Who nose what flows: Shark narial denticle morphology and hydrodynamic implications.

Dermal denticles (scales) are important in influencing the movement of water around a shark's body. To date, most of the research on denticle morphology and their impacts on hydrodynamics has focused on the lateral flank of fast-swimming species. One understudied region where these interactions may be important is the nares of sharks. The constant flow of water through the nares is critical to olfaction and therefore a shark's survival. We imaged dermal denticles all around the incurrent and excurrent nares of the Pacific spiny dogfish (Squalus suckleyi), a benthopelagic species inhabiting eastern Pacific waters. At the incurrent naris, we quantified denticle morphological traits such as length, width, aspect ratio, ridge number, and angle of rotation off the anterior-posterior axis. We found that denticles at the incurrent naris display two primary morphologies: elongated with ridged crowns and rounded with smooth crowns. Moreover, we show denticles rotated to nearly 180-degrees off the anterior-posterior axis as denticles enter the incurrent naris at the cranial region. Using particle image velocimetry (PIV) over a 3D printed model of a micro-computed tomography scan of the incurrent naris, we visualized the effects of this rotation on flow and found preliminary data for a reverse circulating vortex in addition to laminar flow into the olfactory chamber. Finally, we propose two hypotheses on the importance of this phenomenon. This work highlights the diversity of shark denticle morphology, particularly with respect to their role in fluid mechanics. Our findings challenge our current understanding of dermal denticle orientation and function, further supporting the need to investigate areas of interest across shark bodies that have not yet been studied in the literature.