Shark brains: how are they wired?

SHOW NOTES

Have you ever wondered what a shark’s brain looks like? How the brain of a goblin shark compares to a mako? Or…what it’s like to get a shark brain in the post? These are all things that Dr Kara E. Yopak, associate professor of biological sciences, shark brain expert and zombie Queen knows a thing or two about! Kara is interested in what different shark brains look like and how these differences have evolved over time. She looks at variations in brain size between species, as well as differences in how the brain is organised and structured. This information can give us insight into how sharks live, their habitat, and even their behaviour. Grab your labcoats, because neuroscience is served…

Before we get into the fascinating world of comparative neuro-anatomy, we go back to the beginning and learn how Kara go into studying shark brains in the first place. Her obsession with sharks goes back to childhood, where Kara developed a lifelong passion for the ocean [5.10]. Although she didn’t grow up by the sea, trips to the beach were a family event, and Kara remembers her family struggling to get her out of the water! At the tender age of 5, Kara announced she wanted to be an ichthyologist – the fancy word for a scientist who studies fishes [6.15]. Kara’s mother has written proof of this. In a Dr Seuss ‘All About Me’ book, underneath a section titled “when I grow up…”, Kara had scrawled “I want to be an ichthyologist, who studies sharks like Eugenie Clark”. Most impressively, ichthyologist was spelt correctly – quite the achievement for a 5-year-old, and a sign of the career to come…

Kara’s passion for sharks started very young, but her love for studying their brains came much later [10.00]. While studying at the University of Boston, Kara became interested in shark behaviour and sensory cues. She wanted to know where they went and why. She went on to study for a PhD at the University of Auckland, New Zealand, where her prospective supervisor told her to “read lots of papers” to decide on a topic for her thesis. Kara did just that – devouring literature on shark behaviour and sensory cues. But every time, her research led her back to the brain. It was during this time that Kara came across a 1978 paper, that looked at variation in brain size across 12 species. It showed that contrary to popular belief, sharks didn’t have tiny brains – in fact, they had brains that were of similar size to birds and mammals. This was a revelation to Kara, and she decided she wanted to explore this area of research further. She started out studying electrophysiology – a branch of neuroscience that explores the electrical activity of neurons, and how they send signals across the brain – but this proved to be incredibly difficult. Struggling to get anything to work, Kara turned to her PhD supervisor, who then imparted a central piece of wisdom that defined Kara’s entire career: “just go and collect some brains”. This is exactly what Kara did next. Her PhD grew into a project that looked at 40-50 shark brains in total, which revealed some patterns in how the brains varied in both size and structure. These variations could be linked to the ecology and life history of the sharks – an area of study that Kara has pursued to this day.

The aptly named ZoMBiE lab, which Dr Yopak directs, was named after Kara observed her students at conferences, introducing themselves to other delegates before politely asking if they had any shark brains lying around [16.00]. It was this mass pursuit of brains that gave Kara the idea for the name of the lab, which officially stands for Zootomical Morphology of the Brain and its Evolution. Over the years, Kara’s lab and its purpose have become so well known that Kara and her team now receive shark brains in the mail, carefully packaged and prepared.

Kara and her team at the ZoMBiE lab are interested in a few different things, all centred around the same theme of comparative anatomy and evolution [19.20]. The first is understanding that variation across species, and making inferences as to the function of this variation. For example, why do some sharks have larger regions of the brain dedicated to movement, or spatial learning, than others? Kara now has a comparative dataset of over 180 species, which is substantial enough to start identifying commonalities that correlate to different habitats, lifestyles, and even behaviours (more on this later). Second, the ZoMBiEs also examine how the brain varies throughout a shark’s life. One of the most interesting things about shark brains is that they have a lot of plasticity [21.30]. This means that they can constantly regenerate neurons throughout their whole life, which is something that we humans cannot do. When we’re born, we have pretty much all the neurons we’re ever going to have, which is why brain injuries are such bad news for us. Sharks, however, have the ability to make new neurons – repairing and changing their brain as they age.

Finally, Kara is very interested in understanding how humans are impacting the brain development of sharks [23.50]. Human-generated stressors, like climate change, could potentially alter a shark’s brain, impairing their abilities.

Before we dive deeper into these questions, we first have to take a very quick anatomy lesson to understand the basics – shark brain 101, if you like [26.40]. As Kara explains, a shark’s brain is modular. Like our own, you can break the brain down into different components, all with different functions. For example, a region of the brain is dedicated to processing odour (in sharks, this is known as the olfactory bulb), while another is linked to vision. Certain regions of the brain control how a shark moves it’s body, while others are responsible for memory and spatial learning. Our brain is the same. In fact, a shark’s brain is similar to that of all other vertebrates, because sharks were the blueprint for all vertebrate brains. They were the first to develop all the key components that we all share, including Kara’s favourite, the cerebellum [30.00]. We still aren’t entirely sure of all the functions of the cerebellum – we think it is linked to things like motor control, but the rest remain a mystery.

