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
Cowtail stingray ("Himantura uarnak"), likely because the whiprays are better able to detect predators due to their longer tails. The serrated spine of the cowtail stingray is potentially dangerous, and this species is regarded as particularly dangerous to handle as its long tail is capable of reaching over its back to strike someone gripping it from the front. Small to moderate numbers of cowtail stingrays are caught as bycatch in trawls and marketed for meat throughout their range, and their tough skin is used to polish wood. This species is also the primary source of shagreen or galuchat, a type of leather,