After autopsies of stranded sharks, researchers have discovered that more ‘warm-blooded’ sharks exist than previously thought. And that is actually bad news: these species may find it very difficult in the warming world.
The white shark and the extinct megalodon are some of the most famous (and infamous) sharks. It is also striking that they belong to the exceptionally small group of only about 1 percent of all shark species that are warm-blooded. This means they have the ability to regulate their body temperature, which is unusual among sharks. Although… researchers have now discovered that this may be slightly less exceptional than previously thought.
Hot-bloodedness
As mentioned earlier, being warm-blooded means that an organism is able to generate its own body heat and control its body temperature. This contrasts with cold-blooded organisms, which rely on external heat sources to regulate their body temperature. The fact that animals became warm-blooded offered an evolutionary advantage. It is not without reason that mammals have managed to spread into almost every global ecosystem. In the sea, warm-bloodedness is less common. The majority of marine organisms, including fish, reptiles and invertebrates, are cold-blooded. It was always thought that only top predators, such as the great white shark or giant tuna, are warm-blooded. This would be necessary for their very active and predatory lifestyle.
The smalltooth sand tiger shark
A new study now sheds new light on this issue. The team performed autopsies on dead smalltooth sand tiger sharks that unexpectedly washed up on the coasts of Ireland and the United Kingdom. This is a fairly old, but still extant shark species that split from the megalodon about 20 million years ago – and is therefore a living close relative. After a thorough inspection, the researchers make a remarkable discovery. For example, the smalltooth sand tiger shark appears to have anatomical features that suggest the shark is ‘regionally endothermic’. This means that this shark can regulate the temperature in certain parts of its body and can therefore no longer be classified as cold-blooded animals.
Basking shark
But it is not only the smalltooth sand tiger shark that appears to have surprises in store for us. The same researchers discovered this before the impressive basking shark, the second largest living fish, is regionally endothermic. The team therefore believes that there are more ‘warm-blooded’ sharks than previously believed in science. “In the past, we assumed that only top predators were warm-blooded,” says researcher Nicholas Payne. “But now we have evidence that the smalltooth sand tiger shark and even the plankton-eating basking shark are also regionally endothermic. And if these sharks are, there’s a good chance that more regionally endothermic sharks exist. This leads to many new questions about why warm-bloodedness evolved.”
Evolution
In short, the discovery has led to a rethinking of the understanding of warm-bloodedness in sharks. It suggests this is much more widespread than previously thought. The researchers suspect that this property evolved a long time ago. “Our knowledge in science is constantly expanding,” says researcher Haley Dolton. “And it is becoming increasingly clear that whenever warm-bloodedness has emerged in the past, it has persisted in a growing number of shark species with very different lifestyles.”
Worrying
Although the discovery is very interesting, marine biologists are also concerned about the survival of many regionally endothermic sharks. Because with the continued warming of the oceans due to climate change, these sharks may face serious challenges in regulating their body temperatures in the future. In the extreme doomsday scenario, they may suffer the same fate as the megalodon. “We suspect that environmental changes in the distant past played a major role in the megalodon’s extinction,” Dolton explains. “We think this large, warm-blooded animal could no longer meet its energy needs.” Now we see the seas warming again at a worrying rate. It has led to the smalltooth sand tiger shark appearing in new areas, such as off the coast of Ireland. “Their habitat is shifting, possibly due to warming oceans,” Dolton continues. “This sets off alarm bells.”
The study therefore points to a possible threat to many warm-blooded and regionally endothermic sharks, of which there may be far more of them swimming around in the seas than previously thought. “When we realized that the smalltooth sand tiger shark is probably regionally endothermic, I was quite shocked,” says Dolton. “But maybe the next time we see this in another species, I’ll be a little less surprised.”
