Ask Us Anything About Sharks!

SHOW NOTES

In this special season opener, host Isla is diving headfirst into the podcast mailbag — joined by Save Our Seas Foundation CEO Dr James Lea — to answer your burning shark and ocean questions.

Do any sharks, other than the megamouth shark, use mimicry? (04:54–14:00)

The episode begins in the deep sea with one of the ocean’s strangest residents: the elusive megamouth shark. Listener Tristan wrote in to highlight a scientific study that demonstrates megamouth sharks have a reflective band above their mouths, which reflects the bioluminescence emitted by their planktonic prey. The theory is that plankton is then attracted to the mouth of the filter-feeding megamouth, making it easier for them to find prey in the dark.

It could be argued that this is a form of mimicry – an evolved adaptation where one organism (the mimic) resembles another, or an inanimate object, to gain a specific advantage. “Like a harmless species will evolve like the patterns of a more toxic one to avoid being eaten, or even they’ll evolve to look like an inanimate object – so like stick insects, for example,” Isla explains. Mimics can also recreate the sounds or scents of others. Considering bioluminescence specifically, the lure of the anglerfish is considered ‘aggressive mimicry’, where the mimic is deceptively signalling a benefit (e.g. the presence of fellow planktonic organisms) to manipulate another organism’s behaviour. Isla and James wonder if megamouth sharks could also be classed as aggressive mimics…

But, is there evidence of mimicry in other species of shark? The short answer is yes, although it’s a poorly studied area. Zebra sharks were the first recorded examples of mimicry in sharks. As juveniles, they are striped (giving them their name) with long, thin tails – thought to mimic the appearance of a venomous marine species, the banded sea snake. Wobbegongs engage in a behaviour called ‘caudal luring’, waggling their tail to mimic the movements of a small fish or worm in order to attract other fish towards them. As ambush predators, wobbegongs lie still on the seabed, waiting for unsuspecting prey to cross their paths. By using their tail to mimic other fish, they are effectively speeding up that process.

Are megamouth sharks regional endotherms? (14.15 – 19.10)

Tristan also had a second question, around evidence for regional endothermy in megamouth sharks. Typically, sharks are ectotherms, or ‘cold-blooded’, meaning their internal temperature is regulated by how warm or cold the surrounding seawater is. Regional endothermy is a special adaptation where some shark species can warm up particular parts of their bodies – such as their muscles, eyes or brain – to above the ambient water temperature and maintain that higher temperature within that body part for a period of time. In doing so, regionally endothermic sharks gain certain advantages; they can swim faster and for longer, explore and inhabit colder, deeper waters, and retain sensory function even at depth.

All known sharks with traits of regional endothermy come from the order Lamniformes, also known as the mackerel sharks. This group is dominated by fast-moving, agile, apex predators, including mako sharks, great white sharks, and thresher sharks. As they are often hunting large or nimble prey, these sharks benefit from being able to put on fast bursts of speed. But the lamniforms also include more unusual species, like basking sharks and the megamouth. These more slow-moving, filter-feeding species were previously assumed not to be regionally endothermic – until recent work led by Dr Hayley Dolton found regionally endothermic traits in the basking shark. As a species with a similar feeding strategy, it is not unlikely that megamouth sharks would also share those traits.  “[Regional endothermy] allows [sharks] to basically stay at depth(s) where it’s colder for longer,” Says James. “So if you are a megamouth shark who’s looking to dive down deep for Mesopelagic plankton or whatever, you might be able to sustain certain temperatures for longer if you are sort of regionally endothermic.”

However, there just isn’t evidence for regional endothermy in megamouths yet. As James explains, proving it would require detailed anatomical study and live temperature measurements — something that’s incredibly difficult given how rarely these sharks are encountered. “I think there’s, it’s either fewer or maybe just a little over 300 recordings ever of this type of shark,” he adds.

Do magnetic shark deterrents work? (19:22–29:54)

From there, the conversation moves on to the topic of water safety and shark deterrents. Listener Sam asks whether magnetic or electrical devices used in fisheries could double as safety tools for surfers and divers.

As James explains, the answer is: it depends. Some devices can disrupt sharks’ electroreception and reduce interactions, but effectiveness varies widely by species, range and motivation. Sharks can habituate, and hungrier sharks may ignore discomfort in favour of a meal. Furthermore, evidence suggests that as a shark’s electroreceptive senses are more designed to be used over a short range, the deterrents also only work when the shark is close enough to detect it. “If it’s only working at short range and a shark’s made a decision from a distance – even if it wants to abort, by the time it enters that short range, it might not have the opportunity to fully abort what it’s doing before it sort of makes contact,” James says. There are examples where sharks have turned around, but again, it very much depends on certain factors. “It’s all about managing risk,” he adds.

Isla likens deterrents to carrying bear spray: “You hope you don’t have to use it, but you have it if you do, and you’re prepared. But it’s not totally eliminating the risk”. They may provide extra peace of mind — but they are not foolproof solutions.

