113.4 Saturday, Jan. 7 Hydrodynamic Function of Dorsal Fins in Sharks MAIA, A.*; LAUDER, G.V.; WILGA, C.D.; University of Rhode Island; Harvard University; University of Rhode Island firstname.lastname@example.org
Kinematic and electromyographic data from dorsal fins in sharks indicate that these structures are actively controlled during steady swimming. Here, we investigate how the two dorsal fins interact with the surrounding flow in spiny dogfish and bamboo sharks during steady swimming by means of particle image velocimetry (PIV). A horizontal laser sheet intersecting the dorsal fins was filmed from above with a high-speed camera (500 fps). Cross correlation analysis of consecutive images was used to calculate stroke averaged velocity and vorticity variables. Results reveal the presence of vorticity in the wake of all fins. In the wake of the first dorsal fin of spiny dogfish the flow is decelerated with strong lateral velocities in the opposite direction to the movement. The first dorsal fin is thus not contributing thrust but rather is stabilizing the shark. Accelerated flow was observed in the wake of both dorsal fins in bamboo sharks with some lateral losses. In addition in bamboo sharks, the fluid flow in the wake of the second dorsal fin had higher longitudinal velocities compared to the wake of the first dorsal fin. This may result from vortex interaction between the first and second dorsal fins through an additive effect. Fluid flow patterns in bamboo sharks indicate that both dorsal fins are adding m to the swimming shark. These results are consistent with previous studies on fin kinematics and muscle function that show different functions in shark dorsal fins depending on fin and species. Shark dorsal fins are capable of interacting with the flow and producing thrust in a manner similar to that of bony fishes, despite a relatively simpler musculoskeletal arrangement. In addition, this study refutes the long standing idea that dorsal fins in sharks are only used as stabilizers.