Abstract
Evolutionary patterns of intrinsic caudal musculature in ray-finned fishes show that fine control of the dorsal lobe of the tail evolved first, followed by the ability to control the ventral lobe. This progression of increasing differentiation of musculature suggests specialization of caudal muscle roles. Fine control of fin elements is probably responsible for the range of fin conformations observed during different maneuvering behaviors. Here, we examine the kinematics of the caudal fin and the motor activity of the intrinsic caudal musculature during kick-and-glide, braking and backing maneuvers, and compare these data with our previous work on the function of the caudal fin during steady swimming. Kick-and-glide maneuvers consisted of large-amplitude, rapid lateral excursion of the tail fin, followed by forward movement of the fish with the caudal fin rays adducted to reduce surface area and with the tail held in line with the body. Just before the kick, the flexors dorsalis and ventralis, hypochordal longitudinalis, infracarinalis and supracarinalis showed strong activity. During braking, the dorsal and ventral lobes of the tail moved in opposite directions, forming an ;S'-shape, accompanied by strong activity in the interradialis muscles. During backing up, the ventral lobe initiated a dorsally directed wave along the distal edge of the caudal fin. The relative timing of the intrinsic caudal muscles varied between maneuvers, and their activation was independent of the activity of the red muscle of the axial myomeres in the caudal region. There was no coupling of muscle activity duration and electromyographic burst intensity in the intrinsic caudal muscles during maneuvers, as was observed in previous work on steady swimming. Principal-component analysis produced four components that cumulatively explained 73.6% of the variance and segregated kick-and-glide, braking and backing maneuvers from each other and from steady swimming. The activity patterns of the intrinsic caudal muscles during maneuvering suggest motor control independent from myotomal musculature, and specialization of individual muscles for specific kinematic roles.
Highlights
Electromyographic protocol The surgical insertion methods for electrodes used in these experiments were the same as we presented in Flammang and Lauder (Flammang and Lauder, 2008) for the study of steady swimming in the same bluegill sunfish
Bluegill sunfish are able to modulate the shape of their tail fins into a variety of configurations different than that exhibited during steady swimming (1.2 L s–1) (Fig. 2A), depending on the behavior being performed
Steady swimming at 1.2 L s–1 resulted in a lateral excursion from the fish median axis of approximately 1 cm in both the right and left directions (Flammang and Lauder, 2008)
Summary
Teleost fishes are defined as a monophyletic group by their characteristic caudal skeleton (de Pinna, 1996; Gosline, 1997; Lauder, 1989; Lauder and Liem, 1983; Rosen, 1982). The interradialis muscles in bowfin insert only onto the seven dorsal-most, but not ventral, fin rays Teleost fishes, such as the bluegill sunfish, are the first fishes to have a flexor dorsalis muscle in the tail and infracarinalis muscles and interradialis muscles between the ventral fin rays (Fig. 1). This phylogenetic pattern shows that a key transition in the evolution of caudal fin structures in ray-finned fishes involves the addition of first more dorsal and ventral control elements to caudal fin rays. The end result, accomplished with the addition of the infracarinalis, interradialis and supracarinalis muscles in teleost fishes, is hypothesized to be a more maneuverable tail fin that can be formed into different shapes, rather than acting as a rigid propulsive foil (Lauder, 1982; Lauder, 1989)
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