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
Porcupine ray of leather called shagreen, had many historical uses. In particular, it was used to cover the hilts of various melee weapons, as its extremely rough texture prevented slippage during battle. For example, the Japanese deemed it the only species whose skin was acceptable for covering sword grips. The Malayans used it to cover shields. The skin was also used ornamentally, such as by the Chinese, who dyed it and ground down the thorns to yield a mottled pattern. The native inhabitants of Funafuti Atoll used dried portions of the ray's tail as a rasp-like tool. Presently, the porcupine ray is