Few fast-swimming apex fishes are classified as regional endotherms, or partially warm-blooded, having evolved a relatively uncommon suite of traits (e.g. elevated body temperatures, centralized red muscle, and thick-walled hearts) thought to facilitate a fast, predatory lifestyle. Unlike those apex predators, basking sharks (Cetorhinus maximus) are massive filter-feeding planktivores assumed to have the anatomy and physiology typical of fully ectothermic fishes. New research led by Trinity College Dublin scientists suggests basking sharks are not full ectotherms, instead sharing several traits used to define a regional endotherm, thus deviating from current understanding of the species and questioning the link between physiology and ecology of regionally endothermic shark species.
“The basking shark is a shining example of how little we know about shark species in general,” said Haley Dolton, a Ph.D. candidate at Trinity College Dublin.
“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.”
“Regional endotherms are thought to use more energy, and possibly respond differently to ocean warming than other fish species.”
“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.”
“Hopefully this kind of research will continue the momentum needed to effectively protect these incredible animals in Irish waters and further afield.”
To make the discovery, Dolton and colleagues first undertook dissections of dead basking sharks that washed up in Ireland and the UK.
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.
Next, the authors designed a new low-impact tagging method to record body temperature of free-swimming basking sharks off the coast of Co Cork, Ireland.
They 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 by the team.
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.
“These 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,” said Trinity College Dublin’s Dr. Nicholas Payne.
“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.”
The team’s paper is published in the journal Endangered Species Research.
Citations:
Haley R. Dolton et al. 2023. Regionally endothermic traits in planktivorous basking sharks Cetorhinus maximus. ESR 51: 227-232; doi: 10.3354/esr01257
This article was first published by Sci-News on 20 July 2023. Lead Image: The basking shark (Cetorhinus maximus). Image credit: Chris Gotschalk.
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