Unsuspected strategy of hammerhead sharks to stay warm in cold waters

Sharks, like most fish, are fully ectothermic and their body temperatures are largely regulated by their immediate environment.

This can pose a physiological challenge for large predatory fish, which must maintain certain body temperatures to maintain their functions, but venture into cooler, deeper waters to find their prey.

For example, the common hammerhead (Sphyrna lewini), which occupies warm surface waters in temperate and tropical coastal regions, occasionally dives to depths exceeding 800 meters, where water temperatures can be as low as 4 degrees Celsius. , to hunt a prey.

Given their lack of morphological and vascular adaptation to actively conserve body heat, how these sharks can maintain the proper temperature during deep and icy dives has long been an enigma.

To explore this mystery, Mark Royer of the University of Hawaii at Manoa and colleagues developed modern remote biorecorders, implanted internally in adult sharks, that measured ambient and deep water temperatures, activity indices, body movements, and internal body temperature.

The study authors found that the sharks maintained elevated body temperatures (up to 20 degrees Celsius above ambient temperature) during the deepest leg of each dive and began to rapidly lose heat only upon returning to the surface.

Hammerhead sharks off the coast of the Big Island of Hawaii. (Photo: Cory Fults)

Royer and his colleagues have concluded that sharks effectively maintain their body heat by “holding their breath” while submerged.

By tightly closing the mouth or gill openings to reduce the flow of cold water through the gills, sharks can minimize the loss of body heat if they “breathe” cold water. The rapid loss of body heat during their ascent to warmer waters reflects the reopening of the gill openings and convective heat transfer. The authors note that further research is needed to confirm this thermoregulation strategy.

The study is titled “Breath holding” as a thermoregulation strategy in the scalloped hammerhead”. And it has been published in the academic journal Science. (Source: AAAS)