Dorsal fin function in spiny dogfish during steady swimming

Dorsal fins of actinopterygian fishes are known to function to varying degrees as stabilizers and as propulsive elements that augment thrust from the caudal fin. However, little is known about the ability of elasmobranchs to control three-dimensional conformation of the dorsal fins during swimming, which may alter the force balance during locomotion. In this study, dorsal fin function was investigated in spiny dogfish, Squalus acanthias, swimming steadily at 0.5 and 0.75BLs(-1), using three-dimensional kinematics and electromyography. Points on the dorsal and caudal fins were tracked in dorsal and lateral views with dual high-speed video at 125fs(-1). Electrodes were implanted in three points along each dorsal fin muscle and in the adjacent red epaxial muscle. Conformational changes were detected in both dorsal fins at both speeds. Speed was found to influence lateral displacement of the first dorsal fin relative to trunk undulation, with larger magnitudes at 0.5BLs(-1). The first dorsal fin oscillates at a different phase lag than predicted by position on the body, while the second dorsal fin moves in synchrony with the axial musculature. Muscles of the first dorsal fin show synchronous bilateral activation, while there is no clear pattern in the second dorsal fin. This study provides evidence that spiny dogfish control movements of the first dorsal fin during steady swimming to stabilize body position. In contrast, the second dorsal fin appears to be capable of thrust generation. Thus, there is a dual dorsal fin function in spiny dogfish during steady swimming.