Abstract
There are approximately 50 muscles that control tail fin shape in most teleost fishes, and although myotomal muscle function has been extensively studied, little work has been done on the intrinsic musculature that controls and shapes the tail. In this study we measured electrical activity in intrinsic tail musculature to determine if these muscles are active during steady rectilinear locomotion, and to compare intrinsic muscle recruitment patterns to previous data on myotomal muscle fibers. Five bluegill sunfish (Lepomis macrochirus) were anaesthetized and electrode wires surgically placed into a total of 24 intrinsic caudal muscles, up to 13 at a time, and activity was correlated with synchronous recordings from myotomal fibers in the caudal peduncle. After recovery, fish swam steadily at speeds of 0.5, 1.2 and 2.0 L s(-1), while filmed from lateral, posterior and ventral views simultaneously at 250 frames s(-1). Comparison among speeds confirmed that muscle recruitment varies significantly with speed. At 0.5 L s(-1), the caudal fin was generally not used for propulsion, and swimming was accomplished primarily through body undulations. Intrinsic caudal muscle activity at this speed was intermittent and variable. At 1.2 and 2.0 L s(-1), the supracarinalis and infracarinalis muscles acted on the dorsal- and ventral-most fin rays, respectively, to expand the surface area of the caudal fin. The interradialis muscles adducted individual fin rays, dorsally to ventrally, following activation of the hypochordal longitudinalis. Contralateral muscle activity of interradialis muscles occurred as the caudal fin crossed the mean direction of travel and fin height was greatest, whereas ipsilateral activity of carinalis muscles occurred near points of maximum excursion of the fin, at speeds of 1.2 and 2.0 L s(-1), after fin height was lowest. Burst intensity increased with swimming speed, suggesting stiffening of the tail fin against imposed hydrodynamic loads. Activity patterns of intrinsic caudal muscles suggest that these most posterior muscles in fishes, located within the tail, are among the very first recruited as swimming speed increases, and that slow undulatory swimming is powered by muscle fibers located posteriorly in the caudal peduncle and tail.
Highlights
The caudal fin of fishes, with its large surface area, is of major importance for propulsion, and general patterns of fin motion have often been studied to quantify the frequency and amplitude of tail beats during locomotion (Bainbridge, 1963; Grove and Newell, 1936; Lauder, 1989; Nursall, 1958; Videler, 1975; Webb and Smith, 1980)
Anatomical studies of caudal fin structure (Lauder, 1982; Lauder, 1989; Liem, 1970; Nag, 1967; Videler, 1975; Winterbottom, 1974) have shown that approximately 50 discrete muscles are present within the caudal fin, and that these muscles could potentially control tail fin shape during swimming and generate propulsive waves on the caudal fin itself independently of the myotomal muscle fibers that generate body bending anterior to the tail
Muscular activity patterns that could control the shape and orientation of the caudal fin in teleost fishes have yet to be described [ some preliminary data have been presented by Lauder (Lauder, 1989) and Videler (Videler, 1975)], and there are effectively no data on recruitment patterns for intrinsic tail musculature in fishes
Summary
The caudal fin of fishes, with its large surface area, is of major importance for propulsion, and general patterns of fin motion have often been studied to quantify the frequency and amplitude of tail beats during locomotion (Bainbridge, 1963; Grove and Newell, 1936; Lauder, 1989; Nursall, 1958; Videler, 1975; Webb and Smith, 1980). Anatomical studies of caudal fin structure (Lauder, 1982; Lauder, 1989; Liem, 1970; Nag, 1967; Videler, 1975; Winterbottom, 1974) have shown that approximately 50 discrete muscles are present within the caudal fin, and that these muscles could potentially control tail fin shape during swimming and generate propulsive waves on the caudal fin itself independently of the myotomal muscle fibers that generate body bending anterior to the tail. Muscular activity patterns that could control the shape and orientation of the caudal fin in teleost fishes have yet to be described [ some preliminary data have been presented by Lauder (Lauder, 1989) and Videler (Videler, 1975)], and there are effectively no data on recruitment patterns for intrinsic tail musculature in fishes
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