Ask Us Anything About Sharks (Round 2)!
SHOW NOTES
This episode we’re diving back into our mailbag to answer even more of your burning questions, with the help of Save Our Seas Foundation CEO and shark scientist Dr James Lea…
Q1: Are there any shark species that are showing signs of adapting well to climate change? [3.36 – 10.21]
Our first query of the episode is a difficult one – namely because, as sharks (in general) have long lifespans and slow reproductive rates, it is tricky to say whether any have truly ‘adapted’ to the rapidly accelerating changes we are seeing in our oceans as a result of climate change. There is some evidence of shifts in their movements and behaviour, however. Some highly migratory species, like white and bull sharks, have been observed in places they don’t usually go, altering their migratory patterns. While it’s difficult to say exactly why, some scientists believe this is a response to changes in ocean temperature or shifts in the movements of prey species. “The best evidence we have of sharks actually responding and adapting to changing environments is through movement,” says James, “that’s what they can do in the timeframe that they have. If they are able to move as the range of their prey or suitable habitat changes or extends, then that’s what they do.”
Migratory species may still be able to move to cooler waters, but for those species that are less mobile – sharks with small home ranges, who are dependent on a particular habitat, particularly those in shallow, coastal environments – the impacts of climate change may be felt more acutely. While there is research looking into their responses to threats like ocean warming, most studies are done under laboratory conditions and it is still too early to say whether these sharks are adapting. However, evidence points towards negative consequences, at least in the short term, on the metabolic rate and reproduction of some shark species. A study published in 2021 by scientists at the Rummer Lab suggests that at temperatures nearing 31°C, the growth and development of embryonic and neonate epaulette sharks were negatively impacted. And, as Isla points out, if the epaulette sharks are showing signs of being affected by higher temperatures, there is no doubt other sharks will be as well. “Epaulette sharks…are kind of like the canary in the coal mine, right?” she says. “They live in these very extreme environments, hanging out in rock pools, for example. At low tide, they can be stranded. It’s a low oxygen environment, the temperature of the water can really heat up. And so a lot of studies that have been done on how sharks might respond to climate change come from epaulette sharks, because if they can’t cope with temperature shifts, then it doesn’t look good for other species because they’re already adapted to live in these quite extreme environments.”
You can read more about climate change and how it may affect sharks here, listen to our episode with Dr Jodie Rummer and Dr Carolyn Wheeler of the Rummer Lab here, or our episode on shark movements and ocean warming with Dr David Sims and Dr Freya Womersley, from the Global Shark Movement Project, here.
Q2: Has there ever been more observations of any of the two species of Mollisquama (pocket sharks)? [10.31 – 13.20]
The short answer is no. There still remains only two known specimens of pocket sharks, each from separate species – Mollisquama parini and Mollisquama mississippiensis, the American pocket shark. They were both found on deep sea trawls, over 40 years apart. M. parini was found first, 1,200km off the coast of Chile in 1979. This tiny shark, reaching just 40cm in length, was a biological marvel with two pocket-like glands beneath its pectoral fins, containing bioluminescent fluid (hence the name ‘pocket shark’). Then, in 2010 during an expedition searching for sperm whales, scientists found yet another pocket shark. This one was smaller than the last – an immature male of just 14cm from snout to tail. Closer inspection revealed this to be a completely different species! While M. mississippiensis resembles M. parini, there are a few subtle differences: namely the location of a pit organ on its lower jaw, and numerous light-producing organs (photophores) covering its body, in addition to the bioluminescent pocket glands.
Q3: How did bioluminescence in species like the pocket shark evolve? Was it independent, or did their last common ancestor have bioluminescent fluid? [13.40-15.11]
This question is also difficult to answer! As Isla says, this is mainly because bioluminescent organs tend to be soft tissue, which doesn’t preserve well in the fossil record – so it’s difficult to say for sure. “The only paper that I could find on this subject…which states that there was a single origin of bioluminescence in a bathydamersal ancestor,” Isla explains. “Which basically means a deep sea species – meaning it lived at depths greater than 200 meters – evolved bioluminescence as a way to adapt and thrive in this environment. And based on genetics work of current bioluminescent species, they’ve traced it back to that single ancestor. So it didn’t evolve independently. Bioluminescent species all come from one ancestor.” But, she stresses, this is one single paper – the answer might change, as with pretty much all discussions around prehistoric life!
