Regional Endothermy: The Secret Superpower of the Mackerel Sharks
SHOW NOTES
We start by getting to know a little more about Haley [6.00]. Her most memorable ocean experience comes from one of her PhD fieldwork expeditions in Ireland, which involved the complicated task of employing brand new equipment in the pandemic, with limited help. Haley was attempting to attach biologgers – a small device inserted into the surface of the shark’s skin – on the second largest species of shark in the world. This meant that Haley had to get in the water with basking sharks to see whether the loggers had attached properly. Upon entering the water, Haley found herself surrounded by enormous sharks (basking sharks can reach up to 12 metres in length)! It was the most sharks Haley had ever seen, right under her feet. She felt like she was in blue planet!
Sharks came into Haley’s life very early on [10.10]. She was saying the word ‘shark’ almost as soon as she could talk, which she attributes to a children’s TV programme called Sharkie and George: Crimebusters of the Sea. Haley jokes that her career could have gone one of two ways: marine biologist or detective! But, growing up in landlocked Oxford, she didn’t know that marine biologist was a profession. So, she started out as a terrestrial biologist. The possibility to get into marine biology arose after graduation, when Haley saw an opportunity to work on the Isle of Man with the Manx Wildlife Trust, Manx Whale and Dolphin trust and Manx Basking Shark Group. Here she learned a lot of research and survey techniques for marine megafauna, including basking sharks. Haley’s passion for sharks started the first time she saw a basking shark while on survey, which pushed her to go back into academia. She headed to the University of Exeter, where she got involved in basking shark research. She started with satellite tagging to understand their movements around the Isle of Man, and how that overlapped with human pressures.
This experience later led her to a PhD at Trinity College Dublin [15.11]. For her PhD, Haley focussed on the biology and ecology of Ireland’s regionally endothermic (warm-bodied) fish. She first looked at Atlantic bluefin tuna and how they responded to catch-and-release fishing, before moving onto regionally endothermic traits in the lamniform sharks. Her research involved a lot of “bucket science” and dissections (more on this later!) along with state-of-the-art biologging techniques.
But before we get stuck into Haley’s research, let’s explore regional endothermy. First, what is an ectotherm vs an endotherm [17.20]? An ectotherm, or an ectothermic fish, is what we might call ‘cold-blooded’. They cannot regulate their own body temperature, and so tend to match the external temperature of their surrounding environment. Endotherms, on the other hand, have the ability generate their own body heat, and – more importantly – maintain that heat. Most fish (around 99%) are ectothermic; being endothermic, or ‘warm-blooded’, is a very rare trait in the fish world. There is one fish that is considered a full endotherm, and is capable of keeping its entire body warm – the opah, or ‘moonfish’. But most ‘warm-blooded’ fishes are actually regionally endothermic, which means that they can only heat certain parts of their body.
There are different types of regional endotherms, depending on the part or region of the body that is heated. Orbital endothermy, for example, is where the animal is able to maintain heat around its eyes. The big-eye thresher is thought to have this trait, which might enhance visual perception while hunting. Other fishes have visceral endothermy, where the stomach area is heated to aid in digestion, or cranial endothermy which keeps the brain nice and toasty in colder waters. Most common in sharks though is red muscle endothermy, which Haley specialises in – and we’ll come back to later in the episode.
But before we get into that, how does this all work? How are some sharks able to heat just one part of their body [22.00]? As Haley explains, it’s all to do with the vascular system. They have capillaries, arteries and veins concentrated around that region of the body, that are arranged in a counter-current heat exchange system. Essentially, cold blood and warm blood meet, and the exchange of the two through the capillaries “takes the sting” out of the cold. It’s like when you run the cold and hot tap at the same time to make water that is just the right temperature. This is how sharks are able to heat certain regions of the body, and maintain that temperature.
As we mentioned earlier, Haley’s work concentrates on the red muscle of regionally endothermic sharks [22.50]. In these sharks, red muscle is concentrated around the vertebrae. The counter current exchangers are located just underneath the skin to keep this red muscle warm, which allows these sharks to move quickly and powerfully even in deeper, colder waters.
So, how many sharks can do this [24.50]? So far, only a handful of species have been found to have regionally endothermic traits, and they all reside in the same family: the Lamnidae. This family (also known as the ‘mackerel sharks’) contains some of the most famous, powerful, predatory sharks; the species that spring to mind when you think of the word ‘shark’! These include the white sharks, mako sharks and thresher sharks, all the apex macro-predators that are capable of hunting for fast-moving prey. However, there are some anomalies. The Lamnidae also include the filter-feeding basking shark and megamouth, as well as deep sea species like the goblin and small-tooth sand tiger shark.
There are a couple of schools of thought as to what advantages regional endothermy might offer, and why it evolved [26.40]. One is the niche expansion hypothesis, which states that regionally endothermic sharks would have been able to expand their range to colder waters – waters where ectothermic sharks would have gotten too cold and died. The other is that it might increase their speed, which more recent research supports. Studies are showing that regionally endothermic fishes can swim faster, and migrate for longer distances in shorter amounts of time. This makes sense for the Lamniforms, who are considered the “Olympians of the shark world”, known for their athleticism and roaming nature.
