Function of the dorsal fin in bluegill sunfish: Motor patterns during four distinct locomotor behaviors

The median fins of fishes are key features of locomotor morphol- ogy which function as complex control surfaces during a variety of behaviors. However, very few studies have experimentally assessed median fin function, as most workers focus on axial structures. In particular, the dorsal fin of many teleost fishes possesses both spiny anterior and soft posterior portions which may function separately during locomotion. We analyzed the function of the soft region of the dorsal fin and of the dorsal inclinator (Di) muscles which are the primary muscles responsible 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 during four locomotor behaviors: braking and three propulsive behaviors (steady swimming, kick and glide swimming, and C-starts). During the three propul- sive swimming behaviors, the timing of Di activity was more similar to that of ipsilateral red myomeric muscle rather than to contralateral myomeric activ- ity, whereas during braking the timing of activity of the Di muscles was similar to that of the contralateral myomeric musculature. During the three propul- sive behaviors, when the Di muscles 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 body being swept laterally through the water. In contrast, activity of the Di muscles during braking was consistent with the function of actively flexing the soft dorsal fin towards the side of the fish that had Di activity. Activity of the Di muscles during steady speed swimming was generally sufficient to resist lateral bending of the soft dorsal fin, 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 suggest that the Di muscles can be activated independently relative to the myomeric musculature rather than having a single phase relationship with the myomeric muscle common to all of the observed behaviors. o 1996 Wiley-Liss, Inc. The median fins of most fishes are large, conspicuous structures whose evolutionary origin within the chordate clade probably preceded that of appendicular structures, and yet we presently know remarkably little about how the median fins function during fish locomotion. This is unfortunate because hy- drodynamic modeling suggests that interspe- cific variation in median fin morphology is related to differences in the longitudinal dis- tribution of surface area that tend to corre- late with different locomotor tasks such as fast steady swimming, rapid acceleration, and maneuvering (Webb, '84). Our current knowl- edge of the median fins of teleosts is derived mainly from a small number of studies which are mostly structural and evolutionary in nature, such as Winterbottom's ('74) com- parative descriptions of the median fin muscles. With the exceptions of the papers by Arita ('71) and Roberts ('69), the overview of Harris ('53), and the work of Geerlink and Videler ('87), who focused on the relation between fin ray structure and its mechanical