FUNCTION OF THE DORSAL FIN IN BLUEGILL SUNFISH - MOTOR PATTERNS DURING 4 DISTINCT LOCOMOTOR BEHAVIORS

The median fins of fishes are key features of locomotor morphology whi ch function as complex control surfaces during a variety of behaviors. However, very few studies have experimentally assessed median fin fun ction, as most workers focus on axial structures. In particular, the d orsal fin of many teleost fishes possesses both spiny anterior and sof t posterior portions which may function separately during locomotion. We analyzed the function of the soft region of the dorsal fin and of t he dorsal inclinator (Di) muscles which are the primary muscles respon sible for lateral flexion. We used electromyography to measure in vivo Di activity, as well as activity of the red myomeric muscles located at a similar longitudinal position. We quantified motor patterns durin g four locomotor behaviors: braking and three propulsive behaviors (st eady swimming, kick and glide swimming, and C-starts). During the thre e propulsive swimming behaviors, the timing of Di activity was more si milar to that of ipsilateral red myomeric muscle rather than to contra lateral myomeric activity, whereas during braking the timing of activi ty of the Di muscles was similar to that of the contralateral myomeric musculature. During the three propulsive behaviors, when the Di muscl es had activity, it was consistent with the function of stiffening the soft dorsal fin to oppose its tendency to bend as a result of the bod y being swept laterally through the water. In contrast, activity of th e Di muscles during braking was consistent with the function of active ly flexing the soft dorsal fin towards the side of the fish that had D i activity. Activity of the Di muscles during steady speed swimming wa s generally sufficient to resist lateral bending of the soft dorsal fi n, whereas during high speed kick and glide swimming and C-starts, Di activity was not sufficient to resist the bending caused by resistive forces imposed by the water. Cumulative data from all four behaviors s uggest that the Di muscles can be activated independently relative to the myomeric musculature rather than having a single phase relationshi p with the myomeric muscle common to all of the observed behaviors. (C ) 1996 Wiley-Liss, Inc.