Tunas, billfishes, and some sharks, such as white sharks and shortfin mako sharks, have an ability known as regional endothermy, which allows them to maintain body temperatures higher than the ambient water. This ability has been regarded as an adaptation to cold environments. However, its role in warm-water species such as shortfin mako sharks, bigeye tuna, and swordfish has been unclear.
In this study, we attached data loggers to shortfin mako sharks caught off southeastern Taiwan to record water temperature, body temperature, and swimming depth (Fig. 1). During repeated deep dives, the body temperatures slowly decreased in deep cold waters, and rapidly increased when they returned to warm surface waters. Their warming rates were over 10 times higher than their cooling rates, only comparable to bigeye tuna and swordfish among fish species studied to date. Since deep waters contain abundant food resources, regional endothermy likely helps them stay longer in prey-rich deep waters while minimizing recovery time at the surface.
One shark showed unique thermoregulation before a deep dive. After quickly warming at the surface, it stayed there even longer, raising its body temperature above the ambient water before starting a deep dive. This suggests intentional "pre-dive warming" in preparation for deep cold waters. To our knowledge, no other fish species has been reported to show similar thermoregulation.
Shortfin mako sharks showed enhanced thermoregulation abilities during repeated dives. For warm-water species like shortfin mako sharks, regional endothermy allows not only heat retention but also flexible body temperature control. Our findings help explain the success of regionally endothermic fishes as apex predators across the world's pelagic oceans.
