Whale sharks are filter feeders that have long been observed eating krill at Ningaloo Reef in Western Australia. However, when a team of researchers led by the Australian Institute of Marine Sciences (AIMS) analyzed biopsy samples from these sharks, they were surprised to discover that the animals also ate a significant amount of plant material. This makes whale sharks, which can be up to 18 meters long, the largest omnivores in the world.
“This makes us rethink everything we thought we knew about what whale sharks eat,” said lead study author Mark Meekan, a fish biologist at AIMS. “On land, all larger animals have always been herbivores. In the sea we’ve always thought that animals that got really big, like whales and whale sharks, were feeding a step up the food chain of shrimp-like animals and small fish. It turns out that maybe the evolutionary system on land and in water isn’t so different after all.”
In order to find out what the sharks eat, Dr Meekan and his colleagues collected a variety of samples of possible food sources on the reef, from small plankton to large algae. They then compared the amino acids and fatty acids in the plankton and plant material with those sampled from the sharks, and found that the whale shark tissue contained compounds found in Sargassum, a species of brown algae common to Ningaloo, which breaks down the reef and float on the surface of the water.
“We think that over evolutionary time, whale sharks have developed the ability to digest some of this sargassum that gets into their guts,” Dr Meekan said. “So the view we have of whale sharks coming to Ningaloo just to feast on this little krill is only half the story. In fact, they’re also eating a fair amount of algae.”
Studying shark tissue using specific stable isotope analysis of a compost allows scientists to clarify which animals use for energy and growth, rather than what. they are eating
“Something like a whale shark, swimming through the water with its mouth open, will ingest many different things,” he explained. “But you don’t know how much of that the animal has used and how much is just coming straight out the other end. Whereas stable isotopes, because they’re actually incorporated into the body, are a much better reflection of what animals are using to grow up”.
In the case of whale sharks, this analysis revealed that plant material is essential to providing them with energy and helping them grow, perhaps even more so than krill. More research is needed to better understand the evolution and behavior of these fascinating creatures.
The study is published in the journal Ecology.
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By Andrei Ionescu, Earth.com staff writer