How powerful is a shark’s sense of smell?
SHOW NOTES
This week we are diving into the fascinating world of snoot science with aquatic sensory biologist Dr Lauren Simonitis! This episode includes the ins and outs of shark noses, a shark that eats its greens, and a type of whale that inks while it poops. And we break down some famous myths about sharks…do they really go into a feeding frenzy from the mere scent of blood?
The episode starts as always with our guest’s most memorable ocean experience [4.59]. Lauren remembers a very special field trip to the Galapagos during her undergraduate degree. She was exhausted from some intense chemistry and mathematics courses, but the trip reminded her of why she got into the field in the first place. Especially when the world’s largest shark – a whale shark – cruised past during a snorkel!
So, what is an aquatic sensory biologist [8.00]? Lauren investigates how marine animals sense their environment, particularly through sense of smell. As they swim through the water, they are constantly feeling their environment, through a variety of ways. Sharks have senses that we don’t possess, such as electroreception, and this constant stimulation for a human would be sensory overload! Lauren is interested in how sharks are able to manage all of these senses, and how they use them together to understand their world.
But, Lauren’s journey into the world of snoots didn’t start with sharks [9.45]. Rather, it was a sea slug! Lauren was very interested in neurobiology, which investigates what we can learn from other species to help inform human health, especially regarding things such as neurological diseases. California sea hares are a model species to study the human nervous system. But this field of study required some pretty intense chemistry modules, which didn’t quite work for Lauren. Instead, she turned to the field of neurophysiology; the understanding of how our brains and sensory systems work. For her PhD, Lauren joined a research team who were studying pygmy sperm whales, a relatively small species of whale that can – wait for it – ink as a form of defence. Not only that, but they ink while also excreting at the same time, sending a cloud of red-coloured ink and poop at any unsuspecting predator. What a combination! Lauren’s thesis looked at the neurobehavioural mechanisms of ink as an anti-predator strategy, which involved seeing how an actual predator of inking organisms would react to ink – enter the bonnethead shark. And the rest is history. Lauren now studies the form and function of shark olfactory systems, as well as the role of ink in predator-prey relationships.
So how does she do this [14.30]? Lauren has a few ways of looking inside a shark’s head to examine the olfactory system. One is CT scanning. This is the same technique that a doctor would use at the hospital – lots of x-rays are taken while the subject is slowly rotating, and these images are then compiled to generate a 3-D image. However, sharks are lacking bones, so lauren has to first ‘dunk’ them in a special staining substance that allows the scan to pick up their tissues. Another method is dissection, taking tiny fragments of the shark’s tissues and examining them through a Scanning Electron Microscope to see the surface of the samples in hi-definition. And finally, Lauren also uses histology. This involves taking incredibly thin slices of the shark’s tissue, to look at all the layers that make up the nose.
Although the exact structure of the olfactory system differs between species, there is a basic pattern that all sharks – and teleost fishes – follow when it comes to the nose [17.30]. They have paired nostrils, meaning one at each side of the head. Unlike our own noses, which breath in and out with both nostrils, water goes in one shark nostril (the incurrent) and out the other (excurrent). Therefore shark’s don’t sniff! The water passively moves into the incurrent nostril as they swim. Once in, the water flows past the olfactory rosette. This is a very cool structure that Lauren describes as shaped like a flower, with petals made of lamellae – the sensory cells that send signals to the brain when they detect a chemical.
That is the basic ‘blueprint’, but with over 500 species of shark, the layout of the olfactory system varies widely [19.20]. For example, hammerhead sharks – with their elongated head – have long, tubular noses. In fact, most of their head is a nose. Whereas dogfish have more of a ball-shaped structure. The species Lauren has studied the most closely is the bonnethead [21.20]. This is a small species of hammerhead, with a very large, shovel-shaped head that gives the impression that the shark is wearing a bonnet (hence the name!). A cool fact about them is that they are the first known omnivorous shark; they have been observed eating seagrass! With such a big head (and therefore big nose) they are relatively easy to study. Lauren has looked at their noses in great detail, and describes their olfactory system as looking like plates stacked in a dishwasher, with many lamellae laid out across the head in rows.
