Tuna use sharks as back scratchers despite risk of being eaten

Large fish prefer to scrape against sharks over members of their own species, possibly to pry off parasites

Fish opt to scrape themselves against sharks over members of their own species, potentially because shark skin offers an ideal texture for sloughing off external parasites.

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

To learn more about the interactions between sharks and other fishes, Thompson and his team deployed 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.

The thousands of hours of footage revealed that yellowfin tuna (Thunnus albacares) were doing around 44 per cent of the scraping. Blue sharks (Prionace glauca) were most likely to be scraped against, being the rasp of choice 58 per cent of the time.

In almost all interactions, the fish scraped against the rear half of the shark, often along its 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 scraping events, fish rubbed up on members of their own species. Smaller fishes 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.

Journal reference: PLoS One, DOI: 10.1371/journal.pone.0275458