Locomotor muscle morphology of three species of pelagic delphinids.

The locomotor muscle morphology of diving mammals yields insights into how they utilize their environment and partition resources. This study examined a primary locomotor muscle, the longissimus, in three closely related, similarly sized pelagic delphinids (n = 7-9 adults of each species) that exhibit different habitat and depth preferences. The Atlantic spotted dolphin (Stenella frontalis) is a relatively shallow diver, inhabiting continental shelf waters; the striped (Stenella coeruleoalba) and short-beaked common (Delphinus delphis) dolphins are sympatric, deep-water species that dive to different depths. Based upon comparative data from other divers, it was hypothesized that the locomotor muscle of the deepest-diving S. coeruleoalba would exhibit a higher percentage of slow oxidative fibers, larger fiber diameters, a higher myoglobin concentration [Mb], and a lower mitochondrial density than that of the shallow-diving S. frontalis, and that the muscle of D. delphis would display intermediate values for these features. As expected, the locomotor muscle of S. coeruleoalba exhibited a significantly higher proportion of slow (57.3 ± 3.9%), oxidative (51.7 ± 2.5%) fibers and higher [Mb] (8.2 ± 0.7 g/100 g muscle) than that of S. frontalis (41.3 ± 3.9%, 31.0 ± 3.2%, 4.7 ± 0.05 g/100 g muscle, respectively). There were no differences in fiber size or mitochondrial density among these species. Like other deep divers, S. coeruleoalba displayed locomotor muscle features that enhance oxygen storage capacity and metabolic efficiency but did not display features that limit aerobic capacity. These results suggest a previously undescribed muscle design for an active, small-bodied, deep-diving cetacean. HIGHLIGHTS: The locomotor muscle features displayed by the striped dolphin, which are unique among deep divers, enhance oxygen stores but do not limit aerobic capacity. This novel muscle design may facilitate the active lifestyle of this small-bodied deep diver.