Abstract
This paper describes the structure and operation of the fibres in the locomotor muscle bands of several salp species. In many species, for example Thalia democratica or Pegea confoederata , all the muscle fibres of the locomotor muscle bands are similar in width and structure. In others, for example Salpa fusiformis and S. maxima, although fibre structure is similar, the marginal fibres edging the bands may be some 3-4 times the width of those in the centre of the band. In Ihlea punctata , not only is there a more striking difference in width between the marginal and central fibres of the bands, but also the two differ in structure. The marginal fibres are up to 10 times the width of the central fibres and the two differ in myofibrillar and mitochondrial content. Intracellular recordings from the fibres show that the normally compound spike potentials do not overshoot resting potentials (up to -70 mV), and are decremental. The two types of fibre may be separately activated. It is suggested that in Ihlea punctata , the wide marginal fibres may be involved in slow swimming, the central narrow fibres in `escape´ swimming.
Highlights
Salps swim by rhythmic contractions of the muscle bands which partially or completely encircle their tubular bodies; contraction of these muscle bands sends a jet of water out of the posterior or anterior openings and propels the salp forwards or backwards
Our observations suggest the possibility that in I. punctata the two different types of muscle fibre may be involved in different types of swimming
Van Beneden and De Selys-Longchamps (1913) and Foxton (1967) observed special wide marginal fibres in Ihlea racovitsai and I. magalhanica, as did Streiff (1908) but in his taxonomic study of a large number of salp species, Metcalf did not describe a similar peculiarity in the muscle bands of other genera, nor was it noted by other authors, such as Fedele (1938) and Van Soest (1974, 1975)
Summary
Salps swim by rhythmic contractions of the muscle bands which partially or (in some species) completely encircle their tubular bodies; contraction of these muscle bands sends a jet of water out of the posterior or anterior openings and propels the salp forwards or backwards. Structural details of the fibres in several species were described at the light microscope level by Fedele (1932, 1938), whilst more recently their ultrastructure and development were examined by Bone and Ryan (1973) and Toselli and Harbison (1977)
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