So, now we know the basic structure of a shark brain, how does this blueprint differ between species [35.10]? They differ in many varied and nuanced ways, not only in overall size, but also in the size and organisation of their components. Kara has studied a lot of brains over her career, and is now in the position where she could be handed any random brain and predict what species, or at least what type of shark, it belongs to. Brains that look similar tend to come from sharks that share similar habitats, lifestyles, and even behaviours. For example, deep sea sharks – whether they are closely related or not – all seem to converge on a very similar brain pattern. They are relatively small in comparison to their body size, but have a large region to process scent, and even larger regions dedicated to processing information from their sixth senses: lateral line and electroreceptors. In contrast, the part of the brain responsible for vision is comparatively small, which makes sense. If you’re living in deep, dark waters, where light doesn’t penetrate, your other senses will be much more advantageous than sight when it comes to finding prey, or a potential mate. In contrast, reef sharks – who live in 3D, spatially complex, and well-lit environments – have a brain that is almost the complete opposite of a deep sea shark. The largest regions of their brain are those that process visual cues, and those responsible for spatial learning and memory. When hunting on a reef, you’d have to remember all those little nooks and crannies to find where dinner is hiding!

Kara has also studied the largest species of shark in the world, the whale shark, and found that their brains are small in comparison to their body size. This isn’t unique to whale sharks, either – many species, including the great white, have relatively small brains. But does size matter? Well, the answer is more complex. Kara’s whale shark study is a prime example. It looked entirely different to how she’d expected, not least because it had the largest cerebellum she had ever seen. This part of the brain was extremely complex and highly folded, which at first stumped Kara. She had always associated large, complex cerebellums with highly active and mobile species, given its function in motor control and learning (essentially, how a shark moves its body). Whale sharks, on the other hand, are not known for their agility. However, as Kara mentioned previously, much of what the cerebellum does is still a mystery. She proposed that the large, highly folded cerebellum of the whale shark served to coordinate its enormous body, helping it to manoeuvre in such a complex environment as the open ocean. Further studies added weight to this theory, showing that the size and complexity of the cerebellum correlated, among other variables, with body size. In summary, there is so much more to understanding shark brains than their size – it also involves looking at the degree of folding, complexities, and the size of the different components.

But what does this all mean for shark intelligence [49.20]? How smart are they? This is Kara’s least favourite question, partly because there isn’t really a proven answer. Defining intelligence in other non-human species is incredibly difficult, and we can’t base it off our own idea of what intelligence is. For example, sharks aren’t solving math equations every day, but they are processing huge amounts of sensory information that we can’t even detect. There is still a myth that sharks are mindless killers, or lack intelligence. But we do know that they are capable of incredible, complex behaviours – such as recognising familiar sharks, or even problem solving – and Kara believes that many people would be surprised at their ability to learn and adapt. She wonders if we should base our measure of intelligence on behavioural flexibility; the ability to adapt to new challenges and environments. And given how successful sharks have been, surviving through five mass extinctions and occupying a huge diversity of niches, it’s safe to say they aren’t as cognitively simple as we might think.

ABOUT OUR GUEST

DR KARA E. YOPAK

ASSISTANT PROFESSOR AND DIRECTOR OF THE ZoMBiE LAB, UNIVERISTY OF NORTH CAROLINA WILMINGTON

Dr.  Kara Yopak received her B.A. in Biology (with a specialization in marine science) from Boston University in 2002 and completed her PhD at the University of Auckland in New Zealand in 2007 where she first started to explore variation in the brain of sharks. She was fortunate to serve as a Postdoctoral Fellow at the University of California San Diego and later the University of Western Australia. Now, her “ZoMBiE Lab” is based at the University of North Carolina Wilmington within the Department of Biology and Marine Biology.

Her research interests lie in the evolution of the brain within and across cartilaginous fishes, particularly the ways in which variation in brain size, structure, and cellular composition underlies complex behaviors and even “intelligence” in sharks and their relatives. Dr. Yopak and her students uses novel techniques, such as magnetic resonance imaging (MRI) and flow cytometry, coupled with traditional methods, to describe variation in the brain across this fascinating group of fishes and how we can use this variation to make predictions about their life history, ecology, and behavior.

@ProfSharkbrain; @YopakZoMBiELab

https://yopaklab.com