Sharks often get cast as the villains of the ocean.
They are portrayed as deadly and merciless killing machines that just attack hapless
swimmers and “infest” coastal waters, when really, ya know that’s where they live.
While it is true that they are predators, they’re generally pretty uninterested in eating us.
And there’s one shark that’s even less interested in seeing us as prey, because it
would rather eat seagrass, instead.
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Say hello to the bonnethead shark.
The bonnethead is the smallest member of the hammerhead family, with a head that reads
more “shovel” than “hammer.”
The females are larger than the males and they max out at around 1.5 meters long.
In adult bonnetheads, you can tell the difference between the males and females by looking at
the shape of their heads.
In females, the front edge of the head is smoothly rounded, while in males, the edge
is more pointy.
And they’re the only known shark species where the sexes have different head shapes!
They’re found in coastal waters of the Pacific Ocean, from southern California to Ecuador,
in the Atlantic from Rhode Island to Brazil, and throughout the Gulf of Mexico and the Caribbean.
And many of these waters are home to seagrass meadows,
“the most widespread coastal ecosystem on earth.”
Now, for a long time, we thought bonnetheads were just hanging out in seagrass meadows
to forage for prey, like crabs, shrimp, and snails.
Other hammerheads usually don’t eat invertebrates like these, but bonnetheads have flat, molar-like
teeth in the back of their mouths for crushing hard shells.
And, while the bonnetheads were foraging, scientists would see them suck up and consume
seagrass, way more seagrass than they would expect any shark to eat.
But you can eat all kinds of things that don’t actually do anything for you, nutritionally,
like when your cat eats grass.
And if you’re a shark, scientists are going to assume that you’re a carnivore, because
that’s how sharks are supposed to work.
They don’t seem to have any of the adaptations we’d expect a non-carnivore to have.
But, not only are bonnetheads eating a lot of seagrass, up to 62% by mass of their gut
contents, they are digesting it.
That makes them the only omnivorous shark that we know of.
And how we figured out that they are actually digesting the seagrass is pretty cool.
First, the then-grad student behind the research, Samantha Leigh, had to catch some sharks.
She set up a net in the Florida Keys and snagged five live bonnetheads to bring back to the
lab for controlled feeding experiments.
She fed them a diet of 90% seagrass and 10% squid, and found that the sharks gained weight!
By comparing the carb, lipid, and protein contents of what she was feeding the sharks
to what quantity of those things they pooped out, she could calculate what percentage of
each macronutrient the sharks were digesting.
And it turns out, they’re about as good at digesting seagrass as juvenile green sea
turtles are, and about twice as good at it as pandas are at digesting bamboo!
They also have enzymes and gut microbes that seem to help with breaking down cellulose,
the structural component of the cell walls of plants, which other sharks aren’t known to have.
So bonnetheads are eating seagrass and digesting it, but the researchers wanted to
show that the sharks are assimilating the plant molecules into their tissues, too.
If they’re doing all of these things, it’d be pretty hard to argue
that bonnetheads aren’t true omnivores.
So the researchers carried out what’s called ‘stable isotope analysis,’ which is basically
a scientific method to show that ‘you are, indeed, what you eat.’
Before they fed the sharks, they tagged the seagrass with an isotope of carbon called carbon-13.
An isotope is just a form of an element with a different number of neutrons than it usually has.
Carbon usually has 6 neutrons; carbon-13 has 7.
And after their feeding experiments, the researchers found an increase in the carbon-13 signature
in the blood and liver tissue of the sharks.
This shows that the bonnetheads were incorporating the carbon-13 from their diet into their tissues.
Taken together, all the different lines of evidence make it pretty clear:
bonnetheads sharks are grass-eating sharks.
Which is definitely weird enough on its own to make bonnetheads an excellent bizarre beast,
But it’s not the only very weird thing about them!
They’ve also been observed to reproduce, on their own.
In 2001, a female bonnethead at a zoo in Nebraska gave birth to a pup, without mating with a male.
She’d been collected from the wild as a juvenile three years earlier and there were
no male bonnetheads in the zoo’s tank, though there were males of other shark species.
To say the staff there were pretty surprised is an understatement.
Unfortunately, the pup didn’t make it, it was bitten by a stingray in the tank,
which led to its death.
But genetic testing of the pup later revealed that it was not a hybrid.
And that it also had no father.
All of its DNA could be matched with its mother’s genetic profile.
The female bonnethead had reproduced through a process called parthenogenesis,
which literally means ‘virgin birth.’
And while parthenogenesis has been seen before in other vertebrates, like some reptiles,
amphibians, and birds, this was the first known case of it in sharks!
But it wasn’t the last; shortly after that paper was published, several more cases of
different species of sharks in aquariums giving birth without mating were reported.
So, it looks like this thing that, at first, seemed incredibly strange, might just be a
thing that’s actually kind of common in sharks.
Between this and the bonnethead’s odd plant-based diet, it looks like we should be wary not
so much of sharks themselves, but of making assumptions about them!
The Bizarre Beasts pin club subscription window is open from now through the end of May 8th.
Due to some delays with manufacturing, these fantastic, adorable little sharks will start
swimming to your house at the end of May.
But once you’re in the pin club you should see the pins after that around the time each
new video goes live.
This bonnethead shark pin has the best little face!
I am so excited to get mine!
You can always find us on Twitter @BizarreBeasts, and on Instagram and Facebook @BizarreBeastsShow,
where we will share more shark facts.
And, as always, profits from the pin club and all our merch go to support our community’s
efforts to decrease maternal mortality in Sierra Leone.
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