CAPE FERGUSON, Australia — Sharks are notorious carnivores, but noteworthy new research reveals that the whale shark is actually Earth’s largest omnivore. Marine scientists from the Australian Institute of Marine Science have discovered that these massive fish also eat plants.
Marine biologists have long considered whale sharks to be “filter feeders” who eat krill along Western Australia’s Ningaloo Reef. However, when study authors analyzed a series of biopsy samples collected from whale sharks at the reef, it became clear they had been eating quite a lot of plant material.
“This causes us to rethink everything we thought we knew about what whale sharks eat,” says Australian Institute of Marine Science fish biologist Dr. Mark Meekan in a media release. “And, in fact, what they’re doing out in the open ocean.”
Scientists have found whale sharks that measure up to an astounding 18 meters (59 feet) in length.
“On land, all the biggest animals have always been herbivores,” Dr. Meekan adds. “In the sea we always thought the animals that have gotten really big, like whales and whale sharks, were feeding one step up the food chain on shrimp-like animals and small fishes. Turns out that maybe the system of evolution on land and in the water isn’t that different after all.”
Whale sharks have an appetite for brown seaweed
To narrow down what the sharks could have been eating, researchers collected samples of various possible food sources around the reef, ranging from tiny plankton to large seaweed. Next, they compared the amino acids and fatty acids found in the plankton and plant material to those in the whale sharks.
Sure enough, the whale shark tissue indeed displayed compounds found in sargassum, a variety of brown seaweed common at Ningaloo, known to break off the reef and float to the surface.
“We think that over evolutionary time, whale sharks have evolved the ability to digest some of this Sargassum that’s going into their guts,” Dr. Meekan explains. “So, the vision we have of whale sharks coming to Ningaloo just to feast on these little krill is only half the story. They’re actually out there eating a fair amount of algae too.”
Dr. Andy Revill, a CSIRO Oceans and Atmosphere organic biogeochemist, analyzed the whale shark tissue via a compound-specific stable isotope analysis. He explains that this technology opened the door for scientists to study what animals use for energy and growth, not simply what they are eating.
“Something like a whale shark, which swims through the water with its mouth open, is going to ingest a lot of different things,” he says. “But you don’t know how much of that has been used by the animal and how much just goes straight out the other end. Whereas stable isotopes, because they’re actually incorporated into the body, are a much better reflection of what the animals are actually utilizing to grow.”
Shark poo finds something surprising
Meanwhile, biological oceanographer Dr. Patti Virtue, from the University of Tasmania’s Institute for Marine and Antarctic Studies, admits it was surprising to see the whale shark’s biochemical signature.
“It’s very strange, because in their tissue they don’t have a fatty acid or stable isotope signature of a krill-feeding animal.”
In the interest of covering all their bases, researchers even gathered up whale shark feces with a net for further analysis.
“The poo did show that they were eating krill,” Dr. Virtue concludes. “But they’re not metabolizing much of it.”
The study is published in the journal Ecology.