Researchers have discovered that basking sharks are as warm-bodied as great white sharks, in spite of having a more sedentary lifestyle.
An international team led by scientists from Trinity College, Dublin, says the “surprising” discovery has implications for the conservation of this species, which gained legal protection in Irish waters last year.
Their findings are published in the international journal, Endangered Species Research.
The scientists explain that approximately 99.9% of fish and shark species are “cold-blooded”, meaning their body tissues generally match the temperature of the water they swim in.
However, “the mighty basking shark is a one-in-a-thousand exception”, they say.
Basking sharks keep the core regions of their bodies warmer than the water – just like the most athletic swimmers in the sea such as great white sharks, mako sharks and tuna ,”they state.
Great whites, mako sharks and tuna are all fast-swimming “apex predators” at the top of their food chain, known as “regional endotherms”.
It has long been believed that their “athletic predatory lifestyle” was helped by this ability to keep warm – and that evolution had shaped their physiology to match their requirements.
The research team, including scientists from University of Pretoria, Marine Biological Association, Queen’s University Belfast, Zoological Society of London, University of Southampton, and Manx Basking Shark Watch, first undertook dissections of dead basking sharks that washed up in Ireland and Britain.
They found that the sharks have cruise-swimming muscles located deep inside their bodies as seen in white sharks and tunas; in most fish this “red” muscle is instead found toward the outside of the animals.
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.
The team says it then designed a new low-impact tagging method to record body temperature of free-swimming basking sharks off the coast of Co Cork.
The researchers says were able get close enough to 8 m 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 and were collected.
“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,”they state.
Lead author of the study Haley Dolton, who is a PhD candidate in TCD’s School of Natural Sciences, said the basking shark is “a shining example of how little we know about shark species in general”.
“That we still have lots to uncover about the second biggest fish in the world – such a huge, charismatic animal that most people would recognise it – just highlights the challenge facing researchers to gather what they can about species to aid in effective conservation strategies,”Dolton said.
“Regional endotherms are thought to use more energy, and possibly respond differently to ocean warming than other fish species,”Dolton continued.
“So lots more work will need to be done to work out how these new findings regarding an endangered species might change previous assumptions about their metabolism or potential distribution shifts during our climate crisis, which is something marine biologists are focusing on as our planet and its seas continue to warm.”
The study’s senior author, Nicholas Payne, who is an assistant professor in TCD’s School of Natural Sciences, said the results “cast an interesting new light on our perception of form versus function in fishes because until now we thought regional endothermy was only found in apex predatory species living at high positions in the marine food web”.
“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,”Payne said.
“It’s a bit like suddenly finding that cows have wings,” he said.
Haley Dolton is funded by the Irish Research Council, with support from the Fisheries Society of the British Isles and Dr Nicholas Payne was funded by Science Foundation Ireland.
The journal article can be read at: https://www.int-res.com/abstracts/esr/v51/p227-232/. A PDF copy is available on request.
