title: Slow muscle function of Pacific bonito (Sarda chiliensis) during steady swimming.
abstract: 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 patterns were determined during steady swimming (tailbeat frequency 1.2-3.2 Hz) at four locations on the body of Sarda chiliensis using sonomicrometry and electromyography. Both strain and the phase of electromygraphic activity were independent of tailbeat frequency. The strain of superficial slow-twitch muscle increased from +/-3.1 % l(0) at 0.35FL to +/-5.8 % l(0) at 0.65FL, where l(0) 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-twitch muscle relative to the strain cycle. Muscle activity patterns are comparable with those of tuna. At 0.58FL, the onset of activity in deep slow-twitch muscle was approximately synchronous with the onset of activity in superficial 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 albacares, was also measured. Slow-twitch muscle appears to become more concentrated at approximately 0.5FL as swimming kinematics become more thunniform.
authors.affiliation: School of Biology, The University of Leeds, UK. bgydje@leeds.ac.uk
authors.forename: D J
authors.initials: DJ
authors.lastname: Ellerby
authors.forename: J D
authors.initials: JD
authors.lastname: Altringham
authors.forename: T
authors.initials: T
authors.lastname: Williams
authors.forename: B A
authors.initials: BA
authors.lastname: Block
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D018657:Muscle Fibers, Slow-Twitch / Q000502:physiology; D018482:Muscle, Skeletal / Q000033:anatomy & histology / Q000502:physiology*; D010473:Perciformes / Q000033:anatomy & histology / Q000502:physiology*; D013550:Swimming / Q000502:physiology*