Arapaima gigas is one tough fish. So tough, in fact, that the scales on this Amazon Basin freshwater fish protect it from the powerful jaws of snapping piranhas. How these scales serve as natural dermal armor has eluded scientists. Robert O. Ritchie of Lawrence Berkeley National Laboratory and the University of California, Berkeley, and coworkers designed an experiment in which they probed the scales’ microstructures via a synchrotron-based X-ray scattering method while subjecting the scales to various types of mechanical forces (Nat. Commun. 2013, DOI: 10.1038/ncomms3634). The team found that the scales consist of a hard mineralized outer shell and an inner core of micrometer-sized collagen fibrils arranged in a unique pattern of twisted lamellae (bundles of fibrils). As force is applied to the scales, the lamellae reorient but not in a uniform way. Some of the bundles deform along the direction of applied force through stretching and sliding motions. Others rotate away from that direction and are compressed. The team explains that the concerted motions enhance the scales’ ductility and toughness, which prevent penetration and fracture but do not impair flexibility.