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Shark scales and drag reduction

Phil manually erecting single scales on the shark

Laura using a catheter to pressurize the skin, then erecting the scales

Phil examining the scales of a hammerhead shark

Laura viewing dermal denticles
Microscopic view of the dermal denticles on a mako

 

Our latest National Science Foundation funded research investigates the passive boundary-layer separation control methodology derived from shark skin. Working in conjunction with Dr. Amy Lang of the University of Alabama, and Dr. Robert Hueter at Mote Marine Laboratory we quantified placoid scale morphology and flexibility in the shortfin mako Isurus oxyrinchus and the blacktip shark Carcharhinus limbatus. The shortfin mako shark has shorter scales (169 to 299 µm) than the blacktip shark (166 to 435 µm). The majority of the shortfin mako shark scales have three longitudinal riblets with spacing from 29 to 45 µm and groove depth of 2 to12 µm. In comparison, the blacktip shark scales have five to seven longitudinal riblets with an average spacing of 34 to 83 µm and an average depth of 4 to 25 µm. Manual manipulation of the scales at 16 regions on the body and fins revealed a range of scale flexibility, from regions of non-erectable scales such as on the leading edge of the fins to highly erectable scales along the flank of the shortfin mako shark body. The flank scales of the shortfin mako shark can be erected to a greater angle than the flank scales of the blacktip shark. The shortfin mako shark has a region of highly flexible scales on the lateral flank that can be erected to at least 50°. The scales of the two species are anchored in the stratum laxum of the dermis. The attachment fibers of the scales in both species appear to be almost exclusively collagen, with elastin fibers visible in the stratum laxum of both species. The most erectable scales of the shortfin mako shark have long crowns and relatively short bases that are wider than long. The combination of a long crown length to short base length facilitates pivoting of the scales. Erection of flank scales and resulting drag reduction is hypothesized to be passively driven by localized flow patterns over the skin.