Heterogeneity of myoglobin distribution in the locomotory muscles of five cetacean species

Myoglobin is an important storage site for oxygen in the swimming muscles o f diving marine mammals, However, little is known about its distribution wi thin muscles since previous studies have relied on single samples. The goal of this study was to determine the distribution of myoglobin within the sw imming muscles of five species of cetacean: dusky dolphin, false killer wha le, striped dolphin, humpbacked dolphin and bottlenose dolphin. The entire dorsal (epaxial) and ventral (hypaxial) swimming muscles were removed from each animal and weighed. Transverse sections were taken from the cranial, m iddle and caudal regions of each muscle and sampled along a circular grid w ith a minimum of 30 sites per section. Spectrophotometric analysis was used to measure the myoglobin concentration of each sample. Contour maps of myo globin concentration were made for each transverse section, Myoglobin conce ntration was found to be non-uniformly distributed within the muscle. The i nterior of the muscle lying closest to the vertebrae showed a significantly higher (11 %) mean myoglobin concentration than the exterior of the muscle for all five species. In the epaxial muscles, the mean myoglobin concentra tion was significantly higher in the caudal region closest to the flukes. T he two deep-water species (false killer whale and striped dolphin) had sign ificantly higher myoglobin concentrations than the three species (dusky, hu mpbacked and bottlenose dolphins) that occur in shallow, coastal waters. Th ese results show that myoglobin is not homogeneously distributed in the loc omotory muscle of cetaceans and that levels may be highest in those areas t hat produce greater force and consume more oxygen during aerobic swimming. Enhancing oxygen stores in those areas of the muscle that work the hardest would theoretically lengthen the aerobic dive limit of the animal during su bmerged swimming.