Over 60 Feet Long – Scientists Discover New World’s Largest Omnivore

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Researchers were surprised to find that whale sharks ate algae as well as krill at Ningaloo Reef in Western Australia. Credit: Andre Rerekura, Australian Institute of Marine Science

Marine biologists have discovered that whale sharks eat plants, making the famous species the largest omnivore in the world.

According to marine biologists, whale sharks eat plants, making the iconic animal the world’s largest omnivore. Whale sharks are filter feeders, and in Ningaloo Reef, Western Australia, they have long been seen consuming krill.

Marc Meekan

Australian Institute of Marine Science fish biologist, Dr Mark Meekan. Credit: Andre Rerekura, Australian Institute of Marine Science

However, scientists found that reef whale sharks consumed a lot of plant material when they analyzed the animals’ biopsy samples.

“It makes us rethink everything we thought we knew about what whale sharks eat,” said Australian Institute of Marine Science fish biologist Dr Mark Meekan. “And, in fact, what they do in the open sea.”

The discovery makes whale sharks, which have been measured up to 18.8 meters (61.7 feet) in length, the largest omnivores in the world.

“On earth, all the larger animals have always been herbivores,” Dr Meekan said.

“In the sea, we’ve always thought that animals that got really big, like whales and whale sharks, fed on a rung of the food chain with shrimp-like animals and small fish. Turns out, maybe the system of evolution on land and in water isn’t so different after all.

Whale shark tissue analysis

Australian researchers have analyzed tissue from whale sharks to analyze what they use for energy and growth. Credit: Andre Rerekura, Australian Institute of Marine Science.

The study has just been published in the journal Ecology. The researchers collected samples of potential food sources on the reef, ranging from small plankton to giant algae, to determine precisely what the whale sharks were consuming. Next, they compared the amino and fatty acids in whale sharks to those in plankton and plant matter.

Dr Meekan said substances found in sargassum, a form of brown seaweed common to Ningaloo that breaks off the reef and floats to the surface, were present in the tissues of the whale shark.

“We believe that during evolution whale sharks evolved the ability to digest some of this sargassum that enters their guts,” he said.

“So the vision we have of whale sharks coming to Ningaloo just to feast on these little krill is only half the story. In fact, they also eat a fair amount of algae.

Whale Shark Plants

The researchers found that the whale sharks ate plants as well as krill. Credit: Andre Rerekura, Australian Institute of Marine Science

CSIRO Oceans and Atmosphere organic biogeochemist Dr Andy Revill, who has analyzed whale shark tissue using compound-specific stable isotope analysis, said the technology allows scientists to study which animals were used for energy and growth, not just what they ate.

“Something like a whale shark, swimming in the water with its mouth open, is going to ingest a lot of different things,” he said.

“But you don’t know how much was used by the animal and how much goes straight to the other end. Whereas stable isotopes, because they are actually incorporated into the body, reflect much better what animals actually use to grow.

Biological oceanographer Dr Patti Virtue, from the University of Tasmania’s Institute of Marine and Antarctic Studies, said she was surprised by the whale shark’s biochemical signature.

“It’s very strange because in their tissues they don’t have any fat acid or the stable isotopic signature of an animal that feeds on krill,” she said.

The researchers also caught whale shark poo with a net and analyzed it.

“The poo showed they were eating krill,” Dr Virtue said. “But they don’t metabolize a lot of it.”

Reference: “The world’s largest omnivore is a fish” by MG Meekan, P. Virtue, L. Marcus, KD Clements, PD Nichols and AT Revill, July 19, 2022, Ecology.
DOI: 10.1002/ecy.3818

This AIMS whale shark research project is supported by Santos and INPEX as joint venture participants in the development of Van Gogh.

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