As Mark Meekan bobbed amongst swells within the Indian Ocean, he noticed an enormous shadowy determine shifting via the water. The tropical fish biologist dove to satisfy the light 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. But “having an encounter with something that feels like it’s from prehistory is an experience that never gets old,” he says.
Averaging round 12 meters lengthy, the whale shark (Rhincodon typus) is the most important dwelling fish species on Earth and among the many most mysterious. Since these sharks spend nearly all of their lives within the deep ocean, Meekan says, analyzing the chemical make-up of their tissues might be vastly highly effective for studying extra concerning the animals’ elementary biology (SN: 7/14/20) and conduct — together with what they wish to eat.The pores and skin samples that Meekan and colleagues collected and analyzed reveal that whale sharks, lengthy considered 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 crops, making them the most important omnivores on Earth. The earlier record-holder, the Kodiak brown bear (Ursus arctos middendorffi), averages about 2.5 meters in size.
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Algae has turned up earlier than within the stomachs of beached whale sharks. But due to how whale sharks feed — by swimming mouth-open via swarms of zooplankton — “everyone though it was just accidental ingestion,” Meekan says. Carnivores sometimes can’t digest flowers, so some scientists suspected that algae handed via whale sharks’ guts with out being digested.
To discover 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. Since the large fish are effectively camouflaged from the ocean floor, the workforce positioned 17 people that had surfaced to feed from an airplane. They then intercepted the sharks by boat and jumped into the water to snap footage, scrape off parasites and accumulate tissue samples.
Meekan says most whale sharks don’t react after they get jabbed by the spear, which is roughly the width of a pinky finger. Some even appear to get pleasure from consideration from researchers, he says. It’s as if they assume: “This isn’t threatening. In fact, I quite like that.”
The whale sharks at Ningaloo reef have been wealthy in arachidonic acid, an natural molecule present in brown algae referred to as sargassum, the evaluation confirmed. Since the sharks can’t make this molecule themselves, they most likely acquired it by digesting algae, Meekan says. It’s not but clear how arachidonic acid impacts whale sharks.The new work helps earlier analysis by an unbiased group that discovered plant vitamins within the pores and skin of whale sharks off the coat of Japan. Taken collectively, the findings recommend that digesting greens is widespread observe for whale sharks.
But that doesn’t imply that whale sharks are true omnivores, says shark biologist Robert Hueter of Mote Marine Laboratory in Sarasota, Florida. “Whale sharks take in a lot of other things than the food they’re targeting,” he says. “This is a bit like saying that cows are omnivores because they eat insects while feeding on grass.”
While Meekan admits that he can’t say for sure that whale sharks particularly search out sargassum, the quantity they eat isn’t incidental both. His workforce’s evaluation clearly exhibits, he says, that plant materials the truth is makes up a really giant a part of their food plan. It’s so giant, he notes, that whale sharks and the zooplankton that additionally they eat appear to occupy related rungs on the marine meals chain, each only one rung above the phytoplankton they each feast upon.
Regardless of whether or not whale sharks actively search out plant snacks, the animals do have the power to digest them, Meekan says. “We don’t see whale sharks that often, but their tissues hold a remarkable record of what they’ve been up to,” he says. “We’re now learning how to read this library.”