Fish scale the surface, interacting only with the riblet tips, not causing any high-veloctiy flow in the valleys. Since this high velocity flow now only interacts with the riblet-tip, which is a very small surface area, the momentum transfer which causes drag is now much lower than before, thereby effectively reducing drag. Also, this reduces the cross-stream velocity fluctuations, which aids in momentum transfer too. The rough, sandpaper-like texture of shark and ray skin, coupled with its toughness, has led it to be valued as a source of rawhide leather, called shagreen. One of the many historical applications of shark shagreen was
Shagreen as a fancy leather for book bindings, pocketbooks and small cases, as well as its more utilitarian uses in the hilts and scabbards of swords and daggers, where slipperiness is a disadvantage. In Asia, the Japanese Tachi, Katana, and Wakizashi swords had their hilts almost always covered in undyed rawhide shagreen, while in China, shagreen, whose use dates back to the 2nd century CE, was traditionally used on Qing dynasty composite bows. Typically the ears and the spaces above and beneath the grip were covered by polished shagreen (in which the calcified papillae are reduced to equal height and form