Back-scratching sharks

Itching is an experience shared by the entire animal kingdom. Flies and snails, we humans and fish scratch themselves. If you have paws and nails scratching is easy, and likewise if you have something rough available, for example, you can rub yourself on a bark or a rock. But how do you get rid of an itch if you find yourself in the middle of the ocean and there is nothing around you but an endless expanse of water? Yellowfin tuna have solved the problem by scratching themselves on sharks. By analysing underwater film footage, an Australian research team noticed that large tunas rub their sides and heads on the sharks' skin. This unusual behaviour has an anatomical explanation: the skin of sharks is very rough because it is bristling with small, tooth-sharp scales. Indeed, as is well known, the skin of these animals was once used as sandpaper.