Why have sharks been so successful?
SHOW NOTES
This very special episode of World of Sharks is not only our 50th episode, but also the last episode of 2023, the year in which the Save Our Seas Foundation celebrated its 20th anniversary. So what better way to honour the occasion than by discussing just how amazing sharks are; recognising their evolutionary success and how many challenges they have overcome over the last 400 million years. Dr James Lea – shark scientist and CEO of the Save Our Seas Foundation – returned to the podcast to chat with host Isla about surviving mass extinctions, super-powered adaptations and strange prehistoric species.
We start by discussing the impressive evolutionary history of sharks, which extends over 440 million years [5.32]. This means that they were around long before the dinosaurs (240MYA), and are even older than trees, which first appeared ~390MYA. We know from preserved shark skin (also known as dermal denticles) and teeth that sharks likely descended from an ancient group of fishes, known as the Acanthodians or “spiny sharks”, and have evidence of early shark-like forms dating back to 420 MYA. Although it wasn’t until ~380MYA that fishes that looked similar to the predatory sharks we know today – with the classic ‘shark shape’ – first appeared. Since then, sharks have gone on to diversify and occupy almost every niche imaginable, from deep waters 3,000m below the surface to extremely shallow rocky reefs that become almost entirely exposed at low tide.
Sharks have even managed to survive four of the five mass extinctions [11.34]. This is especially impressive considering what happened during these catastrophic events. One occurred at the end of the Devonian Period, beginning roughly 380MYA. It was caused by a combination of global cooling, potentially caused by volcanic eruptions, and the arrival of trees – which may seem surprising, given how much good trees do for the climate now. But back then, the planet wasn’t used to these strange organisms. The trees drew nutrients out of the soil, and when they eventually died and decomposed, an excess of these nutrients leached into the ocean. Although we can’t say for certain, scientists believe that this evoked an enormous algal and bacterial bloom that consumed much of the dissolved oxygen in the water – causing 70% of marine life to suffocate and die out. But this wasn’t the worst! Over 100 million years later, the end Permian extinction occurred [20.27]. This was an event so severe it has been nicknamed the “great dying”, because an astonishing 96% of marine life and 70% of terrestrial life was lost. Of the 4% of marine life that survived, some were sharks. Scientists believe that this is because of their astonishing capacity to adapt. Much like the End Devonian extinction, the oceans lost a large amount of oxygen, so much so that almost half of the seabed was inhospitable. But sharks had already begun to occupy many different niches, and were able to move to other oxygenated areas.
So what is it exactly about sharks that has made them so resilient and successful as a group of animals [24.00]? James describes many aspects of their biology and anatomy that allow them to be incredibly efficient. For example, their cartilaginous skeleton is much lighter than one of bone, meaning they don’t have to fight as hard to stop sinking. Their large, oily liver also helps with buoyancy, meaning less energy is spent trying to stay afloat. They are also incredibly hydrodynamic, thanks to the way their skin is designed. Sharks have hard plates in place of scales, called dermal denticles, that are made out of a tough, enamel-like substance and overlap across the shark like tiles on a roof. The arrangement and shape of these denticles affect how water flows across them as the shark swims, reducing drag and again reducing the amount of energy needed to swim. Their teeth and jaws also play a role. Sharks are one of the few marine predators capable of eating prey larger than themselves, as they can extend their jaws wider and have teeth that are specially adapted for cutting and tearing. This means that sharks can have highly calorific meals less often and are then able to spend the rest of their time effectively ‘resting’.
Sharks are also able to occupy a diversity of niches and habitat types because of their ‘superpowered’ adaptations [27.35]. These include their suite of powerful senses, which vary depending on the environment. Some sharks have incredibly powerful senses of smell, like those that live in the dark of the deep sea, whereas others, like reef sharks, have excellent vision capable of locating prey in the complex landscape of a coral reef. And most sharks can sense electricity, picking up tiny electrical signals given off by other living organisms. They can essentially feel your heartbeat! This special extra sense allows sharks to locate prey that would otherwise be hidden, and to navigate their environment. As well as being extremely well equipped in the senses department, some species – including thresher sharks, white sharks and basking sharks – can also heat up specific parts of their bodies to aid them in certain activities like sudden bursts of speed while hunting, or cruising the cold ocean depths. Known as regional endothermy, this adaptation contributes further to their efficiency. Keeping your whole body warm uses a lot of energy, but only warming certain parts when needed expends a lot less, while allowing you to do things other fishes can’t.
Essentially, sharks have a suite of adaptations that allow them to diversify and be incredibly successful – outlasting countless other animal groups and truly become guardians of the ocean. We think they’re amazing!
ABOUT OUR GUEST
DR JAMES LEA
CHIEF EXECUTIVE OFFICER OF THE SAVE OUR SEAS FOUNDATION
He 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 SOSF 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.
