Diductor muscles of brachiopods: activation and very slow contraction

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Abstract
The diductor muscles of brachiopods run from the ventral valve to an insertion posterior to the hinge on the dorsal valve: thus they serve to open the shell. Spontaneous electrical activity is recorded from the muscles when they are contracted, i.e., the shell is open. A burst of impulses is recorded shortly after the antagonist smooth adductor muscles begin to contract to close the shell. A second burst may occur at the time the valves begin to reopen.The diductors contract spontaneously and open the shell in 1–5 min. The rate of isotonic contraction is in marked contrast with the very low rate of isometric development of tension which may require as long as 186 min. If the contracted diductors are stretched the tension holding ability abruptly drops to near zero, then begins to redevelop immediately at a low rate. This sudden compliance is termed 'slip.' The burst of impulses recorded during shell closure is seen to follow slip and may serve to reactivate the diductor muscles. Slip is induced by stretch and by vibration and also occurs spontaneously from high levels of tension. It is suggested that the phenomenon of slip represents a sudden breakage of all cross bridges in this slowly contracting muscle.The diductor muscles do not appear to possess a 'catch' mechanism as found in molluscan muscles, but rather appear to exist in a tonically contracted active state.

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