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VEGETARIAN BONNETHEAD SHARKS MAINLY FEED ON PLANT MATERIAL AND ARE ABLE TO DIGEST IT.
Self-medicating orangutans and vegetarian sharks
Sharks are surely the last animals you’d expect to enjoy their vegetables. But bonnethead sharks are able to get by on little else.
Back in 2007, it was discovered that up to 62 per cent of the gut contents of these sharks was plant material from their preferred seagrass meadow habitat. What wasn’t clear was whether the sharks reap any nutritional benefit from all that salad. It was speculated that their short guts – typical of an exclusive meat-eater – might not even be able to digest it.
It turns out that they can. Biologists have now found that bonnetheads can thrive on a 90 per cent seagrass diet. They are able to break down the vegetable matter by means of an enzyme in their guts – missing in carnivorous animals – that targets the plants’ tough cell walls. What’s more, the biologists showed that the digested material becomes incorporated into the sharks’ body tissues.
So is it possible that omnivory has been overlooked in other predatory shark species? “It’s hard to say,” says Samantha Leigh of the University of California, Irvine. “We know that the scalloped hammerhead, a very close relative of the bonnethead, is carnivorous, despite thriving in similar regions and habitats.”
Plankton-feeding species such as basking and whale sharks, which inevitably ingest significant quantities of algae might also provide clues. “We do not know the extent to which planktivorous sharks can digest phytoplankton,” Leigh says.
She is currently investigating the source of the plant-digesting enzyme. Her suspicion is that it is produced by mutualistic micro-organisms living in the bonnetheads’ guts.
There is also the matter of what this quirky foodchain means for the ecology of the seagrass meadows. “Bonnethead sharks are abundant in seagrass meadow habitats,” says Leigh. “However, the role that they are playing is unclear now that we know they are not acting as the top predators.”