New research sees basking shark lose 'cold-blooded' tag

Researchers designed a new low-impact tagging method to record body temperature of basking sharks off the coast of Cork. Picture: Irish Basking Shark Group

The mighty basking shark is a one-in-a-thousand exception to the ‘cold-blooded’ shark, after scientists discovered the gentle creatures are physiologically closer to the warm-bodied apex predators of the sea.

With approximately 99.9% of fish and shark classified as cold-blooded, meaning their body tissue generally matches the temperature of water they swim in, a small number of species keep core regions of their bodies warmer than the water.

Usually positioned at the top of the food chain, scientists have long believed that these fast-swimming “regional endotherms” have the ability to keep warm to help with their predatory lifestyle and that evolution shaped their physiology to match their requirements.

Now, in a shock discovery, a team of international researchers, led by scientists at Trinity College Dublin, has discovered plankton-feeding basking sharks keep the core regions of their bodies warmer than the water too, just like great white sharks, mako sharks, and tuna.

This is despite having very different lifestyles to these fast-moving creatures, raising implications for conservation, as well as a plethora of ecological and evolutionary questions.

The research team first dissected dead basking sharks that washed up in Ireland and the UK. They found that basking sharks have cruise-swimming muscles located deep inside their bodies. In most fish this “red” muscle is instead found toward the outside of the animals.

Muscular hearts

They also discovered basking sharks have strong muscular hearts that probably help generate high blood pressures and flows. Most fish species have relatively “spongy” hearts, whereas basking shark hearts are more typical of the regional endotherm species.

Next, the team designed a new low-impact tagging method to record body temperature of basking sharks off the coast of Cork. 

Researchers were able to get close enough to basking sharks to safely deploy the tags, which recorded muscle temperature just under the skin for up to 12 hours before they automatically detached from the animals.

These tags revealed that basking shark muscles are consistently elevated above water temperatures, and to almost exactly the same extent as their regionally-endothermic predatory cousins.

Nicholas Payne, assistant professor in Trinity’s School of Natural Sciences, was senior author of the study. The results cast an interesting new light on our perception of form versus function in fish, he said.

“Now we have found a species that grazes on tiny plankton but also shares those rather uncommon regional endotherm features, so we might have to adjust our assumptions about the advantages of such physiological innovations for these animals. 

"It’s a bit like suddenly finding that cows have wings," he said.