Hungry Sharks? How a Shark’s Digestive System Works
SHOW NOTES
Like most parts of a shark, the digestive system is a highly evolved, finely tuned system that is built for efficiency [7.45]. This drew Dr Leigh to the field of shark physiology and anatomy: “I’ve always been interested in how animals adapt to their environment, but sharks are particularly intriguing because they have evolved to be such successful predators over millions of years.” The variety in diet across all species raises interesting questions about how different sharks are able to optimise their digestive systems in order to get all the nutrients and energy they need to survive. “Obviously, the energy that they take in is important for all of their physiological functions, right? You can’t move, you can’t reproduce, you can’t do much of anything if you don’t have energy,” says Dr Leigh. “So I was really interested in trying to figure out how sharks got to be so good at getting that energy out of the very diverse types of food that they’re eating.”
With over 538 species of shark to choose from, Dr Leigh has no shortage of dietary types and digestive system morphologies to study [11.40]. There are the strict carnivores, who eat a range of prey species including fish, seals, sea lions, large marine mammals, and even smaller sharks. Then, there are the more opportunistic feeders who take advantage of whatever comes their way – invertebrates, seabirds, and even the carcasses of land animals that have fallen into the ocean (Greenland sharks, for example, have been found with polar bear and moose in their stomachs!). The planktivores feast on the smallest organisms in the ocean: plankton. They also happen to be the largest species of sharks in the world, the whale shark, basking shark and megamouth shark. And some sharks even eat plants! The bonnethead, for instance, is a small member of the hammerhead family that consumes seagrass (more on this later).
In order to eat such a diverse range of things, sharks have evolved a number of different mechanisms to ingest their food [16.10]. The plankton eaters, for example, have filter feeding mechanisms that allow them to separate tiny organisms – such as krill and copepods – from the surrounding seawater. They swim with their mouth agape, allowing water to enter the mouth and pass over the filtering mechanism. The seawater runs straight through and exits via the gills, but all the plankton of a certain size gets caught and swallowed by the shark in one big gulp. Carnivorous sharks, on the other hand, rely on very sharp, strong teeth, which are designed for biting, grasping, and tearing. A great white shark has teeth like a steak knife, which are serrated around the edges. This means they can cut through large chunks of flesh very efficiently. Sharks with more flexible diets have differently shaped teeth, allowing them to consume a greater range of items. Some species, for example, have flatter, more plate-like teeth for crushing shells (handy if you’re eating a lot of crustaceans). Or ambush predators, like the wobbegong, have long, very thin, needle-teeth, specially designed to help them grab slippery, fast-moving prey.
Once swallowed, the food then passes down the oesophagus and into the stomach [19.10]. Sharks aren’t exactly known for their table manners – many species gulp down large chunks of food, or even swallow their prey whole. This means they need stomachs capable of breaking down things that you or I would find pretty hard to digest, like bone and cartilage. “Sharks in particular have a very acidic stomach, even more acidic than ours,” explains Dr Leigh. “Ours is in the range of pH two to three – theirs is a little bit more acidic with a pH range of about one to two.” This allows them to process much larger volumes of protein, and break down harder-to-digest materials. But they have another tactic up their sleeve: turning their stomach inside out! “Some sharks have a really unique behaviour known as stomach eversion, which is a process that involves the shark basically turning its stomach inside out to expel any indigestible material,” Dr Leigh says. “It’s a really interesting adaptation because it helps the shark get rid of that unwanted or unprocessed material so that its digestive system can stay efficient. It can stay focusing on the stuff that it can break down.” Weird, but brilliant.
Beyond the stomach is another strange but fascinating adaptation, the spiral intestine [21.13]. Unique to the sharks, skates and rays, the spiral intestine looks like a spiral staircase in that there is a flap of tissue that moves in a spiral shape throughout the intestine. It has two main functions. One is to slow the rate that food passes through the intestine, allowing more time for nutrients to be extracted from it. The other is to increase surface area. “You have all these folds that the digesta is going to be touching and interacting with as it goes through, which is going to allow the shark to absorb a lot more of those nutrients. So it’s a really efficient way to make sure you’re getting as much as you can out of the food you eat.” It’s another reason in the very long list of how sharks have become so successful.
Dr Leigh is particularly interested in the spiral intestine, and exactly how it works [23.40]. In her research, she describes it as working like a Tesla valve, the invention the Tesla car company is named after. Invented by Nikola Tesla, the valve directs fluid in one direction without using any moving mechanical parts or energy input. Dr Leigh believes the spiral intestine works in a similar way. “Based on the direction and orientation of those spirals in the intestine, they’re able to very efficiently move things along.” She says. In addition, the smooth muscle of their intestine moves segmentally. This differs to our own, which has a peristaltic movement, almost like a wave. “It’s almost like squeezing a toothpaste tube to get the toothpaste to move from one end to the other.” In sharks, there are contractions happening all along the intestine, rather than one long movement. “You get this very slight back and forth movement to kind of help with the mixing, and help with retaining that digesta in there for longer periods of time. But ultimately still pushing them in the direction you’d want the digesta to go.”
Another important element of a shark’s digestive system is the liver [25.26]. Shark livers are very large relative to their body size in comparison to other animals, and filled with lots of oils and fats. The primary reason for this is that sharks use their liver to maintain neutral buoyancy in the water. Sharks are slightly heavier than water and so need something to stop them from sinking. Their liver is filled with a low-density oil, also known as squalene, which is lighter than water, helping them to stay at the depth they want to be at. It also plays an important function in digestion, storing nutrients, producing bile (which aids in the digestion of fats) and the removal of waste products and toxins. And, the liver can help sustain sharks in the absence of a meal. Some sharks go for long periods without eating: “I think the longest recorded is over a year…fifteen months or so that it was confirmed a shark was not eating,” says Dr Leigh. This happens during long migrations, as sharks cover vast distances of open ocean where food is potentially scarce. In these times of scarcity, sharks are able to use the nutrients stored in their livers as an emergency energy source.
