What did the whale sharks do (Rhincodon typus) tell your waiter at the restaurant? “Can I have a side of salad with that?”
It is true! The whale shark has dethroned Kodiak bears (Ursus arctos middendorffi) to now claim the title as ‘the largest omnivorous animal.’ A slow-moving, filter-feeding animal found in tropical oceans around the world, whale sharks are notorious for turning up at Western Australia’s Ningaloo Reef each year to take advantage of the ecosystem’s remarkable abundance of food thanks to annual coral spawning and nutrient upwelling -rich water. In particular, they feed on plankton and krill, small oceanic crustaceans that are high in nutrients.
The fringing Ningaloo Reef is a UNESCO World Heritage Site and one of the world’s largest aggregation sites for whale sharks, making it a great place for scientists studying these animals to find them easily (well, easier). Researchers from the Australian Institute of Marine Science (AIMS), CSIRO and the University of Tasmania recently analyzed biopsy samples from these starry visitors and discovered that they eat a lot – not just plankton and krill, but plant material too! “This is making us rethink everything we thought we knew about what whale sharks eat,” Australian Institute of Marine Science fish biologist Dr Mark Meekan said in a press release. “And actually what they do out in the open ocean.”
This isn’t the first time a shark has been known to be omnivorous, with pansharks (Sphyrna tiburo) sort of broke the internet when they were found to be eating and digesting sea grass. In fact, it’s not the first time whale sharks have been found to be omnivores either! Previous research in Japan showed that whale sharks had blood and tissue chemistry, showing that about half of their diet came from plant materials, such as algae or sea grass.
But the whale shark tissue of these Ningaloo whale sharks was different. Not only were they found eating sargassum, a type of floating brown algae, but these plants contributed to their energy and growth. The whale shark tissue was analyzed by CSIRO Oceans and Atmosphere organic biogeochemist Dr Andy Revill who used compound-specific stable isotope analysis to study what the animals used for energy and growth, not just what they ate. Revill said that while the whale shark swam with its mouth open and ingested many things, it was interesting to see what was specifically used by the animal and what just passed through the animal’s body: “Because [stable isotopes are] actually incorporated into the body, [they] is a much better reflection of what the animals actually use to grow.”
Biological oceanographer Dr Patti Virtue, from the University of Tasmania’s Institute for Marine and Antarctic Studies, said she was surprised by the whale shark’s biochemical signature. “It’s very strange, because in their tissues they don’t have a fatty acid or stable isotope signature of a krill-feeding animal,” she said in the news release. However, the researchers also caught whale shark poo with a net and analyzed it, showing that they were eating krill but not metabolizing much of it.
“We think that over evolutionary time, whale sharks have developed the ability to digest some of this Sargassum that goes into their guts,” 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.” And that makes Meekan wonder about the history of evolution and how it differs – or doesn’t – between two different ecosystems: “On land, all the biggest animals have always been herbivores. In the sea, we’ve always thought that the animals that have become very large ones, such as whales and whale sharks, fed one step up the food chain on shrimp-like animals and small fish. It turns out that perhaps the evolutionary system on land and in the water is not so different after all.”
The academic paper, available here, is also exciting to another whale shark researcher around the world: “What I love most about whale sharks is that they challenge everything we think we know about sharks,” Alistair Dove, a marine biologist and the vice president for science and education for the Atlanta-based Georgia Aquarium, Mashable told. How this will shake things up in the world of whale shark conservation is yet to be determined, but comes at an interesting time for these struggling gentle giants.
The biggest danger they face? Us. While unsustainable fishing pressure – from accidental catches or targeted attempts – is known to be a threat to these animals, it is the ships themselves that are also the problem. With whale sharks spending a lot of time at the ocean’s surface devouring plankton and other delicious seafood delicacies, they are easily hit by commercial fishing vessels. If it’s a fatal blow, their bodies gently sink to the bottom of the ocean—never to be seen or counted in declining population numbers. In fact, Dove believes commercial shipping can be a silent killer. And there might be another killer to add to that list too!
Rated “endangered” by the International Union for Conservation of Nature (IUCN), there is little predate on this shark due to its large size. However, this knowledge has been challenged with recent footage of killer whales teaming up to take down a young whale shark. Maybe someone should give the killer whales the idea to add lettuce to their menu instead of potentially preying on whale sharks – no way to get the vegetable count for the day!