Australian marine scientists have discovered that the massive whale shark (Rhincodon typus) eats plants, making it officially the largest omnivore on Earth.
Whale sharks can grow up to 18 metres long and weigh up to 43 tonnes. They’re filter feeders that have been known to eat krill – small, shrimp-like crustaceans – but now we know that they actually eat a lot of plant material too.
Tissue samples from whale sharks at Ningaloo Reef off the coast of Western Australia also indicate that while they eat krill they may not be metabolising a lot of it, according to a new study published in the journal Ecology.
“This causes us to rethink everything we thought we knew about what whale sharks eat,” says first author Dr Mark Meekan, a fish biologist from the Australian Institute of Marine Science (AIMS). “And, in fact, what they’re doing out in the open ocean.”
“On land, all the biggest animals have always been herbivores,” explains 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.”
Whale sharks are the largest omnivores on Earth
In marine systems, the largest animals are filter feeders that eat nekton (actively swimming aquatic organisms) which form dense patches that allow for harvesting large amounts of food at once.
But in coastal regions these patches also accumulate floating debris like macroalgae (seaweeds and algae visible to the naked eye).
Scientists wanted to find out how whale sharks deal with ingesting this material. So, to determine exactly what the whale sharks were eating, they collected samples of possible food sources at the reef.
From tiny phytoplankton to seaweed, they compared the amino acids and fatty acids in the potential food sources to those found in whale shark tissues.
The team found that whale shark tissues were rich in N-6 polyunsaturated fatty acids (PUFA), particularly arachidonic acid (ARA), which are compounds found in relatively high proportions in Sargassum – a type of brown seaweed that breaks off of the reef and floats at the water’s surface at Ningaloo.
“We think that over evolutionary time, whale sharks have evolved the ability to digest some of this Sargassum that’s going into their guts,” says 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.”
They eat krill but don’t metabolise much of it
Going a step further, they then analysed the tissue samples using compound-specific stable isotope analysis (CSIA), a method used to measure the ratios of naturally occurring stable isotopes incorporated into tissue from the diet.
Co-author Dr Andy Revill, an organic biogeochemist at CSIRO Oceans and Atmosphere, Australia, who analysed the samples using CSIA, says whale sharks swim through the water with their mouths open.
“It’s going to ingest a lot of different things.
“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.
“Stable isotopes, because they’re actually incorporated into the body, are a much better reflection of what the animals are actually utilising to grow.”
And what they found was pretty surprising.
“It’s very strange, because in their tissue they don’t have a fatty acid or stable isotope signature of a krill-feeding animal,” says co-author Dr Patti Virtue, a biological oceanographer from University of Tasmania’s Institute for Marine and Antarctic Studies.
Though samples of collected whale poo showed that whale sharks were indeed eating krill, their tissues indicated that they didn’t metabolise much of it.