As we follow the shark’s food on its journey through the digestive system, we inevitably end up…well, at the end! What goes in must come out, and for a scientist like Dr Leigh, shark poop is actually pretty exciting [27.40]. “As someone who has worked with shark poop…I can tell you that it is not easy to collect.” She laughs. “But I mean, they’re eating a lot of different things, right? And it contains remnants of undigested material. If some little bits of shell or bone make its way through the shark digestive system, then those things will come out and you can very clearly tell most of the time what they are.” This can give us important clues into a shark’s life, and provide vital information on what they’re taking in, and what they can’t digest.
Collecting shark poop was a large part of Dr Leigh’s PhD, which sought to understand how bonnetheads – those unusual, plant-eating sharks – are digesting and processing seagrass [31.20]. “So there was a study that had originally come out in 2007 that did the original gut content analysis for bonnethead sharks, and showed that their diet was, especially in juvenile sharks, up to 60 % seagrass,” Dr Leigh recalls. “When I read that paper, I thought that that was very strange. And decided I wanted to try to figure out how they’re able to do this. Because…we think of sharks as these kind of stereotypical top predators eating only meat. And here’s the shark that over half of their diet, in some cases, is a plant.” In order to understand this, Dr Leigh conducted a series of laboratory experiments where bonnethead sharks were collected from the wild, and brought into a controlled setting. There, they were fed a diet of 90% seagrass, and 10% squid. “We achieved this by – if you think of like sushi rolls, it was kind of like that!” Dr Leigh explains. “We had this little thin sheath of squid, and we would stuff a bunch of seagrass in the centre of it.” This meant that Dr Leigh knew exactly how much was going in. Then, by collecting the bonnethead’s faecal matter, she could ascertain how much had come out.
This unusual sushi restaurant produced some very interesting findings. Firstly, they were able to find that the bonnetheads were digesting a compound called cellulose – the main component of plant cell walls. “It ended up being over 50% of the cellulose that they were able to digest. Which, to put that in perspective, is on par with sea turtles,” says Dr Leigh. So how were they able to digest such large volumes? Well, Dr Leigh then looked into their digestive enzymes, and found that they had a specific enzyme dedicated to breaking down cellulose material. The more unusual thing is that there are no known vertebrates that can produce this enzyme on their own, meaning that the bonnetheads are relying on a symbiotic relationship between microbes or bacteria in their gut to make the enzyme, and help them break down the grass.
This study also had significant implications relating to the ecological role of bonnetheads in seagrass ecosystems [38.20]. “I think a lot of people probably hear that bonnethead sharks are eating grass and maybe that would actually be a bad thing for the grass that, you know, they’re destroying seagrass meadows or something, but that’s not the case at all,” Dr Leigh explains. “They’re actually likely helping to maintain the health of the seagrass ecosystems…by promoting the growth of new seagrass shoots, and working as a like nutrient distributor. So by consuming grass, locally migrating to maybe some other seagrass meadows and excreting, you know, their waste and their nitrogen and their nutrients into those seagrass meadows.” So, you can almost think of bonnethead sharks as ecological engineers, helping to restore and maintain the health of a critically threatened ecosystem – which is pretty cool!
And there is yet another way that sharks could potentially help the marine environment. Dr Leigh’s current project – funded by the Save Our Seas Foundation – looks at whether the shark’s spiral intestine can be modelled to create a kind of filtration system, that could be used to filter and remove microplastics from our waterways. “So it seems like as the [water] flows, as it’s moving through the spiral intestines, it’s able to kind of piece out like certain sizes of particles so that they’re kind of like sequestered in very specific parts of that structure, if that makes sense. So it’s almost like categorizing things of different sizes, if you will, as it’s coming through.” Dr Leigh, collaborating with engineers, has been experimenting with 3D models created from CT scans of shark intestines, to see if a bio-filtration system can be built from it. The idea is to scale-up the model so that it can be implemented into systems like stormwater run-off or wastewater treatment plants, to catch microplastics and prevent them from entering the ocean. “And this was something that we kind of stumbled upon on accident, honestly,” Dr Leigh recalls. We were looking at shark intestines to just try to figure out how they worked, because there really isn’t a lot of information about them in the scientific literature. When I was looking at papers, a lot of the papers kept saying the same thing over and over again and citing a paper from over a hundred years ago that had little sketch drawings of spiral intestines from a dissection, which was an amazing starting point and we learned a lot from that. But it was, you know, interesting to think, okay, how do we progress from this? How do we learn more about the structure that we haven’t really looked at very deeply in over 100 years?”
Another example of how sharks continue to surprise us.
ABOUT OUR GUEST
DR SAMANTHA LEIGH
Dr. Samantha Leigh is an Assistant Professor of Biology at California State University, Dominguez Hills, specializing in animal physiology. With a Ph.D. in Ecology and Evolutionary Biology from UC Irvine, her research focuses on the intricate relationships between animals and their environments. Dr. Leigh has studied the omnivorous bonnethead shark, gaining insight into its ecological role and behavior. Additionally, her work examines the impact of microplastics on commercially important fish species, shedding light on the environmental challenges facing marine ecosystems. Passionate about conservation and science communication, she works to bridge the gap between research and the public’s understanding of these critical issues.
Here’s Dr. Leigh’s Instagram.