So what do these differences mean [24.10]? The answer is, we don’t really know yet! Lauren’s colleagues have studied the sensitivity of different shark noses, using an electrode placed next to the shark’s head. The electrode would pick up any electrical activity generated by the sensory cells sending a message to the brain. They found that all sharks, regardless of head size or olfactory arrangement, had the same sensitivity. Not only that, but the sensitivity matched that of teleost (bony) fishes. It seems that the idea of sharks as ‘super noses’ may not actually have that much weight to it!
That being said, Lauren and her team do have a second theory. They believe the different nose shapes might influence how water flows through the nose, which in turn might impact their sense of smell. But more research is needed…
We now take a detour into Lauren’s fascinating PhD research, looking at the neurobehavioural mechanisms of ink as an anti-predation strategy across three unrelated taxa: the pygmy sperm whale (that species that inks and poops at the same time), sea hares, and cuttlefish [29.25]. Sea hares are able to recycle the pigments in red algae, and use it to produce a rich purple-red ink. Cuttlefish on the other hand produce ink using melanin, the same skin pigment found in humans. Lauren’s research sought to understand how these inking mechanisms evolved independently of one another, and found that they all had a similar origin – as a detoxifier. In all three species, the inking mechanism could be linked to the gut, suggesting that the ink first started as a way to get rid of nasty stuff from the body. However, it also served a useful purpose to stop yourself getting eaten, so those that were able to produce ink survived to pass on their genes. It’s actually incredible if you think about it – three completely unrelated groups of animals, all evolving the same strategy independently of one another.
And here is where the bonnethead re-enters our podcast [35.20]. Lauren wanted to test how predators responded to the ink, so she experimented with bonnetheads in tanks at a local aquarium. Her methods included shooting different substances into the water whenever the shark approached her ‘injection station’. She tested a few different hypotheses by injecting different scents and smells (such as the shark’s preferred food, to test that they were still capable of having a positive reaction to scent), food colouring (to see if they were responding to colour rather than smell), plain seawater (to see if it was the action of getting sprayed in the face put the shark off) and finally, the ink. She found that the bonnetheads would be either indifferent or show interest in all the substances except for ink. In fact, they showed a strong negative response to ink – in Lauren’s words, they hated it.
The next step was to find out what it was about the ink that deterred the sharks [38.00]. One theory was the consistency. In all three inking animals, the ink was released with a certain amount of mucus. By altering the amount of mucus, they could change the shape of the ink cloud that was produced – even creating ‘decoy’ animals, like octopus! Sea hares also have a special compound that can cause the ink to coat the sensory organs, which you can imagine would be very unpleasant. But Lauren’s current research has proved that bonnetheads are smelling the ink, and having quite an intense response, which suggests that the main deterrent is how bad it smells.
Lastly, we address a popular myth about sharks – do they go into a feeding frenzy because of the smell of blood [45.52]? There is a common misconception that if you go into the ocean with an open cut on your leg, that all the sharks for miles around will flock to you, an idea that is still perpetuated by Hollywood. But there isn’t much truth to this. Lauren’s studies have shown that sharks are perfectly capable of ignoring the scent of blood. There are a lot of factors that influence whether a shark will go for the source of blood, including hunger and food availability. And what’s more, that’s for fish blood! Although it’s hard to study, it seems likely that sharks don’t actually like the taste of humans – so, the chances of them going “Bruce-from-finding-Nemo-crazy” after catching a whiff of blood from your papercut are pretty slim.
ABOUT OUR GUEST
DR LAUREN-EVE SIMONITIS
Lauren is an aquatic sensory biologist interested in how animals use their sensory systems to understand their environment and impact their role in predator-prey relationships. From the prey perspective, she focuses on how animals use ink as a chemically mediated antipredator defence. Lauren also studies the olfactory system of a common ocean predator- sharks- by describing the general morphology, distribution of sensory structures, and fluid dynamics of different Chondrichtyhan nasal morphologies. Lauren employs a suite of biovisualization techniques including histology, scanning electron microscopy (SEM), and computed tomography (CT) scanning to visualize the noses of sharks and their relatives. Lauren received a dual B.S. in Marine Science and Biology from the University of Miami and a Ph.D. in Marine Biology from Texas A&M University.
Instagram and twitter: @explauren
