Whale sharks may be the world’s largest omnivores

As Mark Meekan bobbed amongst swells within the Indian Ocean, he noticed a large shadowy determine shifting by way of the water. The tropical fish biologist dove to fulfill the mild big  — a whale shark — and took some samples of its pores and skin. Swimming alongside these aquatic giants is nothing new for Meekan, who works on the Australian Institute of Marine Science in Perth. However “having an encounter with one thing that feels prefer it’s from prehistory is an expertise that by no means will get outdated,” he says.

Averaging round 12 meters lengthy, the whale shark (Rhincodon typus) is the most important residing fish species on Earth and among the many most mysterious. Since these sharks spend the vast majority of their lives within the deep ocean, Meekan says, analyzing the chemical make-up of their tissues will be vastly highly effective for studying extra about the animals’ fundamental biology (SN: 7/14/20) and conduct — together with what they prefer to eat.

The pores and skin samples that Meekan and colleagues collected and analyzed reveal that whale sharks, lengthy regarded as strict meat-eaters, additionally eat and digest algae. The findings, described July 19 in Ecology, add to a rising physique of proof that whale sharks intentionally eat vegetation, making them the largest omnivores on Earth. The earlier record-holder, the Kodiak brown bear (Ursus arctos middendorffi), averages about 2.5 meters in size.

Algae has turned up earlier than within the stomachs of beached whale sharks. However due to how whale sharks feed — by swimming mouth-open by way of swarms of zooplankton — “everybody although it was simply unintentional ingestion,” Meekan says. Carnivores sometimes can’t digest plants, so some scientists suspected that algae handed by way of whale sharks’ guts with out being digested.

To search out out if that assumption held up, Meekan and colleagues adopted whale sharks because the fish gathered close to Ningaloo reef off the coast of Western Australia in 2017. Because the gigantic fish are effectively camouflaged from the ocean floor, the crew situated 17 people that had surfaced to feed from an airplane. They then intercepted the sharks by boat and jumped into the water to snap photos, scrape off parasites and acquire tissue samples.

Meekan says most whale sharks don’t react once they get jabbed by the spear, which is roughly the width of a pinky finger. Some even appear to take pleasure in consideration from researchers, he says. It’s as if they assume: “This isn’t threatening. In truth, I fairly like that.”

The whale sharks at Ningaloo reef had been wealthy in arachidonic acid, an natural molecule present in brown algae known as sargassum, the evaluation confirmed. Because the sharks can’t make this molecule themselves, they in all probability acquired it by digesting algae, Meekan says. It’s not but clear how arachidonic acid impacts whale sharks.

The brand new work helps earlier analysis by an unbiased group that found plant nutrients within the pores and skin of whale sharks off the coat of Japan. Taken collectively, the findings counsel that digesting greens is frequent follow for whale sharks.   

However that doesn’t imply that whale sharks are true omnivores, says shark biologist Robert Hueter of Mote Marine Laboratory in Sarasota, Florida. “Whale sharks soak up quite a lot of different issues than the meals they’re concentrating on,” he says. “It is a bit like saying that cows are omnivores as a result of they eat bugs whereas feeding on grass.”

Whereas Meekan admits that he can’t say for sure that whale sharks particularly hunt down sargassum, the quantity they eat isn’t incidental both. His crew’s evaluation clearly reveals, he says, that plant materials in actual fact makes up a really massive a part of their weight loss program. It’s so massive, he notes, that whale sharks and the zooplankton that in addition they eat appear to occupy comparable rungs on the marine meals chain, each only one rung above the phytoplankton they each feast upon. 

No matter whether or not whale sharks actively hunt down plant snacks, the animals do have the power to digest them, Meekan says. “We don’t see whale sharks that always, however their tissues maintain a exceptional report of what they’ve been as much as,” he says. “We’re now studying the best way to learn this library.”

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