to the next, which the researchers believe is what gives the material such great resistance (Matter, October 16). Bulletproof vests have a similar structure to pirarucu scales.