For more information on the subject, check out this article: https://saveourseas.com/worldofsharks/do-shark-repellents-work

Are hydrothermal vent species able to move around? (30:28–32.53)

The episode then takes a slight detour to the deep sea, following a question by listener Alyssa. Inspired by a previous episode, Alyssa asks about hydrothermal vent ecosystems and whether the species living there can move away from the vent. Isla explains that many vent species are so specialised that they cannot survive away from the extreme conditions of superheated, mineral-rich water. Some disperse as larvae or through fissures in the ocean crust, but many are permanent residents of this hostile, alien environment.

How do we know what prehistoric shark fossilised poop came from? (32.54 – 36.52)

In another question inspired by a previous episode on prehistoric sharks, Isla and James explore the curious (and likely very specialised) role of a palaeontologist trying to decipher clues from shark coprolites – or, to give it its less scientific name, fossilised shark faeces. “The cool thing about sharks, as we’ve discussed on this podcast, is that they have spiral-shaped intestines, which means that their faeces, when it comes out, is also spiral-shaped. So a palaeontologist can kind of look at that and go, I think that’s come from a shark,” Isla says. “They can also look at the size… so obviously a bigger shark would have a larger dropping. Megalodon, for example, imagine how massive their poops would be. And I guess they can also have a look inside to see what fragments of food they contain. So it can give clues as to what the shark was eating. So that gives them clues as to the lifestyle. And there might also be tooth fragments from the shark itself, or if it was eating other sharks, what sharks they were eating.”

James and Isla then briefly find themselves Googling images mid-conversation — proof that shark science can be both rigorous and wonderfully weird.

How do basking sharks find their food? (41:13–47:49)

Next up: basking sharks.

“How do basking sharks find their food?” asks one Reddit listener. James dives into optimal search strategies, describing how large basking sharks use a pattern known as a Lévy flight — intensively searching one area before making large movements to another. Sense of smell is also important, at closer distances. Plankton, it turns out, is very smelly, giving off a “wonderful plankton-y bouquet”. It’s thanks to Dimenthyl Sulfide (DMS), a compound that is produced as a byproduct when phytoplankton is eaten by zooplankton. Alongside this, basking sharks likely use temperature cues to locate plankton patches. “Plankton, as I said, often accumulates in certain patches, particularly around tides and things, but also around temperature,” James explains. “You get these fronts of temperature in the water where different temperature bodies meet, and plankton relies on its primary productivity, so sunlight, and where you get productive fronts meeting, you’re often going to get more plankton forming. And so the basking sharks probably also use temperature cues as well.”

Despite their enormous size and relatively small brains — what Scottish poet Norman McCaig once called “that room-sized monster with a matchbox-sized brain” — basking sharks are far from simple. Research has also suggested that they learn from experience, returning to the same plankton-rich sites year on year. They then home in on specific patches using environmental cues. “It’s almost like, yeah, they vaguely know their favourite restaurant is there…but it might have moved along the street somewhere, and they’ve got to find it once they get there,” Isla jokes.

Which is the fastest shark? (47:49–52:05)

Another listener wants to know: what is actually the fastest species of shark?

James highlights a challenge in giving a definitive answer: “How do you define fastest and how do you measure it?” It depends on which behaviour you are talking about – burst speed, cruising speed, and even tail-slap velocity all complicate the picture. While the shortfin mako generally holds the top spot in recorded cruising speeds (6.9 km/hr), species like salmon sharks and even breaching basking sharks can achieve impressive athletic feats. Both have been recorded cruising at 3.9km/hr, and basking sharks have been known to breach with speeds of 5.1m/s!

However, measuring true top speeds in the wild remains notoriously difficult. For now, mako sharks come out on top; alongside scientifically recorded cruising speeds, they have been observed keeping up with boats travelling at 70km/hr (around 45 mph).

You can read more on this topic here: https://saveourseas.com/worldofsharks/which-sharks-are-the-fastest

If you could pass one international law… (52:34–01:02:37)

The episode closes with the biggest question of all: if given the executive power to pass one international law to help sharks, what would it be?

 “It’s so complex,” James begins. Blanket bans on shark fisheries sound simple, but they are tied to livelihoods and food security worldwide. Instead, the discussion turns to strengthening what already exists. With more than 90% of the global fin trade now technically covered under CITES regulations, attention shifts from listing species to enforcing those listings. James suggests that one powerful international law would require proof of sustainable harvest before any shark product could be exported — effectively making sustainability verification mandatory for global trade.

But that, too, comes with complications. What qualifies as “proof”? Who collects the data? Who pays for monitoring? Isla highlights the critical role of traceability and DNA testing to verify species identity throughout supply chains, noting that without standardised and well-resourced traceability systems, regulations can be difficult to implement effectively.

They also discuss gear modifications, observer coverage on industrial vessels, and bycatch mitigation measures — all valuable tools, but each with trade-offs. A rule that benefits sharks might inadvertently affect seabirds. Increased oversight may strain already limited resources in developing nations. Every solution reveals new layers of complexity.

Ultimately, the conversation lands on a central theme: conservation success requires attention to not just legislation, but implementation and enforcement. Laws need funding, governance structures and political will to be meaningful.

 Stay tuned later in the season for part 2 of Ask Us Anything About Sharks, where we have more listener questions waiting in the wings!

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.