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Scientists have discovered that some fish use sharks as backscratchers!

If you've got an annoying itch that needs scratching, you might ask a friend to help you out, well that's what some species of tuna do... except in this case their friend is a shark, who might eat them!

Researchers from the University of Western Australia sent underwater cameras out to 36 different parts of the Pacific, Indian and Atlantic oceans to study how fish interact with sharks there.

From watching the footage back, they were surprised to discover that big fish like yellowfin and bluefin tuna were rubbing themselves on sharks to get rid of any annoying parasites (like flatworms) that were stuck to their head or gills.

Despite the risk of potentially becoming a snack, the fish rub themselves on sharks because of their skin.

"Shark skin is really smooth in one direction and it's like sandpaper in the other," said Chris Thompson, one of the authors of the study.

The researchers sent cameras on 6,000 missions to study the relationships between underwater creatures.

From looking at the footage the team recorded around 117,000 animals from 261 different species!

They also spotted 106 cases where fish rubbed themselves on sharks or other fish.

Just under half of these scraping incidents were made by yellowfin tuna, and blue sharks proved to be the most popular choice for scraping, being chosen 58 percent of the time.

Yellowfin tuna and silky sharks also proved to have a pretty tight relationship.

The researchers said the interesting thing about this was that the sharks appeared mostly unbothered by being used as a backscratcher!

"I was kind of surprised at how nonchalant the sharks were," said Chris Thompson.

It was only the smaller species of tuna who stayed away from the sharks, and researchers think this could be because they were a bit more worried they'd be on the menu, than the larger species of tuna.