Whale sharks are filter feeders that have long been observed eating krill at the Ningaloo Reef in Western Australia. However, when a team of researchers led by the Australian Institute of Marine Science (AIMS) analyzed biopsy samples from these sharks, they were surprised to discover that the animals were in fact eating a significant amount of plant material too. This makes whale sharks – which can be up to 18 meters long – the world’s largest omnivores.
“This causes us to rethink everything we thought we knew about what whale sharks eat,” said study lead author Mark Meekan, a fish biologist at AIMS. “On land, all the biggest animals have always been herbivores. 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.”
In order to find out what the sharks eat, Dr. Meekan and his colleagues collected a variety of samples of possible food sources at the reef, from small plankton to large seaweed. Then they compared the amino acids and fatty acids in the plankton and plant material to those sampled from sharks, and found that the whale shark tissue contained compounds found in Sargassum, a species of brown seaweed common at Ningaloo, which breaks off the reef and floats to the surface of the water.
“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 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 out there eating a fair amount of algae too.”
According to study senior author Andy Revill, a biogeochemist at the Commonwealth Scientific and Industrial Research Organization, Oceans and Atmosphere, studying shark tissue using compound-specific stable isotope analysis allows scientists to clarify what animals are using for energy and growth, instead of just 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 explained. “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.”
In the case of whale sharks, such analysis revealed that plant material is essential for providing them energy and helping them grow – perhaps even more so than krill. Further research is needed to better understand the evolution and behavior of these fascinating creatures.
The study is published in the journal Ecology.