Force production during escape responses: Sequential recruitment of the phasic and tonic portions of the adductor muscle in juvenile sea scallop, Placopecten magellanicus (Gmelin)

Giant scallops, Placopecten magellanicus, respond to the presence of starfish predators with an escape response consisting of a series of rapid valve adductions that allow the scallop to jump or swim away from the predator. To evaluate the coordination of the activity of the tonic and phasic muscles during such escape responses, we recorded their force production by attaching a force gauge to the shell of intact scallops and then stimulating the scallops with starfish. These recordings showed series of phasic contractions (claps) separated by prolonged tonic contractions. Numerous characteristics could be quantified from these recordinos including the maximal force, mean force during the first minute, force, frequency and number of claps per series, as well as the force and duration of tonic contractions. The number of claps per series declined and the duration of the tonic contractions increased as the escape response continued. For most scallops, phasic and tonic contractions produced similar levels of force that changed little during the escape responses. The alternation between phasic and tonic contractions suggests that periods of tonic contraction allow the phasic muscle to recuperate and facilitate subsequent phasic contractions. Principal component analysis (PCA) confirmed the coordination between the phasic and tonic adductor muscles, because characteristics of each type of contraction, were closely associated. This method combines the advantages of stimulation of scallops by their predators with the simplicity of force gauge measurements. Force production during escape responses by individual scallops was highly reproducible, suggesting these measurements have considerable potential for tracking changes in the physiologic status of giant scallops.