On the Muscle Fibres and Locomotor Activity of Doliolids (Tunicata: Thaliacea) Q. Bone

Abstract

Doliolids are pelagic tunicates which swim by very rapid contractions of the muscle bands that encircle their barrel-shaped bodies; the resulting jet pulses drive them forwards at instantaneous velocities up to 50 body lengths s-1(Bone & Trueman, 1984). The obliquely-striated muscle fibres within the bands have the unique feature that they lack any kind of sarcoplasmic reticulum or sub-sarcolemmal vesicle system (Bone & Ryan, 1974). Most stages of the rather complex life-cycle of doliolids (Braconnot, 1971a) are small animals, less than 4 mm long; but in one of the species examined the largest may be up to 40 mm. This paper shows that external Ca2+is required for contraction of the locomotor muscle fibres and that the decremental muscle potentials preceding contractions are carried by Ca2+. In younger stages, the muscle fibres within a band are electrically coupled to some extent, but in older animals, the degree of coupling decreases