Haley is at the forefront of our recent understanding of regional endothermy in the mackerel sharks, and her research has challenged our perception of why some sharks have evolved this trait [29.50]. In the last few years, Haley has shown that more unusual members of the mackerel shark family – the basking shark and small-tooth sandtiger shark – have regionally endothermic traits. But Haley’s journey into this area of research began long before her PhD. While working on the Isle of Man, Haley had long suspected that basking sharks might be regional endotherms. She got the chance to borrow an infrared camera from the local fire station, which she used to test out her theory – and, she could see that basking sharks appeared to be warmer than the ambient sea temperature! Although this wasn’t an official scientific study, it did lead to a conversation with her PhD supervisor Nick Payne a few years later, who also shared Haley’s theory. This set Haley on the challenging path to investigate regional endothermy in basking sharks, something that is no small feat considering their size, and how rare it is to find an intact specimen to dissect. But she was determined. Even though basking sharks have a reputation for being slow and sluggish, they are in reality powerful animals and quite active. They are the largest species of shark known to breach, for example, and undertake extensive migrations thousands of kilometres long. The possibility that they might be regionally endothermic was quite high, Haley thought, and she was further encouraged by a singular reference to basking sharks and regional endothermy in the literature. But how she was going to look at this was a different story.
That was until a basking shark washed up just off the shore of Yorkshire, in the UK [35.22]. Haley’s contacts got in touch to ask what samples she might like, which put Haley in the strange position of having a “wish list” of basking shark body parts! What she was particularly interested in was seeing where their red muscle was located, but because this was during the pandemic, Haley had to get pictures of parts of the shark’s body sent to her over WhatsApp! From these, she was able to see that the red muscle was concentrated around the vertebrae – her first sign that there was more going on beneath the surface than scientists had first thought. But she still needed more evidence. Luckily (well, unluckily for the shark, but luckily for science!) three further individuals stranded while Haley was doing her PhD. This allowed Haley to look at not just the red muscle, but also the heart of the shark – another part that could reveal secrets. Regionally endothermic sharks have a high percentage of something called ‘compact myocardium of the heart’, which is basically extremely compact tissue within the ventricle of the heart. It’s thought that this helps to generate power and get blood pumping around the body, to generate that heat and support their active lifestyle. Haley and her supervisor Nick were able to remove the whole heart from the basking shark, and given that basking sharks average around 8-10m long, it was a big heart – around 30cm [41.30]! They were able to see that the basking shark did have that compact myocardium, another sign that they are in fact regionally endothermic.
Haley then combined her ‘bucket science’ with the use of biologgers [44.39]. These are small devices that are inserted beneath the shark’s skin, to gather data on their body temperature. In smaller sharks, this is possible to do by bringing the shark on board a boat and quickly inserting the biologger – but with a 4 tonne animal, it’s slightly trickier! For her PhD, Haley had to design a biologger that could be attached using a long tagging pole, while the shark was swimming alongside the boat. For this Haley worked closely with an engineer, who helped her design a biologger that would eventually ‘pop off’ the shark some hours later to be collected. Their results showed that basking sharks were maintaining a consistent body temperature of 1-1.5 degrees above ambient, which is almost the same as the mako shark. It seemed that Haley’s hypothesis of basking sharks being regionally endothermic rang true!
Haley was also able to discover similar traits in another, more unusual member of the mackerel shark family: the smalltooth sand tiger shark [49.00]. By chance, two individuals stranded in the UK – the most northerly records that we have for this species. Haley was able to obtain samples and found that this species also has centralised red muscle and a high percentage of compact myocardium of the heart. This is particularly exciting because the smalltooth sand tiger is a more basal species, meaning that it sits further back in the family tree [50.00]. This could mean that regional endothermy evolved earlier than previously thought. There has been a lot of interest around the evolution of these traits in sharks, especially because of the fascination with Megalodon, the largest predatory shark to have ever existed. Some believe that the reason Megalodon was able to reach such large sizes was due to regional endothermy, or at least this was part of its success. But the smalltooth sand tiger and the basking shark brings this into question. It indicates that regional endothermy evolved further back in the lamniform family tree, and only once – which have some really fascinating implications for our understanding of this group of sharks.
Haley’s work also has conservation implications [54.00]. As we discussed earlier, one of the major advantages of regional endothermy is to expand into colder waters, and possibly a reason why it evolved in the first place. A current concern is how our warming seas might affect regionally endothermic fishes. In short, we don’t know enough about their physiology and ecology to know what impact this might have, or how it might help these sharks adapt to climate change and other anthropogenic threats. Either way, Haley’s work is furthering our understanding of how these amazing sharks work, and what that could mean for their future.
ABOUT OUR GUEST
DR HALEY DOLTON
Haley is a marine biologist with over 10 years of experience in the field. Having trained as a terrestrial biologist, she has a broad understanding of the ecology and biology of living things and their environments. She has specialised in the biology and physiology of large, regionally endothermic marine fishes including tuna and shark species. Her research interests include understanding how the physiology affects the behaviour of vulnerable species and how they respond and cope to pressures such as climate change and fisheries interactions.
In short, Haley tries and understand how an animal works, and why it does what it does to effectively offer conservation solutions.
You can follow Haley on X/Twitter (@haleydolton) or head to her website, www.haleydolton.com
