At the microscale, shark skin is covered in dermal denticles: rigid, tooth-like structures aligned with the direction of motion. Each denticle has longitudinal riblets (tiny grooves) that interact with the turbulent boundary layer—the thin region of fluid right next to the surface where velocity gradients and shear stresses are highest. In turbulent flow, streamwise vortices (rotating fluid structures) transport high-momentum fluid toward the surface and low-momentum fluid away from it, increasing skin-friction drag. Riblets constrain the lateral (spanwise) motion of these vortices. By limiting cross-stream mixing, they reduce momentum transfer toward the wall, which lowers shear stress and therefore drag. The effect is subtle but measurable. Optimized riblet geometries can reduce skin-friction drag by several percent in turbulent regimes, depending on Reynolds number, riblet spacing, and flow conditions. The key parameter is the non-dimensional riblet spacing in viscous wall units (...