"...each scale is made of three layers on a bone support that all complement one another to defy penetration. The outer coat is the hardest and most resistant to sharp teeth. The middle is softer and dissipates energy by deforming. The last layer has a plywoodlike structure, which prevents cracks from spreading. The precise sequence of these layers critically preserves armor strength—for instance, replacing the outer and middle layers in simulations increased risk of the scale coming apart."
For you ichthyologists, it's Polypterus senegalus and it's not technically an eel. It lives in muddy swamps, and it evolved the tough scales some 100 million years ago, back when predators were fierce and prey were nervous. Tweet