Scalloped hammerhead shark swimming performance and thermoregulation strategies during deep dives into cold water: evidence of a marine mammal-like dive reflex?

Adult scalloped hammerhead sharks (Sphryna lewini) utilize oceanic habitats around the Hawaiian islands where they dive repeatedly at night to depths exceeding 800m and water temperatures as low as 4°C, presumably to forage on deep-dwelling prey. We hypothesized that S. lewini dive duration is limited by ambient water temperature at depth because body cooling associated with excursions into cold water can reduce muscle power output, cardiac function and visual acuity. To determine how S. lewini respond to cold ambient water temperatures experienced during deep dives, we equipped adult individuals with instrument packages capable of directly measuring depth, ambient water temperature, activity rates and swimming muscle temperature. Our specific objectives were to determine whether: (1) S. lewini maintain core body temperature during deep dives via simple thermal inertia, or instead employ active, physiological thermoregulation, and (2) whether swimming performance changes during deep, repetitive dives into cold water. We obtained 180 total days of data from 9 free-swimming adult S. lewini with individual deployment durations ranging from 7 to 23 days. Our data show ‘warm’ core muscle temperatures are maintained throughout the deepest portion of each dive and core muscle cooling only occurs during ascent to the surface. After sharks return to the warm surface layer, it takes 45 to 75 minutes for swimming muscles to fully equilibrate with ambient mixed layer water temperatures, but the shark will begin the next dive while the core temperature is still rising. The delayed onset and rapid rate of cooling followed by the slower rate of rewarming indicate evidence of physiological thermoregulation akin to a marine mammal-like dive reflex. Understanding how S. lewini, a warm-water species, are physiologically able to exploit resources in deep, cold habitats provides important insights into the broader ecology of this regionally endangered shark.