Spatial variation in epaxial muscle activity during prey strike in largemouth bass (Micropterus salmoides)

Epaxial muscle activity during prey strike was measured in two discrete myomeric regions of the largemouth bass Micropterus salmoides to test whether (1) the extreme dorsal region of the epaxial muscle (the epaxial arm region) plays a larger role in prey strike than the region bordering the main horizontal septum (the epaxial cone region), (2) whether the epaxial arm region is more active anteriorly than posteriorly during prey strike and (3) whether the epaxial arm region activity is correlated with the epaxial cone region activity. Electromyographic recordings (EMGs) of four bass were taken from eight different longitudinal, epaxial muscle sites: five sites in the arm region and three sites in the cone region. Selection of electrode sites was based on epaxial muscle dissections in which a set of previously undefined tendons, neurocranial-epineural-epaxial connector tendons, were described. Every strike had some activity in the arm region, while 48 % of the total number of prey strikes had zero cone activity. Muscle activity was recorded consistently from the first four anterior electrodes in the arm region of each fish, while the posteriormost arm electrodes showed varying degrees of activity. Muscle intensity recorded from the anterior three epaxial arm electrodes was consistently higher than from the two posterior epaxial arm electrodes, while the onset times and durations of EMGs were variable. Most notably, the arm region of the epaxial muscle is capable of being active without the adjoining cone region, thus demonstrating that activity in the epaxial muscle mass can be spatially regionalized in a manner dependent on behavior.