The whale shark (Rhincodon typus), which have been reported up to 18 m long, is the largest known extant fish species. This creatrure is a filter feeder and has long been observed eating plankton, including krill, copepods, crab larvae, tiny squid or fish. But new research shows that the floating macroalgae, Sargassum, is a significant source of food for the whale shark.
“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,” said lead author Dr. Mark Meekan, a fish biologist at the Australian Institute of Marine Science.
“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.”
To find out exactly what the whale sharks were eating, Dr. Meekan and his colleagues collected samples of possible food sources at Western Australia’s Ningaloo Reef, from tiny plankton to large seaweed.
They then compared the amino acids and fatty acids in the plankton and plant material to those in the whale sharks.
The whale shark tissue contained compounds found in Sargassum, a type of brown seaweed common at Ningaloo, which breaks off the reef and floats at the surface.
“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.”
“We were surprised by the whale shark’s biochemical signature,” said Dr. Patti Virtue, a biological oceanographer with the Institute for Marine and Antarctic Studies at the University of Tasmania.
“It’s very strange, because in their tissue they don’t have a fatty acid or stable isotope signature of a krill-feeding animal.”
“Something like a whale shark, which swims through the water with its mouth open, is going to ingest a lot of different things,” added co-author Dr. Andy Revill, a biogeochemist at CSIRO.
“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 utilising to grow.”
The findings were published in the journal Ecology.
M.G. Meekan et al. The world’s largest omnivore is a fish. Ecology, published online July 19, 2022; doi: 10.1002/ecy.3818
This article was first published by Sci-News on 25 July 2022. Lead Image: The whale shark (Rhincodon typus). Image credit: Andre Rerekura, Whaleshark Research.
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