Imagine you are a fish, and suddenly your eye itches. Or your side. Or your tail. Perhaps it's because some tiny creature has settled on you or a scratch is healing. You have no hands to scratch, and the suitable rocks are too far away or too deep. Tunas and some other large fish have found a solution: they scratch against sharks.
The scales of sharks and their close relatives, rays, have a unique structure that differs from the scales of other fish — each scale ends with a backward-facing tooth. This type of scale is called placoid; it can be compared to soft sandpaper. By rubbing against it, fish can not only relieve itching but also get rid of parasites.
A study published in PLoS One reports that tunas scratch themselves on shortfin mako sharks, blue sharks, and silky sharks. The fish seeking to scratch against a shark were quite large, which makes sense — even a shark finds it easier to catch and handle prey that is significantly smaller than itself.
The scratching sessions were quick: fish would approach the shark from behind, briefly touch its tail or side, and then immediately swim away. Different species of fish had their preferences: for example, tunas most often chose the shark's caudal fin, while rainbow runners were less selective about where to scratch.
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