Scale Morphology and Flexibility in the Shortfin Mako Isurus oxyrinchus and the Blacktip Shark Carcharhinus limbatus Philip Motta, 1 * Maria Laura Habegger, 1 Amy Lang, 2 Robert Hueter 3 and Jessica Davis 1 1 Department of Integrative Biology, University of South Florida, Tampa, Florida 33620 2 Department of Aerospace Engineering, University of Alabama, Tuscaloosa, Alabama 35487 3 National Center for Shark Research, Mote Marine Laboratory, Sarasota, Florida 34236 ABSTRACT 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 than the blacktip shark. The majority of the shortfin mako shark scales have three longitudinal riblets with narrow spacing and shallow grooves. In comparison, the blacktip shark scales have five to seven longitudinal riblets with wider spacing and deeper grooves. Manual manipulation of the scales at 16 regions on the body and fins revealed a range of scale flexibility, from regions of nonerectable 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 508. The scales of the two spe- cies 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. Erec- tion of flank scales and resulting drag reduction is hypothesized to be passively driven by localized flow patterns over the skin. J. Morphol. 000:000–000, 2012. Ó 2012 Wiley Periodicals, Inc. KEY WORDS: dermal denticles; placoid scales; dermis; collagen; elastin; drag INTRODUCTION The earliest record of elasmobranch fishes is from isolated shark scales that date back to the late Ordovician period, about 455 million years ago (Janvier, 1996; Sansom et al., 1996; Ahlberg, 2001). These placoid scales (dermal denticles) accommodate bioluminescent and sensory organs, and are pur- ported to serve a variety of functional roles including mechanical and biological protection, abrasion re- sistance, and parasite protection as well as antifoul- ing, hydrodynamic drag reduction and increased thrust (Reif, 1985a,b; Bechert et al., 1986; Raschi and Musick, 1986; Raschi and Tabit, 1992; Carman et al., 2006; Schumacher et al., 2007a,b). Mounting evidence indicates that drag reduction most likely occurs by reducing turbulent crossflow near the scale surface, thereby reducing shear stress, and by con- trol of flow separation around the body, which would reduce pressure drag (Bechert et al., 1997b, 2000; Lang et al., 2008, 2011, in press). In addition, increased thrust may occur on the fins by reducing the leading edge vortex thereby increasing anteri- orly directed suction (Oeffner and Lauder, 2012). Elasmobranch skin is composed of a thin epider- mis sitting upon a thick dermis. The dermis is divided into a superficial stratum laxum (S. spon- giosum/S. vasculare/S. superficiale) and a deeper stratum compactum. The stratum compactum of sharks, which is the better studied of the two layers, is composed of type I collagen fibers arranged in a lamellar fashion. Each layer com- prises helically wound and parallel fibers, with alternate layers lying at the same angle (Fig. 1; Kimura et al., 1981; Lingham-Soliar, 2005a,b; Hwang et al., 2007; Meyer and Seegers, 2012). The helically wound fibers course around the body form- ing angles of 50–708 to the longitudinal axis Additional Supporting Information may be found in the online version of this article. Contract grant sponsor: Collaborative National Science Foundation; Contract grant number: 0932352, 0744670, and 0931787; Contract grant sponsors: The University of South Florida and The Porter Family Foundation. *Correspondence to: Philip Motta, Department of Integrative Biology, University of South Florida, 4202 East Fowler Ave., Tampa, Florida 33620. E-mail: motta@usf.edu Received 24 January 2012; Revised 25 April 2012; Accepted 6 May 2012 Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/jmor.20047 JOURNAL OF MORPHOLOGY 000:000–000 (2012) Ó 2012 WILEY PERIODICALS, INC.