Most of us have seen wildlife "spa days" before: typically some large animal (like a hippopotato cod, or sea turtle) waits patiently while some small animal (like a shrimp or wrasse) picks hungrily away at any dead skin or parasites that may be lurking on its body. Sometimes, however, that process happens in reverse: a top predator takes on the role of cleaner. 

Underwater cinematographer Erick Higuera and his colleagues spotted this great white – a four-metre male known locally as "Rudo" – and his fishy followers during a recent dive off the coast of Mexico's Guadalupe Island. It might look like the fish are simply hanging around to catch leftovers, but University of Miami shark scientist Dr Neil Hammerschlag explains that free food isn't the payoff here – their loitering serves a different purpose. And if you look closely you can see it in action!

"These mackerel fish are actually brushing up against the shark using its sandpaper-like skin to remove parasites," he wrote on Instagram. "Pretty amazing and another overlooked example of how sharks are ecologically important. Awesome video capture and observation." 

Rudo might be an unwitting participant in this exfoliation arrangement, but he's certainly tolerating his freeloading "spa" guests.

A shark's skin, a bit like the pumice stone you might use on your feet, is incredibly coarse. It's covered in tooth-like scales known as dermal denticles, and in some species, those structures are rigid enough to cause friction "burn" on human skin if brushed against the grain

White shark denticles have crowns shaped like horseshoe crabs, and they're tiny enough to be almost invisible to the naked eye. But that microstructure also makes them incredibly strong – so strong, in fact, that scientists have compared them to both granite and steel. 

Aside from functioning like built-in chainmail (which is both handy in a fight and keeps parasites from digging in), dermal denticles also give sharks a hydrodynamic boost as they cruise effortlessly through the water. This is made possible by the grooves on the denticles themselves, which, according to ReefQuest Center for Shark Research former director R. Aidan Martin, are akin to those found on the exteriors of today's fighter jets and fastest ships. 

"These grooves must be very closely spaced," he writes. "About as close together as the grooves on an old-fashioned phonograph record (anyone remember those?). Such closely spaced grooves appear to reduce drag by preventing eddies from coming in contact with the surface of a moving body." 

These groovy "skin teeth" are capable of withstanding some serious force, so it's no surprise that they make easy work of plucking parasites from the skin of passing mackerel! 

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Top header image: ...your local connection/Flickr