National Geographic: Algae-eating shark surprises scientists

[Global] Whale sharks are the largest fish in the sea, though as filter feeders, they don’t have the same bloodthirsty reputation as their kin. Still, they are sharks, so it’s long been believed these gentle giants rely almost exclusively on animal protein.

That’s not what an intriguing new study published this month in the journal Ecological Monographs found, though. Careful investigation of blood and tissue samples from over a dozen whale sharks suggests that they actually have a pretty omnivorous diet that includes plants and algae.

The research team, led by University of Tokyo biologist Alex Wyatt, used a combination of samples from captive and wild sharks to demystify the feeding habits of these enigmatic ocean travellers. While previous studies had found seaweed in whale shark stomachs, this is the first study to suggest they might ingest such algae as a dietary staple.

“Whale sharks are a very charismatic creature that is globally threatened, but we still don’t know enough about their ecology for effective conservation,” Wyatt tells National Geographic. “I am very keen to contribute to an improved understanding of the species.”

Studying what an animal eats is “fundamental stuff,” says whale shark biologist and vice president of research and conservation at the Georgia Aquarium Alistair Dove, but “it’s also central to the sort of population models that are necessary when you are trying to develop enlightened conservation plans for an endangered species.”

Those making conservation decisions need concrete information about a species’ habits, including details like how individuals move around and what they need to eat, to determine the best ways to protect it. Tagging studies have elucidated the movement piece of the puzzle, but it’s basically impossible to follow these very mobile fish around all the time to see what they’re eating. And that’s why Wyatt and his team’s approach is so appealing—it uses samples of blood and other tissues that can be opportunistically collected to look back at what an animal has been eating.

“It’s a great addition to knowledge of whale shark foraging and diet,” says Clare Prebble, a senior scientist with the Marine Megafauna Foundation.

Feast or famine lifestyle

To determine what the animals eat, the research team measured the different forms, or isotopes, of key atoms like nitrogen and carbon in the blood and tissue samples. The proportion of these isotopes differs in different food sources like algae, zooplankton, and fish, so looking at the ratios of these atoms in the sharks’ tissues can tell researchers a lot about what the animals are eating.

 

View original article at: Veggie-eating shark surprises scientists

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