Most of us grew up learning that fish are cold-blooded creatures. In fact, about 99.9% of all shark and fish species swimming around our planet are thought to be cold-blooded. But guess what? It turns out that's not entirely true.
Around 50 years ago, scientists had a "wait, what?" moment when they realized some shark and tuna species, like the great white shark and bluefin tuna, aren't as chilly as the water they swim in. Parts of these fish are actually warmer than their surroundings, making them 'regionally endothermic', or warm-blooded. Until now, scientists thought the unique circulatory system that allows for this was exclusive to apex predators - the big bosses of the ocean food chain. But, plot twist! A new study on the basking shark, a slow-moving giant that feeds passively, has turned that assumption upside down. When Payne and his team tagged wild basking sharks with electronic biologgers, they noticed something unexpected. The sharks had higher tissue temperatures than what they had anticipated. This resembled other regional endotherms, like the great white.
But here's the catch - this physiology probably didn't evolve to hunt fast-moving prey or due to the shark's enormous body size, which can reach up to a whopping 12 meters long. After all, the cold-blooded whale shark can grow up to 10 meters long, and yet it has fewer than 15 times the percentage of compact heart muscle as a basking shark. So, why is the basking shark so exceptionally warm? Well, despite moving sluggishly a lot of the time, basking sharks are known to speed up significantly when feeding. Why they do this when their prey is so slow remains a mystery, but it could be related to overcoming drag, thereby "preserving feeding efficiency" in cooler water. Studies have also found that regionally warm-blooded fish can swim 1.6 times faster than their cold-blooded counterparts – providing a possible evolutionary advantage to the unique circulatory system.
"The basking shark is a shining example of how little we know about shark species in general," says zoologist Haley Dolton from Trinity. "That we still have lots to uncover about the second biggest fish in the world—such a huge, charismatic animal that most people would recognize it—just highlights the challenge facing researchers to gather what they can about species to aid in effective conservation strategies." This exciting research was published in Endangered Species Research.