Whale sharks are the largest shark species in the world, and now scientists have discovered that giant sharks are even more amazing food machines than previously thought. In addition to gobbling up large bites of krill—small shrimp-like crustaceans—whale sharks also ingest large helpings of seaweed, enabling the aquatic giants to officially topple Kodiak bears (The Middendorff bear) as the largest omnivores in the world.
The researchers made the discovery by analyzing the whale shark (Rhincodon typus) skin samples collected near the Ningaloo Reef of Western Australia. According to National Ocean Service. Until now, scientists thought the gentle giants were primarily filter feeders, opening their cavernous mouths to swallow roughly 21,200 cubic feet (600 cubic meters) of water every hour. Then, by draining the water through their gills, the sharks are left with morsels of plankton, shrimp, small fish and crustaceans to devour.
But the new discovery, published on July 19 in the journal ecology, has given scientists important new information to chew on.
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“It makes us rethink everything we thought we knew about what whale sharks eat” and calls into question other aspects of shark behavior “out in the open ocean,” lead study author Mark Meekan, a fish biologist at the Australia Institute of Marine Science in Queensland, said in a statement.
Meekan said the discovery contradicts the common assumption that large land creatures are usually herbivores, but those that live in the sea occupy another place in the food chain, feeding on small shrimp and fish.
“It turns out that maybe the evolutionary system on land and in water isn’t so different after all,” Meekan said.
For their study, the scientists collected the sharks’ potential food sources – ranging from tiny crustaceans and plankton to huge clumps of seaweed – and then chemically analyzed the samples to detect their amino acids and fatty acids. After cross-referencing these acids with those found in skin samples taken from whale sharks, the researchers identified high concentrations of sargassum—a type of brown seaweed made up of thousands of microscopic algae—in the sharks’ diet.
Scientists think this omnivorous diet may be the result of sharks evolving to digest accidentally ingested seaweed, saving them the energy cost of spitting it out again.
“We think that over evolutionary time, whale sharks have evolved the ability to digest some of this sargassum that’s going into their intestines,” Meekan said. “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 there eating a fair amount of algae too.”
Having a wider range of food sources may sound like good news for whale sharks, as it may help them cope with potential disruptions to their marine ecosystems caused by climate change. But scientists said it’s more complicated than that. It’s possible that sharks’ tendency to swallow most of what goes into their mouths could make them much more likely to ingest copious amounts of ocean-flowing plastic, according to the study.
“Whale sharks can pass some plastic through their intestines,” but ingesting small or large pieces of plastic can cause sharks to regurgitate their meals and can reduce their intestinal capacity and interfere with digestion, the researchers wrote.
Originally published in Live Science.