Meeting Abstract
Shark pectoral fins are comprised proximally of basal cartilaginous elements that articulate with the coracoid and distally of radial cartilaginous elements and thin, flexible ceratotrichia that overlay the radials. The extent of radials into the fin is known to vary among species, where fins with radials that extend less than 50% into the fin are termed aplesodic and greater than 50% into the fin are called plesodic. Here we examine the skeletal anatomy and flexural stiffness from pectoral fins of 21 species of shark. We hypothesize that there is a range from aplesodic to plesodic, rather than two distinct conditions, that correlates with ecology. We hypothesize that benthic sharks have flexible aplesodic fins useful for gripping the benthos and digging in the sand, whereas pelagic sharks have rigid plesodic fins used to generate lift while swimming. We modeled pectoral fins as cantilever beams by immobilizing the proximal end and applying a load to the free end to determine flexural stiffness (EI). We then dissected each fin to determine how flexural stiffness correlates to skeletal anatomy. We found that coastal-pelagic species such as thresher (Alopias vulpinus) and silky (Carcharhinus falciformis) sharks had the highest flexural stiffness and radials that extended the furthest distally, whereas benthic-coastal species of sharks such as the bonnethead (Sphyrna tiburo) and Atlantic sharpnose (Rhizoprionodon terraenovae) had more flexible fins with radials that do not extend as far distally. Our data describe a range of pectoral fin anatomy from aplesodic to plesodic and support our hypothesis that the skeletal anatomy and flexural stiffness of shark pectoral fins correlate to ecology. Based on our data, we further hypothesize that pectoral fin function will also be highly variable and correlate with fin anatomy among species.
