Tuna sometimes get itchy. Without hands or a back-scratcher, what’s a fish to do? Many species rub up against each other, but some use a different scratching post—sharks. Scientists studying the phenomenon found that big tuna like yellowfin and bluefin almost exclusively use sharks to scratch their itches. Smaller species like skipjack tuna do not. Most interactions took place on the shark’s non-toothy end, a.k.a. near the tail, meaning the fish are likely wary of predation. Based on the relative sizes of the tuna and shark in the picture below, we’re not sure the tuna has much to fear from the shark.
The team think this interesting behavior helps dislodge parasites that latch onto tuna and other fish. Ridding themselves of itchy hitchhikers isn’t unique to fish. Researchers have long believed it’s part of the reason whales jump out of the water. Slamming back down may dislodge lice and other parasites that crawl on the whale’s skin.
We learned about these findings from New Scientist. The team published their data in the peer-reviewed journal PLoS ONE, including more pictures of the shark scratching posts. The paper is titled “Sharks are the preferred scraping surface for large pelagic fishes.” The video below includes a compilation of other observations of fish scratching themselves on sharks, which a different team put together. That group published their data last year in a paper called “ Sharks as exfoliators.” The video even includes smaller sharks scratching themselves on an enormous whale shark, which as we previously learned even have rough armor around their eyes.
It is important to understand relationships like these in vulnerable ocean creatures and if they’re changing with the environment. Many questions remain, like does each species of itchy fish have a favorite shark to scratch themselves on? And do the shark scratchees get anything out of the interaction?
Melissa is Nerdist’s science & technology staff writer. She also moderates “science of” panels at conventions and co-hosts Star Warsologies, a podcast about science and Star Wars. Follow her on Twitter @melissatruth.