‘The Meg’s’ shark cousins are warm-blooded and feeling the heat

A surprise beaching of a shark on the Irish coastline has researchers worried about whether some cartilaginous fish are potentially on course for the same fate as their jumbo-sized ancestors.

Megalodon – popularised most recently in a series of action films – was the world’s largest fish, growing to 20 metres and living in the oceans about 3 million years ago.

While its family, Otodontidae, is extinct, the order of Lamniformes it belonged to consists of mackerel sharks still alive today and include species like the great white, basking, tiger and mako sharks.

Some rare characteristics are still shared between modern and extinct lamnids. In contrast to most fish, a tiny number of sharks – usually apex predators – are warm-blooded, among them the Meg, as well as great whites that swim the oceans today.

But when another lamnid – a smalltooth sand tiger shark (Odontaspis ferox) – washed up on a patch of Ireland’s coast, researchers from Trinity College Dublin made an unexpected finding.

During autopsy, the smalltooth sand tiger shark was found to have anatomical features consistent with being a ‘regional endotherm’ – warm-blooded.

That, the researchers suggest in their study published in Biology Letters, might indicate the ability of animals to maintain their body temperature at a level higher than their environment, evolved much earlier than thought.

“If sand tiger sharks have regional endothermy then it’s likely there are several other sharks out there that are also warm-bodied,” says Dr Nick Payne, the study’s senior author.

“We used to think regional endothermy [warm bloodedness] was confined to apex predators like the great white and extinct megalodon, but now we have evidence that deep water ‘bottom-dwelling’ sand tigers, and plankton-eating basking sharks also are warm bodied.

“This raises plenty of new questions as to why regional endothermy evolved, but it might also have important conservation implications.”

Those conservation implications might be particularly relevant now. Amid increasing ocean temperatures endothermic animals may be forced to respond to their changing environments.

That the smalltooth sand tiger was washed up in the British Isles, its northernmost range being just off France’s west coast, might be indicative of these environmental shifts already being underway. A substantial marine heatwave was registered across the region in June this year, with some coastlines at least 3°C above the 1985-1983 average.

“Our understanding of science continually grows and it’s becoming clear that whenever regional endothermy evolved in the past it has been retained in a growing number of shark species with very different lifestyles,” says Dr Haley Dolton, a marine biologist who led the study.

“The discovery… has major implications from a conservation perspective for regional endotherms. We believe changing environments were a major contributor to the megalodon’s extinction, as we think it could no longer meet the energetic demands of being a large regional endotherm.

“We know the seas are warming at alarming rates again now and the smalltooth tiger that washed up in Ireland was the first one seen in these waters. That implies its range has shifted, potentially due to warming waters, so a few alarm bells are ringing.”  

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