Pairwise modulation of jaw muscle activity in two species of elasmobranchs

Most studies of the function of feeding muscles in fish have implanted electromyogram electrodes unilaterally to understand the motor pattern associated with a behavior. The few studies that have implanted bilaterally have found that paired muscles may be activated asynchronously, often resulting in visible kinematic asymmetry. We investigated modulation of pairwise asynchrony (modulation in the activation patterns of left and right members of a muscle) of feeding muscles during capture and processing of two types of prey in spiny dogfish Squalus acanthias and little skates Leucoraja erinacea (Elasmobranchii). Two asynchrony indices quantified the degree to which muscles in a pair were activated out of phase (lag index, AI_lag) and the degree to which durations differ (duration index, AI_dur). Feeding behaviors for both species were compared according to these indices and total event duration using principal components analysis. Both species modulated pairwise asynchrony according to prey type, exhibiting more asynchronous motor patterns when feeding on more complex prey items (those requiring more processing); however, the motor patterns underlying this asynchrony differed between species. Dogfish process complex prey using head-shaking, which requires alternating activation of contralateral head muscles (i.e. high lag index value). In contrast, little skates process complex prey using a completely unilateral behavior in which prey is moved to one corner of the jaws and jaw muscles are activated on that side only (i.e. high duration index value). Asynchronous behaviors are not detected by unilateral implants; our data demonstrate the importance of bilateral implantation to identify fine details of feeding motor patterns, particularly those involving complex prey. © 2010 The Authors. Journal compilation © 2010 The Zoological Society of London.