Whale sharks may be the world’s largest omnivores

As Mark Meacon leapt among the swells in the Indian Ocean, he saw a huge shadowy figure cascading through the water. Tropical fish biologists dove to meet the gentle giant – a whale shark – and took some samples of its skin. Swimming with these aquatic giants is nothing new for Meekan, who works at the Australian Institute of Marine Science in Perth. But “encountering something that feels like it’s from prehistory is an experience that never gets old,” he says.

On average about 12 meters long, the whale shark (Rhinekodon Typus) is the largest living fish species on Earth and one of the most mysterious. Since these sharks spend the majority of their lives in the deep ocean, Meakan says, analyzing the chemical makeup of their tissues can be extremely powerful for learning more about the animal’s fundamental biology (SN: 7/14/20) and behavior—including what they like to eat.

Meakon and his colleagues collected and analyzed skin samples that showed that whale sharks, long thought to be tough meat eaters, also eat and digest algae. Findings, described in July 19 ecology, add to the growing body of evidence that whale sharks intentionally eat plants, making them the largest omnivores on Earth. Previous record holder, Kodiak brown bear (Ursus arctos midendorphi) has an average length of about 2.5 m.

Algae have come in the stomachs of beach-dwelling whale sharks before. But how whale sharks feed—by swimming through swarms of zooplankton with their mouths open—”everyone even though it was just accidental ingestion,” Meakon says. Carnivores generally cannot digest plant life, so some scientists suspected that algae passed through the guts of whale sharks undigested.

To find out whether that assumption held up, Meakon and his colleagues followed whale sharks as the fish congregated near Ningaloo Reef off the coast of Western Australia in 2017. Since the giant fish is well hidden from the surface of the ocean, the team kept 17 individuals. Came out to feed from an airplane. They then stopped the shark from the boat and jumped into the water to take pictures, scrape off parasites, and collect tissue samples.

Meakan says most whale sharks don’t react when struck with a spear, which is about the width of a pinky finger. They say that few attract the attention of researchers. It’s as if they think: “It’s not a threat. In fact, I quite like it.”

The analysis showed that whale sharks in Ningaloo Reef were rich in arachidonic acid, an organic molecule found in brown algae called Sargassum. Since sharks can’t make this molecule on their own, they probably get it by digesting algae, Meakan says. It is not yet clear how arachidonic acid affects whale sharks.

The new work supports previous research by an independent group that found plant nutrients in the skin of whale sharks from the coat of Japan. Taken together, the findings suggest that it is common for whale sharks to digest greens.

But that doesn’t mean whale sharks are true omnivores, says shark biologist Robert Hueter of the Mote Marine Laboratory in Sarasota, Florida. “Whale sharks take in a lot of other things than the food they target,” he says. “It’s like saying that cows are omnivores because they eat insects while they eat grass.”

While Meakon acknowledged that he cannot say with certainty that whale sharks specifically seek out Sargassum, the amount they eat is not accidental. His team’s analysis clearly shows, he says, that plant material actually makes up a very large portion of their diet. It’s so big, he notes, that the whale sharks and the zooplankton they eat also occupy equal footing on the marine food chain, with both just one notch above the phytoplankton they both feast on.

Even though whale sharks actively seek out plant snacks, the animals have the ability to digest them, Meakon says. “We don’t see whale sharks very often, but their tissues have a remarkable record of what they’re doing,” he says. “Now we are learning to read this library.”

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