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Thursday 31 March 2022

The plant-eating shark

Sharks are normally carnivorous, but there appears to be one that bucks the trend. The bonnethead.

The bonnethead shark eats seagrass, and scientists think it may be omnivorous.

But not everyone agrees: dissenters say the shark is eating the seagrass by accident, and that's why baby bonnetheads (which are immature hunters) eat more than grown-ups. In fact, in babies, 50-60% of their stomach contents is plant! They also argue that the seagrass can't replace hunting; i.e. the protein level is too low by percentage.

So those scientists invested in the omnivorous shark theory designed a few tests.

Seagrass. Sofia Sadogurska via Wikipedia Commons.

The first one tried bonnetheads on seagrass bait: and found they were drawn to it, even though bonnetheads are known to have a good sense of smell, so definitely knew what the bait was.

Dissenters also pointed out that the shark could be eating the seagrass deliberately, but for non-nutritional purposes. For example, cushioning the stomach against the spiny shell of the blue crab, a dietary delicacy for bonnetheads. This is possible, but, notably, it's not seen in other blue crab eaters (and the blue crabs definitely have more to worry about than just the bonnethead).

So researchers did a second test. In captivity, they fed bonnetheads a diet made up on 90% seagrass (more than they usually eat in the wild) and marked it with 13-carbon isotopes. The aim was to see whether they actually incorporated any of the seagrass into their bodies – i.e. whether they digested it – or whether it just passed through. The researchers claimed that if they digested it, it was nutritionally valuable to them, and therefore they were omnivores.

So what did they find?

Bonnethead shark – the only omnivorous shark. By D Ross Robertson via Wikipedia Commons.

Bloods, tissues, and faeces were tested, and all of them contained 13-carbon isotopes. So some of the seagrass was digested, and some of it (about 50%) passed through them! So perhaps they're not very efficient omnivores, but the evidence suggests that onmivores the bonnetheads are.

In a third test, researchers also looked for enzymes that could break down cellulose in the shark guts and found one promising candidate: b-glucosidase, which seems to confirm the isotope findings. The next question to answer is how long these enzymes have been there, and how they developed uniquely in a single shark species.

Read more about sharks in our article on the topic.

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