You can read the full article on the origins of bioluminescence in sharks Isla is referencing here.
Q4: The main populations of white shark are the North Atlantic, Mediterranean, South Africa, Australia, and the Northeast Pacific. We rarely hear about white sharks in the vicinity of South America, either ocean, with the exception of seasonal migrations of NA white sharks into the US South Atlantic coast and Caribbean. My question is: why not? [15.22 – 21.49]
There isn’t a huge amount of research on this particular question, but from what Isla and James could find, scientists have attributed the absence of white sharks in parts of South America to overfishing and the mass extermination of pinniped species (their main prey source) in the 19th and 20th centuries. “Mainly along the coasts of Peru and Chile…it’s gone from having reports of having a [stable] population there to no recent sightings,” Isla says, “and they say this could be attributed to fishing pressure or it could also be attributed to a mass extermination of pinnipeds, so seals and sea lions in the 19th and 20th centuries. I mean, those populations might have recovered, but whether it was worth it for the white sharks to come back, there’s like a question mark over there. And it’s very similar in the South Atlantic, so around Brazil.”
Another factor to consider is that juvenile white sharks require smaller prey to feed on, mainly other fishes. “So the fact that our oceans have been heavily overfished might mean there’s a lack of food for white sharks or especially juvenile white sharks in some places. So that might be a reason why a population hasn’t established,” Isla adds.
Access the article on white shark population trends in South American waters here.
Q5: Which shark species can stay still and breathe through buccal pumping? [21.59-28.44]
Sharks have a few ways to breathe. All require water passing over the gills, to allow oxygen to be exchanged – but there are multiple known ways that sharks can do this. Obligate ram ventilators rely on the movement of swimming to force water into their mouth and across the gills. Therefore, these species have to constantly swim to breathe; if they stop, they will effectively drown. Buccal pumpers, on the other hand, have evolved a mechanism to breathe while stationary. They use muscles around the mouth to pump water in and out. Some sharks also have spiracles – specialised openings behind the eyes – that act as a kind of built-in snorkel, again drawing in water and pushing it out over the gills.
It is a slight myth that all sharks need to keep swimming to breathe. In fact, most sharks use a combination of obligate ram and buccal pumping so that they can breathe while at rest. “I think it’s easier to say: what sharks can’t do this,” says James. Great white, mako, hammerhead, whale, basking, porbeagle, sandbar, salmon, thresher, oceanic white tip and blue sharks are all known as obligate ram ventilators only, with no evidence of them having the ability to buccal pump. But for the vast majority of other species – including more active swimmers like bull and tiger sharks – they are able to do both. “I think it’s probably a bit of a continuum as well,” adds James. “I think some sharks can probably stay still for a lot longer than others, and some that we didn’t think could can do it, which is quite interesting…tigers and bulls, and then we’ve seen it recently in some reef sharks as well. These are species that we thought had to swim to stay alive. And in some instances, they have been seen being able to rest and buccal pump. But the question is, how long can they do that for? …can stay there and meet their oxygen demands indefinitely?”
So, it seems, some sharks are like a hybrid vehicle – switching to buccal pumping when the tank is running low. Whereas others have committed fully to the stationary lifestyle. But either way, it seems buccal pumping is more common than we think!
Q6: Which species of shark and ray are most closely related? [28.54 – 37.50]
Again, a very tricky question to answer. The short answer is, in general, you could consider all sharks and rays as close relatives, given they all come from the same common ancestor. Rays actually diverged from sharks almost 200 million years ago, when sharks started to explore life on the sea floor. Their body plans started to change to suit this bottom-feeding lifestyle; their mouths and gills moved onto the underside of their body, their body shape became flattened and their pectoral fins elongated into the more ‘wing-like’ shapes we know today, giving rise to the superorder Batoidea.
