Tuna use sharks as back scratchers despite risk of being eaten

Sometimes the best way to scratch an itch involves a bit of danger. Large fishes like yellowfin and bluefin tuna have been discovered choosing to scrape themselves against sharks rather than against members of their own species.

By scraping against a rough surface, fish can dislodge painful parasites clinging to their head, eyes and gills.

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

Thompson and his colleagues discovered that some fish will dare to rub themselves against sharks after deploying floating, baited underwater cameras into 36 different regions of the Pacific, Indian and Atlantic oceans, each of which recorded 2 to 3 hours of footage. During more than 6000 deployments of the cameras, the team documented 117,000 individual animals from 261 different species. While the initial aim was to observe more generalized interactions between fish and sharks in the open water, the team’s footage revealed fish scraping themselves against either members of their own species or sharks a total of 106 times.

The thousands of hours of footage revealed that 44 per cent of all scrapes were made by yellowfin tuna (Thunnus albacares). Among sharks, blue sharks (Prionace glauca) were most likely to be scraped against by fish, being the rasp of choice 58 per cent of the time.

When sharks were targeted for scraping, fish usually scraped against the rear half of the shark, often along the tail. Scraped sharks appeared unbothered by the activity. “I was kind of surprised at how nonchalant the sharks were,” says Thompson.

In 17 per cent of all scraping events, fish rubbed up on members of their own species. Smaller fishes such as skipjack tuna (Katsuwonus pelamis) were less likely to use sharks as a personal pumice stone, which the researchers note could be because small fish have a higher risk of being eaten during the brief encounter.

“What makes this paper really interesting is the sheer number of [scraping] observations and… that they’ve got really high quality video recording,” says Iain Barber at Nottingham Trent University in the UK.

Barber notes that the work suggests that the global decline of shark species could have knock-on consequences for fish eager to rid themselves of harmful parasites.