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Big itch: why tuna rub up against sharks

Yellowfin tuna rubs up against a blue shark off Ascension Island (Christopher Thompson)
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Tuna and other large pelagic fish suffering from itches caused by parasites, dead skin or other irritants have no cleaning station available out in the blue – which is why it seems they find relief by rubbing themselves against sharks’ abrasive skin.

Researchers Christopher Thompson and Prof Jessica Meeuwig from the University of Western Australia’s Marine Futures Lab have produced a new study based on sifting thousands of hours of underwater video recorded in the Pacific, Indian and Atlantic Oceans. The footage revealing how fish solved their irritating problem was captured using baited cameras left to drift, as referred to on Divernet last year. 

“Scraping interactions” were found among several fish and shark species and the researchers concluded that fish preferred to scrape on sharks rather than other fish – but only if the shark was not so much bigger that it risked being eaten. The fish tended mostly to scratch their head and sides, because their eyes, nostrils, gills and lateral line were the areas most vulnerable to parasites.

Yellowfin and other tunas were seen to line up in an orderly fashion behind a shark and take turns to brush against its tail, while fish such as rainbow runners would form an unruly school and dart out in turns to bump the back end of the shark’s body. Made up of small “dermal denticles”, shark-skin was at one time used industrially as sandpaper.

Thompson and Meeuwig say they observed the scraping behaviour only in remote regions with relatively healthy populations of sharks and large tuna, but they suggest that if falling shark numbers became too low for fishes to find scratching posts, this could cause a net loss of fitness in the fish species. Their study is published in PLOS One.

Also on Divernet: Sharks Dive Deeper Than We Thought, It’s Official: Sharks Muddle Humans With Seals, Who Says Sharks Never Sleep?, Tiger Sharks Range Far But Don’t Mix

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