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The feeling of an itch in a hard-to-reach place is as annoying as it is uncomfortable. It turns out that this is also a problem in some fish, especially when they have small parasites biting their skin or sucking their blood. But the solution for some fish appears to be finding the nearest shark! It might seem a little dangerous—since sharks, after all, are apex predators—but it turns out that the rough skin of these hunters offers a great surface to slough off pesky parasites.

For the first time, a study has documented the strange scratching behavior of aquatic relief-seekers in the open ocean. In a study published this week in the journal PLoS ONE, Chris Thompson, a research fellow at the University of Western Australia caught the interesting actions all by chance.

[Related: Great white shark sightings are up in the US, which is kinda good news.]

“This study came about through a chance observation. We use baited camera systems around the world to study the status and ecology of pelagic (offshore/blue water) wildlife. But while watching these videos we sometimes see strange behaviors,” Thompson says. In footage from the Revillagigedo Archipelago, a group of islands off the southwestern coast of Mexico, the biologist saw a silky shark swim toward the camera. Suddenly, “a huge yellowfin tuna,” he says, “came up behind it, gently rubbed itself along its tail then cruised off into the blue.” Thompson logged this behavior and continued his work, but his questions about what he’d seen lingered—especially once he started to see similar interactions between fish elsewhere.

Thompson co-wrote the study with Jessica J. Meeuwig.

His team had a lot of footage to review. The underwater cameras were deployed between 2012 and 2019 in 36 different regions of the Atlantic, Pacific, and Indian oceans, and each camera recorded 2 to 3 hours of video. In all, the team observed 117,000 individual animals from 261 different marine species. Roughly 44 percent of the scraping was done by yellowfin tuna (Thunnus albacares). Blue sharks (Prionace glauca) were most likely to be used as a pumice stone, finding themselves being scratched 58 percent of the time.

“We found a strong correlation with larger sharks tending to only be scraped on by larger fish. This suggests that there might be a trade off between benefits from scraping and risk of predation,” says Thompson. “Size seemed to be a limiting factor in determining whether fish scraped with larger fish more likely to scrape on larger sharks and smaller fishes scraping on other fishes instead of sharks.”

[Related: With new tags, researchers can track sharks into the inky depths of the ocean’s Twilight Zone.]

In other words, the bigger fish like yellowfin tuna weren’t as afraid to risk a good scratch from a shark. Scraping has its rewards, though: It can be really helpful for the fish, since painful skin parasites are common and destructive. According to Thompson, the pests can cause significant damage, even damaging fishes’ eyes, the lateral line system, and the gills. 

Itchy fish use sharks as swimming scratching posts
A yellowfin tuna scraping on blue shark at Ascension Island in the southern Atlantic Ocean. Photo: Chris Thompson

Shark skin is one reason why the animals are ideal scratching posts. “Shark skin is made of tooth-like scales called dermal denticles, which feel like sandpaper when rubbed from tail to head,” Thompson says. And as global shark populations continue to decline, Thompson says this relationship raises a novel question about the health of other fish. “What happens when your favorite scratching post disappears? Is there a fitness cost to these fishes? We only saw these interactions in remote areas with healthy shark and tuna populations. This indicates that these behaviors are more likely to occur and persist in relatively intact ecosystems and could be lost as these ecosystems become degraded.”

Large marine protected areas (MPAs), like Papahānaumokuākea Marine National Monument in Hawaii, have proven successful in conserving marine life. MPAs have also been shown to conserve behaviors in fishes and sharks, and may help make sure this fish scratching service continues, according to Thompson.