Meeting Abstract
Shark skin denticles (scales) are diverse in morphology both among species and on the body of single individuals although the function of this diversity is not well understood. Current views indicate that denticles may both reduce drag and increase thrust during swimming, and the morphology of denticles on different regions of a shark may reflect local flow conditions. We systematically measured denticle morphology on the caudal fin of thresher sharks (genus Alopias), and compared thresher tail denticles to those of selected other pelagic shark species. Thresher shark tails are equal in length to the rest of their body and are uniquely used for both locomotion and hunting (by whipping the tail over their head into schools of fish). Using surface profilometry, we quantified 3D denticle patterning and texture in three adult and two embryonic threshers at 27 regions in the adults and 16 regions in embryos. We then measured a suite of variables using these 3D surfaces to compare the surfaces of different regions. The tails of thresher embryos have a membrane that covers the denticles and reduces surface roughness: membrane surface roughness is 4µm, while embryo denticle roughness averages 10 µm. In adult thresher tails, denticles are generally similar to those of other pelagic shark tails, and there is no gradient in roughness from the base to the tip of the tail: tail skin roughness averaged 9 µm. All along the tail there is a leading-to-trailing edge gradient with leading edge denticles larger and lacking ridges (average roughness = 9 µm), while trailing edge denticles are smaller, with 5 ridges, and an average roughness of 7 µm. We found no evidence that the increased undulatory excursions from the base to the tip of thresher tails correlate with surface roughness.
