Whale sharks are regarded as gentle giants but also, like other sharks, as carnivores, albeit filter-feeding on tiny krill. But now marine scientists have discovered that whale sharks in fact derive much of their nutrition from plants – which, they say, makes them the world’s biggest omnivores.
“This causes us to rethink everything we thought we knew about what whale sharks eat – and, in fact, what they’re doing out in the open ocean,” commented fish biologist Dr Mark Meekan of the Australian Institute of Marine Science (AIMS).
“On land, all the biggest animals have always been herbivores,” said Dr Meekan. “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.”
The experts collected samples of possible food sources at Ningaloo, from plankton to large seaweeds, and compared the amino acids and fatty acids they contained to those found in the whale sharks.
The whale shark tissue contained compounds found in Sargassum, the brown seaweed that commonly breaks off the reef and floats at the surface at Ningaloo.
Biological oceanographer Dr Patti Virtue from the University of Tasmania said that the whale sharks’ biochemical signature had struck the scientists as “very strange, because in their tissue they don’t have a fatty acid or stable isotope signature of a krill-feeding animal”.
Analysis of whale shark faeces had confirmed that although they were eating krill, they were not metabolising much of it.
“We think that over evolutionary time, whale sharks have evolved the ability to digest some of this Sargassum that’s going into their guts,” said Dr Meekan. “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.”
The scientists used compound-specific stable isotope analysis to determine what the whale sharks consumed for energy and growth, rather than simply what they were eating.
“Something like a whale shark, which swims through the water with its mouth open, is going to ingest a lot of different things,” said organic biogeochemist Dr Andy Revill of CSIRO Oceans & Atmosphere. “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.”
Until now the Kodiak bear had been considered Earth’s biggest omnivore. The heaviest of these bears weigh in at around 680kg – which would be no contest for whale sharks that can grow to be some 28 times heavier. The research has just been published in the journal Ecology.