If you’re a pelagic fish swimming the open oceans your options are limited when it comes to something to rub up against. Offshore fish suffer parasite infestations just like any other fish, only for reef-associated fish, they have various surfaces to flash and scratch against to try to remove them. A University of Western Australia team deployed underwater cameras in three oceans to document interactions between fish and sharks. They found that in the case of Yellowfin tuna and Blue sharks, the two species were interacting a lot more than anticipated as the Tuna were using the sharks to scrape against. Rub shark skin the wrong way and it’s like sandpaper, so the Yellowfins were approaching the sharks from behind and scraping their heads and flanks against the shark’s tails to try to remove parasites. Blue sharks weren’t bothered by the Yellowfins but for smaller Tuna like Skipjack, they didn’t risk rubbing the sharks which prey upon them and instead rubbed against each other, behavior which will actually spread infection between conspecifics.
Cameras were deployed 6000 times across the Pacific, Indian, and Atlantic Oceans, each recording up to three hours of high-quality footage at a time. 117,000 individuals were recorded from 261 different species. Fish were observed scraping against sharks or their own species 106 times. The global decline of shark species has experts worried that pelagic fish species may be less able to rid themselves of parasites. Parasite numbers may also increase as oceanic waters warm.
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