The Meg was not the only warm-blooded shark
Dr Nicholas Payne and Dr Jenny Bortoluzzi with the smalltooth sand tiger shark that washed up in Co. Wexford earlier this year. Pictures: Dr Jenny Bortoluzzi and Kevin Purves
We are used to hearing of sharks as cold-blooded killers but researchers here have made some surprising discoveries — there may be more warm-blooded sharks than science thought. And these cousins of the infamous megalodon shark may be in real trouble due to warming seas.
Some of the most famous sharks, such as the white shark or the extinct megalodon, are unusual as they are among the ~1% of shark species considered to be warm-blooded or 'regional endotherms'.
Scientists had thought that warmer muscles help fish be more powerful and athletic, with regional endothermy only seen in apex predators such as the great white or giant tuna. But there has been some debate about when regional endothermy first evolved and whether extinct species such as the megalodon was warm-bodied.
The smalltooth sand tiger shark is a relatively ancient (but still-living) shark species thought to have diverged from the Meg at least 20 million years ago. Unlike the megalodon which inspired the 2018 and 2023 action horror films, The Meg, these creatures pose no risk to people. Some of these sharks washed up dead in Ireland and England this year — it was the first time one of these sharks was seen here and may be because the species has expanded its territory due to warming waters.
So in another unexpected turn up for the books, smalltooth sand tiger #sharks also have anatomical traits associated with regional being warm-blooded!!! 🤯 🔥 🦈 Including how this links to the #meg! Check out our new @RSocPublishing paper here: https://t.co/XB4BhfJgY8 pic.twitter.com/thp20VmxvC
— Haley Dolton (she/her) (@haleydolton) November 8, 2023
Dr Nicholas Payne and Dr Haley Dolton from Trinity College Dublin's School of Natural Sciences seized the opportunity to study these sharks when they washed up here. One of the sharks — a 14 feet long specimen — was discovered at Kilmore Quay in Wexford and the other two in southern Britain. These autopsies have, ironically, taken the marine biologists millions of years back in time but also have caused concern for the future of many species of sea inhabitants.
In this new study, researchers have found that the smalltooth sand tiger has anatomic features suggesting it is a regional endotherm. Coming hot on the fins of a similar shock that slow-moving, filter-feeding basking sharks are also regional endotherms, the researchers now believe there are more warm-blooded sharks than science thought, and that warm-bloodedness evolved quite a long time ago.
Dr Nicholas Payne from Trinity’s School of Natural Sciences is senior author of the study, published this week in Biology Letters. He said: "We think this is an important finding, because if sand tiger sharks have regional endothermy then it’s likely there are several other sharks out there that are also warm-bodied. We used to think regional endothermy 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.”
The research team (including scientists from University of Pretoria, ZSL, University of Zurich, Swansea University, Smithsonian Tropical Research Institute, and University College Dublin College of Agriculture Food Science and Veterinary Medicine) were disappointed to see the sharks dead but were delighted to get the opportunity to undertake these dissections of dead smalltooth sand tiger sharks. Dr Payne said: "It was a sad but useful event really. We had so many questions and learned so much. To have three found dead within weeks and in a reasonably close proximity is really strange. The most northerly recorded before that was in the Bay of Biscay and they tend to stay on the sea bed of deeper waters as well."
It's not known if the three sharks had travelled together: "Maybe to some extent they did... of course sharks have to mate so they do have to 'socialise' at times. The one found here had an empty stomach but that can be the norm anyway. The autopsies showed no signs of disease, infection, or physical damage or plastic ingestion — no obvious cause of death at all."
Dr Haley Dolton, also from Trinity, was lead author of the study. She said: “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."
“The discovery itself is very interesting for a marine biologist, but it also has major implications from a conservation perspective for regional endotherms. We believe changing environments in the deep past was 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.”
The scientists are keen to be alerted to any future sightings of dead sharks here. Dr Payne explained: "We only heard about the one at Kilmore Quay because a Swiss tourist saw it on the shore and then googled 'Irish marine scientists' and found my name. The Irish Whale and Dolphin Group has a great network of staff too and they do contact us if they see a shark but really we are asking anyone on the coasts to let us know if they ever see a shark washed up."
- Dr Nicholas Payne's email is paynen@tcd.ie
