Decidedly, the whale shark (from the Latin name Rhincodon typus) is accumulating records. Already the title holder of “the world’s largest fish”, this peaceful marine giant has now been promoted to the top spot on the podium of the largest omnivorous animals, that is, whose diet consists of both plants and animals.
The podium on which the whale shark now takes the place of the Kodiak – the largest subspecies of the brown bear – when this fish was not even at the start, and for good reason: we removed it, from a nutritional point of view, among the carnivores!
“This makes us question everything we thought we knew about what whale sharks eat,” said Dr Mark Mikan, an ichthyologist (fish biologist) at the Australian Institute of Marine Sciences (AIMS) in a press release.
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Indeed, before the publication of the study by Dr. Mikan and his colleagues in the scientific journal Ecology (07/19/2022), it was known that whale sharks feed almost exclusively on krill, a small crustacean that they catch by filtering through their gills. swallowing large amounts of sea water by mouth and releasing only the liquid.
Eating algae by whale sharks until then was considered involuntary, strictly speaking, plants should not have been included in their “diet”.
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The authors of the study collected algae and plankton from whale shark habitat on Ningaloo Reef in Western Australia. They then took skin samples from 17 individuals – a painless process for the animal – to analyze their biochemical composition for fatty acids and amino acids.
Thus, the researchers concluded that certain molecules in the tissues of Ningaloo whale sharks corresponded to molecules found in sargassum, a brown algae, proving that the animal had assimilated these plants for food.
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“We believe that whale sharks have evolved the ability to digest some of the Sargassum that enters their intestines,” explains Dr. Mark Mikan. “So our vision of whale sharks coming to Ningala to feast on these little krill was only half the story. In fact, they also eat a lot of algae.”
Previously, scientists relied mainly on the composition of feces to see what was or was not digested by animals. This new study, on the other hand, uses the specific compound isotope analysis (CSIA) method, which indirectly allows you to determine which foods specifically served the animal for its energy and growth.
“On land, the largest animals are always herbivores. At sea, we thought animals that got very big, like whales and whale sharks, were feeding one step up the food chain — eating animals like crustaceans and small fish,” the lead author clarifies. “It turns out that the system of evolution on land and in water may not be so different after all.”
However, in their study, scientists warn of the danger posed by the plastic present in the water. Indeed, by consuming algae, whale sharks are more likely to ingest this source of pollution at the same time, which is harmful to their digestion.
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Harmless to humans, like the vast majority of shark species, the whale shark is classified as “endangered” on the International Union for Conservation of Nature’s (IUCN) Red List of Threatened Species, largely due to its intentional or accidental capture. fishing boats; and maritime collisions.
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