Fish rubbing themselves against a shark's body may sound as if they have a death wish, but this is precisely what some fish have been spotted doing. And it turns out that such behaviour is more widespread and frequent than one would think.

A study led by the University of Miami (UM) Shark Research and Conservation Program at the Rosenstiel School of Marine and Atmospheric Science uncovered over 40 instances of fish rubbing themselves against a shark’s skin in over ten locations around the globe.

Chafing

This behaviour, called chafing, has been observed between fish and safe, inanimate objects like sand and rocky substrate. Why do fish do it with sharks?

Here's a possible reason: Shark skin is covered in small tooth-like scales called dermal denticles, which provide a rough surface similar to sandpaper. According to UM Rosenstiel School research associate professor and study co-author Neil Hammerschlag, “We suspect that chafing against shark skin might play a vital role in the removal of parasites or other skin irritants, thus improving fish health and fitness.”

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Although instances of fish chafing against sharks have previously been observed, this study finds this cross-species behaviour to be more pervasive than previously understood. The research team examined underwater photos, video, drone footage, and witness reports to find 47 instances of fish rubbing themselves against a shark’s skin. The chafing events, which were documented in 13 locations around the world, varied in duration from eight seconds to over five minutes.

They recorded 12 finfish chafing against eight different species of shark, including great whites. The team even documented silky sharks chafing on the head of a whale shark. The number of fish chafing against sharks ranged from one to over 100 individuals at a time. 

The findings of the study have been published in The Scientific Naturalist journal.