Median fin function during the escape response of bluegill sunfish (Lepomis macrochirus). I: Fin-ray orientation and movement.

The fast-start escape response is critically important to avoid predation, and axial movements driving it have been studied intensively. Large median dorsal and anal fins located near the tail have been hypothesized to increase acceleration away from the threat, yet the contribution of flexible median fins remains undescribed. To investigate the role of median fins, C-start escape responses of bluegill sunfish ( Lepomis macrochirus ) were recorded by three high-speed, high-resolution cameras at 500 frames s−1 and the 3-D kinematics of individual dorsal and anal fin rays were analyzed. Movement and orientation of the fin rays relative to the body axis were calculated throughout the duration of the C-start. We found that: (1) timing and magnitude of angular displacement varied among fin rays based on position within the fin and (2) kinematic patterns support the prediction that fin rays are actively resisting hydrodynamic forces and transmitting momentum into the water. We suggest that regions within the fins have different roles. Anterior regions of the fins are rapidly elevated to increase the volume of water that the fish may interact with and transmit force into, thus generating greater total momentum. The movement pattern of all the fin rays creates traveling waves that move posteriorly along the length of the fin, moving water as they do so. Flexible posterior regions ultimately act to accelerate this water towards the tail, potentially interacting with vortices generated by the caudal fin during the C-start. Despite their simple appearance, median fins are highly complex and versatile control surfaces that modulate locomotor performance. * a : acceleration of the ssCOM parallel to the fish trajectory A 〈 x 〉 : area, where x represents the fin surface between a named fin-ray and the next posterior digitized fin ray, or of the entire named fin/region Ant : anterior trunk ARy# : anal rays, where # indicates its numbered position within the fin ASp# : anal spines, where # indicates its numbered position within the fin C : chord axis of the fin surface cT : tangent to the chordwise curve D : displacement of the center of mass DRy# : dorsal rays, where # indicates its numbered position within the fin DSp# : dorsal spines, where # indicates its numbered position within the fin Fr : frontal axis, a.k.a. normal to the frontal plane L : lateral axis, a.k.a. normal to the fin surface Mid : middle trunk MSE : mean square error Op : operculum Post : posterior trunk r : fin-ray identifier Rs : rostrum S : span axis of the fin surface S1 : stage 1 of the C-start S2 : stage 2 of the C-start Seg : body segment identifier sfA : soft region of the anal fin sfD : soft dorsal fin Sg : sagittal axis, a.k.a. normal to the sagittal plane spD : spiny dorsal fin ssCOM : stretched-straight center of mass sT : tangent to the spanwise curve t : time point during a C-start sequence T : time zero TL : total length tr directional transition event, i.e. change in direction of rotation or orientation Tr : transverse axis, a.k.a. normal to the transverse plane t X : time of a given parameter, where X is the event of a given parameter v : velocity of the ssCOM parallel to the fish trajectory ![Graphic][1] : average span axis angle of a fin ray Δ t X : time difference between a given fin-ray parameter and its corresponding axial event, where X is the event of a given parameter ![Graphic][2] : average elevation of a fin ray θ′ : turning rate, i.e. the first time derivative of yaw ![Graphic][3] : average sweep angle of a fin ray [1]: /embed/inline-graphic-168.gif [2]: /embed/inline-graphic-169.gif [3]: /embed/inline-graphic-170.gif