Thermoregulation Strategies of Deep Diving Ectothermic Sharks
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2020
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Abstract
Temperature is one of the most important factors affecting the distribution, behavior, and physiological performance of animals. The rate of heat exchange between the body and surrounding environment determines the capacity for an individual to exploit various habitats. Some marine ectotherms move between drastically different thermal environments; probably to gain access to foraging opportunities or to find ambient temperatures that are optimal for bioenergetic efficiency. Here I used a novel package of telemetry devices to elucidate the thermoregulatory strategies of two large-bodied ectothermic sharks that routinely move between widely different thermal environments. My results revealed unexpected behavioral and physiological strategies of two relatively understudied species.
The bluntnose sixgill shark (Hexanchus griseus) is a primitive large-bodied vertically migrating shark that occupies deep waters of between 4-6˚C during the day and migrates to shallower depths with temperatures around 15˚C at night. In the first study to measure the body temperature of a deep-water shark in situ, I demonstrate that the large body mass of sixgill sharks provides sufficient thermal inertia to buffer against prolonged exposure to both low or high water temperatures and maintain what are assumed to be optimal body temperatures.
Scalloped hammerhead sharks (Sphyrna lewini) occupy the warm waters of the surface mixed layer during the day but exhibit an extraordinary ability to make repeated nighttime dives to depths exceeding 850m where water temperatures are as low as 4°C Presumably they are foraging on deep-sea squid. I show that scalloped hammerhead sharks maintain ‘warm’ core muscle temperatures throughout the deepest portion of each dive and that core muscle cooling only occurs during ascent to the surface. However, once initiated, this cooling is rapid, suggesting active physiological thermoregulation akin to a marine mammal-like dive strategy (i.e. breath holding). During deep dives, S. lewini exhibit intensive swimming activity which, coupled with reduced respiration, suggests a high capacity for anaerobic metabolism in the white “burst swimming” muscle of this species. Analyses of white muscle poise suggests scalloped hammerhead sharks possess enzyme characteristics that facilitate anaerobic metabolism during deep dives and the necessary aerobic metabolism to allow for rapid recovery (through the breakdown of anaerobic end products) during inter-dive intervals.
These two species use highly contrasting approaches to achieve the same goal of broadening their thermal niche and presumably enabling them to exploit resources that would otherwise be out of reach.
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Physiology, Ecology, Zoology, Biologging, Ectothermic, Hexanchus griseus, Shark, Syphrna lewini, Thermoregulation
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129 pages
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