Tuna use sharks as back scratchers.
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.
Although their original intention was to observe generalised interactions between fish and sharks, they were intrigued when they saw the tuna’s intriguing behaviour a total of 106 times.
In all, 44 percent of the incidents were initiated by yellowfin tuna, while southern bluefin tuna and skipjack tuna accounted for 16 percent and 3.8 percent of the time respectively.
In 17 percent of all rubbing incidents, the tuna rubbed against their own species.
On the other side of the interaction, it was the blue shark that was the preferred shark species 58 percent of the time. Interestingly, the sharks appeared not to be bothered by the activity at all.
Smaller fish like skipjack tuna were less likely to engage in such behaviour, possibly as a precaution to avoid ending up as prey.
It is possible that the decline in shark numbers may reduce the occurrence of such interactions, and this has implications for the benefits that the tuna receive from the rubbing behaviour.