Tuna rub against sharks to get rid of parasites
Tuna often rub against sharks rather than against members of their own species, because shark skin provides a suitable surface for scraping off parasites.
A yellowfin tuna rubs against a blue shark. (Photo: Chris Thompson)
Shark skin is very clean and feels like sandpaper, covered in tiny tooth-like scales, according to researcher Chris Thompson of the University of Western Australia. By rubbing against this rough surface, tuna may rid themselves of painful parasites that attach to their heads, eyes and gills. Thompson and his colleagues published their findings on October 19 in the journal PLOS One.
To learn more about interactions between sharks and other fish, Thompson and his team deployed floating underwater cameras at a total of 36 different locations across the Pacific, Indian and Atlantic oceans, with each camera recording for 2 to 3 hours. Across more than 6,000 camera deployments, the team filmed 117,000 individuals belonging to 261 species. The thousands of hours of footage revealed that yellowfin tuna (Thunnus albacares) accounted for 44% of the rubbing events. Blue sharks (Prionace glauca) were the most frequent targets, at 58% of the time.
In all interactions, the tuna rubbed against the rear half of the shark's body, usually along the tail. The sharks being rubbed did not seem to mind. Thompson said he was very surprised by how unbothered the sharks were. In 17% of rubbing events, the fish made contact with members of their own species. Smaller fish rubbed against sharks less often, most likely out of fear of being eaten.
What makes the study truly remarkable is the diversity of the observations and the quality of the video, according to Iain Barber of Nottingham Trent University in the UK. Barber stressed that declining shark numbers worldwide could affect other fish's efforts to get rid of harmful parasites.
