Thermoregulation abilities of shortfin mako sharks off the coast of Taiwan revealed

A research group consisting of Graduate Student Soma Tokunaga from the Department of Evolutionary Studies of Biosystems at the Graduate University for Advanced Studies, SOKENDAI, Professor Yuuki Y. Watanabe from the Research Center for Integrative Evolutionary Science at SOKENDAI, Nagasaki University, the Okinawa Churashima Foundation, and the Fisheries Research Institute, Ministry of Agriculture, Taiwan announced on September 25 that they have revealed that body temperature warming proceeds more than 10 times faster than cooling in shortfin mako sharks by attaching data loggers measuring water temperature, body temperature, and depth to sharks captured offshore of southeastern Taiwan and releasing them (biologging). The findings showed that mako sharks, which prefer warm waters, can flexibly regulate their body temperature according to conditions. This exceptional thermoregulation ability is believed to enhance their foraging efficiency in cold deep waters where prey is abundant. The results were published in the Journal of Animal Ecology on August 29.

Most fish are ectothermic and their body temperature depends on ambient water temperature. However, tuna, marlin, and some shark species, including great white sharks and shortfin mako sharks, possess a trait called "regional endothermy" that maintains their core body temperature higher than the surrounding water temperature. Regional endothermy is thought to provide survival advantages such as expanding their habitat range into colder waters.

However, shortfin mako sharks and bigeye tuna prefer warm waters, and the benefits of regional endothermy for these species were not understood.

Most fish are ectothermic, and their body temperature depends on ambient water temperature. However, tuna, marlin, and some shark species, including great white sharks and shortfin mako sharks, possess a trait called "regional endothermy" that maintains their core body temperature higher than the surrounding water temperature. Regional endothermy is thought to provide survival advantages such as expanding their habitat range into colder waters.

However, shortfin mako sharks and bigeye tuna prefer warm waters, and the benefits of regional endothermy for these species were not understood.

In this study, the research group investigated mako sharks using biologging, which involves attaching data loggers to animals.

As a result, they found that when moving between cold deep waters and warm surface waters, the sharks' body temperatures declined slowly in deep waters and rose rapidly in surface waters. The sharks prevented heat loss to their surroundings in deep waters and conversely absorbed heat at the sea surface. The warming rate was more than 10 times faster than the cooling rate, comparable to bigeye tuna and far exceeding other fish species.

Since prey is abundant in deep waters, the sharks are thought to enhance their foraging efficiency through dives by slowing heat loss to remain longer in deep waters, then rapidly recovering body temperature at the sea surface before quickly diving deep again.

The recorded data illustrate that one individual exhibited unusual body temperature changes: after rapidly recovering its body temperature at the sea surface, it remained there to raise its temperature even further, then dove into deep waters when its body temperature exceeded surface water temperature. This suggests possible "pre-dive preparation" where the shark raises its body temperature above surface water temperature in anticipation of diving into cold deep waters.

This discovery overturns the conventional understanding that body temperature maintenance and adaptation to cold waters are the primary benefits of regional endothermy. It is expected to provide clues for solving the mystery of how regionally endothermic fish came to thrive in oceans worldwide.

Tokunaga commented: "Although this is a simple study of 'measuring shark body temperature,' tremendous effort went into it behind the scenes. What was particularly difficult was recovering the data loggers detached from the sharks. The Kuroshio Current flows on the eastern side of Taiwan, and when loggers surfaced, they were swept northward at tremendous speed. We devised detailed strategies with local collaborators, sometimes driving north to intercept them, and through trial and error managed to recover them. I am very pleased that we were able to publish these results as a paper. I would like to express my sincere gratitude to everyone who supported this research."

Journal Information
Publication: Journal of Animal Ecology
Title: Enhanced thermoregulation abilities of shortfin mako sharks as the key adaptive significance of regional endothermy in fishes
DOI: 10.1111/1365-2656.70116