Scientists have observed instances in which tuna rub themselves against the side of sharks. Apparently, they do this to dislodge painful parasites that cling to their head, eyes and gills.

“Shark skin is really smooth in one direction and it’s like sandpaper in the other,” said Chris Thompson, a Research Fellow at the University of Western Australia.

Thompson and his colleagues observed this behaviour after studying the footage of floating, baited underwater cameras which they had deployed in 36 regions in the Pacific, Indian and Atlantic Oceans in 2012 to 2019.