Given that the rays have been on their own evolutionary trajectory for the last 200 million years, it’s hard to say exactly who of the modern rays and sharks are most closely related. Some scientists believe the shark-like rays – an order called the Rhinopristiformes which contains guitarfishes, wedgefishes and sawfishes – are evidence of a transitional form between the sharks and rays. While they have the defining traits of rays – mouth on the underside, elongated pectoral fins – they also have a body shape closer to that of a shark, which tapers into a powerful tail with two prominent dorsal fins. “And the first ancestral fossils that we have of guitarfishes are from the same crucial period in the Jurassic period when rays first diverged from the sharks,” Isla says. “So I guess you could argue that these early guitarfishes were the most closely related to sharks. However, there are two big caveats with this. The first is that we definitely don’t know for certain, you know, as with everything with the fossil record and sharks and rays, because they have cartilaginous skeletons that don’t fossilize all that well…And then caveat number two is then you can say that if you’re talking about the prehistoric guitarfishes, but the ones we have today, the modern rhino rays, have undergone over a hundred million years of evolution. And with these ones, scientists have been able to say are more closely related to rays than they are to sharks.”
More information on the chondrichthyan family tree can be found here.
Q7: What is the most surprising thing you’ve learnt in your career (when you first started, vs. most recently)? [37.58 – 44.58]
This question is difficult for different reasons than the last. For two scientists and shark enthusiasts, settling on a favourite fact was a huge challenge! James links it back to the first time his perspective shifted from one of fear and awe to deep respect: “I very clearly remember when I was learning to dive desperate to see these animals, but with more of morbid fascination, like what is this creature that holds this power?… And obviously, over time of spending a lot of time with them and spending a lot of time with individuals, I’ve come to see them completely differently. And that’s sort of flipped on its head. I see them very much as individuals who are very complex, and that sort of fear has completely melted away into complete love for these animals.”
For Isla, who grew up in the northern UK, it’s about their incredible diversity. “One of the most surprising things that I learned personally in the beginning was that sharks can exist in cold water…and that they almost exist in every niche that you can think of in the ocean,” she recalls. “I just thought they existed in like Australia and South Africa and didn’t exist in the Northern Hemisphere pretty much. And then one of the first animals that I saw on my dive was a little dogfish, a little spurdog. And yeah it was amazing to realise that we actually not only had sharks in our waters, in colder waters, but also that they could be that size and that shape.”
Q8: Shark [bite incidents] often go unreported for various reasons despite the best efforts of educational institutes to accurately record them. What do you think the real number of global [incidents] might be? [45.06-52.14]
The final question of our Ask Us Anything episode is perhaps the hardest of all. Shark bite incidents may go unreported for a number of different reasons, but as James says, these are most likely minor. “I’d imagine because of the medical assistance required that the more severe ones are relatively well documented,” he explains, “but there’s instances I know of, of researchers or fishermen that have been bitten while handling sharks…I mean, that we would call provoked.” These bites are likely non-fatal, easily treated and likely not worth the trouble to report. “And especially if you think about where shark bite incidents are likely to happen, which are in coastal areas where people have easy access to the water…it’s likely these incidents are recorded,” Isla adds.
“But it also helps to show just how, I can’t stress this enough, how rare these incidents actually are. And then to add an extra layer to that, how rare it is that they actually are fatal, as awful as it is when that actually does happen, you know?”
You can find more data on human-shark interactions by heading to the International Shark Attack File, by the Florida Museum, here.
We also have resources on the World of Sharks website on this topic, here and here.
ABOUT OUR GUEST
Dr James Lea
Chief Executive Officer
James has had a fascination for the marine realm from a young age, and it was this that sparked his ambition to explore the oceans. Having been humbled by encounters with various shark species, he became keen to learn as much as he could about their behaviour and ecology. James gained a first-class Honours degree in biological sciences from the University of Oxford and then volunteered as a shark researcher at the Bimini Biological Field Station. At Bimini, he cut his teeth catching, tagging and tracking sharks, and working with them so closely consolidated his passion and further motivated him to fight for marine conservation.
James then moved to work as a research scientist for the Save Our Seas Foundation before completing a PhD in marine biology at the University of Plymouth in collaboration with the D’Arros Research Centre. His primary research focus was a comprehensive tagging programme tracking almost 200 sharks of seven different species in Seychelles, aiming to determine the factors that drive their movement behaviour and use this knowledge to inform effective conservation strategies. James’ research has helped to contribute to the design of marine protected areas and has revealed previously unknown open ocean migrations of tiger and bull sharks, highlighting the challenge of managing shark populations that span ocean basins. He continues his research as part of the Evolutionary Ecology Group at the University of Cambridge.
James fully realises the importance of actively promoting awareness of marine conservation issues, so he is particularly excited to lead the Save Our Sea Foundation team to help ensure that we can live with healthy oceans for generations to come.
