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The world’s first omnivorous shark

Though a member of the hammerhead family, the bonnethead shark is much smaller and has a narrower, more rounded head Though a member of the hammerhead family, the bonnethead shark is much smaller and has a narrower, more rounded head
15 Jan
2019
Bonnethead sharks, the second smallest member of the hammerhead family, have been shown to not only eat, but digest seagrass, making them the first omnivorous shark known to scientists

It’s a common complaint among shark-lovers – that these majestic creatures are misunderstood, too often associated with the menacing tones of Jaws and deemed brutal man-eaters. A recent shark-related discovery reveals that this impression could be even further from the truth than their fan-base realised.

Samantha Leigh, a researcher at the University of California Irvine has made it her mission to find out more about the nutritional physiology of sharks. When she embarked upon the project she was surprised to find relatively little information available on shark diets. This was until she came across a research paper that analysed the contents of the stomachs of bonnethead sharks, a small shark common in tropical and subtropical waters with a distinctive shovel-like head. The research revealed that in some populations, the material in the sharks’ guts was up to 62 per cent plant material.

Despite these findings, most people thought little of it. The assumption that sharks could only be carnivores, cemented by their blood-thirsty reputation, proved persistent. ‘A lot of people brushed it off and thought they probably just eat plant material and pass it through and nothing is really digested from it,’ says Leigh. ‘But no one had really tested that before.’

To find out more, Leigh uprooted to the Florida Quays, whose waters contain abundant populations of bonnetheads. In the lab, she conducted feeding trials, offering the sharks a diet made up of 90 per cent seagrass and ten per cent squid. ‘We measured out the amount of seagrass we wanted, measured out the amount of squid and then wrapped the seagrass in a really thin sheath of the squid – almost like a little sushi role. They ate it all right up. They all gained weight on this almost totally seagrass diet,’ she says.

shutterstock 197239355Understanding the diet of the bonnethead sharks is important for protecting the seagrass meadows

The fact that the sharks gained weight suggested that they must have the capacity to digest the seagrass. Analysis of their faeces and blood proved that they were indeed extracting and utilising nutrients from the plants, something previously thought impossible. ‘We found that they were digesting a little over half of the organic matter in seagrass, which may not sound like a lot but is pretty on par with sea turtles who eat the same type of grass and are known omnivores and even herbivores as they age.’ These most famous carnivores of the sea turned out to enjoy their greens.

Leigh’s findings have important implications for understanding the ecosystems of the seagrass meadows in which bonnethead sharks live. ‘I’m looking to use the information to find out how to better protect seagrass meadows,’ says Leigh. ‘A lot of them are not doing so well right now, not due to the sharks eating the grass necessarily but due to warming waters, ocean acidification, fishing trawls that damage meadows on the ocean floor and things like that. The more that we understand about how the ecosystem works in these areas, the better we will be equipped to come up with management strategies that will work efficiently.’ How sharks break down food and what they excrete back into the environment is key to this understanding.

Though the varied diet of the bonnethead sharks doesn’t mean that all sharks are practising fleixitarianism (the Great White is unlikely to forgo its carnivorous diet any time soon), Leigh sees no reason why other, smaller sharks might not also be omnivores. ‘There are plenty of other small, coastal sharks that live in very close quarters to, and with ample access to, different types of vegetation,’ she says. ‘This is the first shark that this has been seen in, but I wouldn’t be surprised if years down the road we find that other sharks are capable of this as well.’

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