Slow muscle function of Pacific bonito (Sarda chiliensis) during steady swimming

The Pacific bonito, Sarda chiliensis, is anatomically intermediate between mackerel and tuna. The specialisations exhibited by tuna are present in the bonito, but to a lesser degree. Slow-twitch muscle strain and activity pat terns were determined during steady swimming (tailbeat frequency 1.2-3.2 Hz ) at four locations on the body of Sarda chiliensis using sonomicrometry an d electromyography, Both strain and the phase of electromygraphic activity were independent of tailbeat frequency. The strain of superficial slow-twit ch muscle increased from +/-3.1% l(0) at 0.35FL to +/-5.8 % l(0) at 0.65FL, where lo is muscle resting length and FL is the body length from snout to tail fork. Between 0.35 and 0.65FL, there was a negative phase shift of 16 degrees in the onset of electromygraphic activity in superficial slow-twitc h muscle relative to the strain cycle. Muscle activity patterns are compara ble with those of tuna. At 0.58FL, the onset of activity in deep slow-twitc h muscle was approximately synchronous with the onset of activity in superf icial muscle in the same myotome at 0.65FL. The distribution of slow-twitch muscle along the body of Sarda chiliensis and four additional fish species , Anguilla anguilla, Oncorhynchus mykiss, Scomber scombrus and Thunnus alba cares, was also measured. Slow-twitch muscle appears to become more concent rated at approximately 0.5FL as swimming kinematics become more thunniform.