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 ray m. It is most common at depths of 200 m, but is found deeper in southern areas. It favors sandy and possibly also rocky habitats. The shagreen ray feeds on a variety of benthic animals, mainly fishes but also crustaceans. Like other skates reproduction is oviparous; the egg capsules are amber in color and oblong-shaped, with long, stiff horns at each corner. One pair of horns are longer than the capsule. The capsules measure 7.5-9.9 cm long and 4.5-4.7 cm wide. Historically this ray was considered of "inferior" quality for consumption. It is caught commercially by longlines and trawls. Shagreen