Despite the risk of being eaten, tuna use sharks as back scratchers.

Large yellowfin tuna prefer to scrape against sharks rather than other members of their own species, possibly to pry parasites off.

A yellowfin tuna rubs up against a blue shark (left) (right).  Thompson, Christopher D. H.
A yellowfin tuna rubs up against a blue shark (left) (right).  Thompson, Christopher D. H.

Sometimes the best way to scratch an itch involves some risk. Large fishes such as yellowfin and bluefin tuna have been observed scraping themselves against sharks rather than members of their own species.

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

"Shark skin is smooth in one direction and rough in the other," says Chris Thompson of the University of Western Australia.

Thompson and his colleagues discovered that some fish are willing to rub up against sharks after deploying floating, baited underwater cameras in 36 different regions of the Pacific, Indian, and Atlantic oceans, each of which recorded 2 to 3 hours of footage. The team documented 117,000 individual animals from 261 different species during over 6000 camera deployments. While the team's initial goal was to observe more generalized interactions between fish and sharks in open water, the footage revealed fish scraping against either member of their own species or sharks 106 times.

Thousands of hours of footage revealed that yellowfin tuna were responsible for 44% of all scrapes (Thunnus albacares). Blue sharks (Prionace glauca) were the most likely to be scraped against by fish, accounting for 58% of the time.

When sharks were targeted for scraping, fish scraped against the shark's back half, often along the tail. The scraped sharks seemed unconcerned by the activity. "I was surprised at how relaxed the sharks were," Thompson says.

Fish rubbed up against members of their own species in 17% of all scraping events. Smaller fish, such as skipjack tuna (Katsuwonus pelamis), were less likely to use sharks as a personal pumice stone, which the researchers speculate could be due to the increased risk of being eaten during the brief encounter.

"What makes this paper really interesting is the sheer number of [scraping] observations and... the really high-quality video recording," says Iain Barber of Nottingham Trent University in the United Kingdom.

According to Barber, the research suggests that the global decline of shark species may have ramifications for fish eager to rid themselves of harmful parasites.

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


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