A new study led by scientists at Trinity College, Dublin, Ireland, has found that the slow-moving, filter-feeding basking shark (Cetorhinus maximus) may be warm-blooded – or at least, partially so – making it just one of a handful of species of fish which are able to raise their body temperatures over that of the surrounding water.
Most fish are ectothermic, or cold-blooded, meaning they are unable to regulate their body temperatures, unlike the warm-blooded – homeothermic – mammals. Instead, the internal body temperature of a fish changes to match the surrounding water, which affects the range of temperatures that particular fish species are able to survive in.
A small handful of fish, including tuna and great white sharks – the basking shark’s closest, if unlikely, living relative – are able to generate body heat through the presence of red muscle surrounding their core, and the evolution of a structure known as the rete mirabile – or ‘miracle net’ – a complex network of blood vessels that helps to prevent heat loss from the muscle tissues.
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Such animals are known as ‘regional endotherms’, meaning part, but not all, of their bodies are warm-blooded. The warmer internal body temperature and increased metabolic rate of the fish are what enable them to swim at great speed, enabling their function as some of the ocean’s apex predators.
Basking sharks are known as neither apex predators nor fast-swimming fish. Although they have been observed swimming at speed and breaching the surface in a fashion similar to great whites, they are mostly regarded as placid, fairly docile fish which, as filter feeders, have no need of the powerful hunting techniques of their relatives.
‘The basking shark is a shining example of how little we know about shark species in general,’ said Haley Dolton, PhD candidate at Trinity College Dublin and lead author of the paper.
‘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.’
The discovery was made when the team – which included scientists from the Zoological Society of London, Manx Basking Shark Watch, University of Pretoria, Queen’s University Belfast, Marine Biological Association and University of Southampton – dissected two male basking sharks, measuring 3.8 and 4.5m in length, that had been stranded in 2020.
In addition to the dissections, hearts were removed from the carcasses of two more stranded basking sharks – a 6.9m female and 4.8m male – in 2021, and four free-swimming animals were tagged with sensors to measure their body temperatures.
The results of the study showed that the red muscle tissue found within the shark’s trunk and tail section displayed similar traits to the shortfin mako (Isurus
oxyrinchus) and great white, already known to be regional endotherms.
The basking shark hearts were found to be thicker-walled and more similar in structure to other regional endotherms, as opposed to exclusively cold-blooded fish, and of particular significance was the discovery that the sharks maintained a subcutaneous (below the skin) body temperature 1.0 to 1.5°C higher than the surrounding water.
The discovery has implications for the future of basking sharks – a globally endangered species, according to the IUCN Red List of Threatened Species – as the distribution of the sharks as ocean temperatures change will potentially be affected in a different manner than it might have been if they were exclusively cold-blooded. It also brings into question the commonly held belief that regional endothermy was exclusive to fast-moving predators.
‘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 Nicholas Payne, Assistant Professor in Trinity’s School of Natural Sciences and one of the study’s co-authors.
‘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 paper ‘Regionally endothermic traits in planktivorous basking sharks Cetorhinus maximus‘ by HR Dolton et al, is published under an Open Access license in Endangered Species Research
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