Roughly 99.9% of fish and shark species are “cold-blooded,” meaning their body tissues generally match the temperature of the water in which they swim—but researchers have just discovered that the great basking shark is one exception in a thousand. Instead, these sharks keep the core regions of their bodies warmer than the water like the most athletic swimmers in the sea like great white sharks, mako sharks, and tuna.
The latter examples are called “territorial heat-eaters” and they are all fast-swimming predators at the top of the food chain. Scientists have long thought that their ability to keep warm aided this athletic predatory lifestyle, and that evolution had shaped their physiology to match their requirements.
However, an international team of researchers led by those from Trinity College Dublin has now shown that the gentle, plankton-eating basking sharks are also territorial heat absorbers despite having very different lifestyles than white sharks and tuna.
This surprising discovery has implications for conservation, as well as raising a slew of ecological and evolutionary questions.
Trinity College of Natural Sciences PhD candidate Haley Doulton was the lead author of the study just published in the international journal, Endangered Species Research. She said:
“The basking shark is a shining example of how little we know about shark species in general. We still have a lot to uncover about the world’s second largest fish – such a charismatic megafauna that most people will recognize – only highlights the challenge for researchers to gather what they can about the species to aid in effective conservation strategies.
Basking sharks only gained legal protection in Irish waters last year, as the species experienced significant declines in numbers across the Northeast Atlantic in the past century. But they still face many challenges in the future.
Haley Doltonadded: “Regional heat is thought to take up more energy, and perhaps respond differently to ocean warming than other species of fish. So much work will need to be done to see how these new findings regarding endangered species can change previous assumptions about their metabolism or potential distribution shifts during our climate crisis, something marine biologists are focusing on as our planet and its seas continue to warm.”
“We hope that this kind of research will continue the momentum needed to effectively protect these amazing animals in Irish waters and beyond.”
To make the discovery, the research team (including scientists from the University of Pretoria, the Marine Biological Association, Queen’s University Belfast, the Zoological Society of London, the University of Southampton and Manx Basking Shark Watch) dissected dead basking sharks that washed up in Ireland and the UK.
They found that sharks have cruise-swimming muscles located deep within their bodies, as seen in white sharks and tuna. In most fish this “red” muscle is found towards the outer surface of the animals.
They also discovered that basking sharks have strong, muscular hearts that may help generate high blood pressure and flow. Most fish species have relatively “squishy” hearts, while basking shark hearts are more typical of endothermic species in the area.
Next, the team designed a new, low-impact labeling method to record the body temperature of free basking sharks off the coast of Co Cork, Ireland. The researchers were able to get close enough to the basking sharks to 8 meters to safely post the tags, which recorded muscle temperature just under the skin for up to 12 hours before they were automatically separated from the animals and collected by the researchers.
These markings revealed that the basking shark’s muscles constantly rise above water temperatures, to about the same extent as its endothermic, territorial predatory cousins.
Nicholas Payne, assistant professor at Trinity College of Natural Sciences, was lead author of the study. He said:
“These findings shed interesting new light on our perception of form versus function in fish because until now we thought regional thermopsies are only found in apex predator species that live high up in the marine food web.
“We have now found a species that grazes on small plankton but also shares those unusual thermoregional characteristics, so we may have to adjust our assumptions about the merits of these physiological innovations for these animals.
“It’s a bit like suddenly discovering cows.”