Scallop Shells Exhibit Optimization of Riblet Dimensions for Drag Reduction

Drag reduction by streamwise surface grooves, or riblets, has been observed by engineers and has been suggested to apply to certain biological systems. Drag reductions as high as 8% have been observed (1), leading to practical nautical and aeronautical applications (2, 3, 4). The shells of several species of scallop, including Placopecten magellanicus, display riblets arranged radially, and therefore roughly parallel to the flow during swimming (Fig. 1a). The dimensions of these riblets on particular scallops fall within the region necessary for drag reduction at experimentally measured swimming speeds. Moreover, the actual spacing of the riblets gradually migrates into the theoretically optimal spacing region as shell length increases beyond 40 mm (Figs. 2, 3). Specimens of P. magellanicus 40 to 80 mm in length demonstrate the greatest swimming ability (5); our data strongly suggest that streamwise riblets may be a contributing factor to the swimming success in scallops of this size range.