The arrangement of plate-like structures within the scales of Arapaima gigas - a freshwater fish found in the Amazon basin - is key to its ability to protect itself from the bite of piranhas. The finding is published in Nature Communications this week.

Piranhas are known to be fearsome attackers, aided by sharp teeth that clamp on to and disable their prey. Certain fish have evolved to avoid death by such an attack; some have developed the ability to swim faster while others have grown tough natural armour - in the form of protective scales - that can absorb the impact of biting teeth. Robert Ritchie and his group find that the distribution of plate-like structures, known as lamellae, in the fish scales of the Arapaima gigas, allows for the absorbing of energy from an applied load. They show that they do this by reorienting and twisting, potentially mitigating the high stresses applied by the teeth of piranhas and preventing the puncturing of the scales. The authors conclude that, while the outer lamellae gives the scale hardness and penetration resistance, the overlapping of scales and the corrugated outer surface of the lamellae allow the scales to bend in this way, transferring tensile stress to the inner lower scales.