We're not far into it yet, but 2018 is already shaping up to be the year of unique whale-shark sightings. Just recently, footage emerged of cormorants stealing suckerfish off whale-shark bellies in Mexico's Baja California, and now, silky sharks in the Galapagos have been filmed stealing free scratches off their giant backs.


This curious case of shark-on-shark abrasion, featured in the BBC's groundbreaking series Blue Planet II, was filmed with the help of a whale shark "fin cam". The camera was deployed by researchers with the Galapagos Whale Shark Project (GWSP), who use the tech in the hope of learning more about how whale sharks use – and interact with – their habitat. (The cameras are non-invasive and programmed to pop off on their own.)

Despite reaching nearly 20 metres in length, whale sharks manage to keep a low profile as they traverse the world's waters. We still don't know where they give birth, and many of their migration routes remain uncharted. 

"The Galapagos is a special place for whale-shark research," the GWSP team wrote on Facebook. "Unlike in other whale shark hotspots, we see here almost exclusively large, apparently pregnant females. Studying them is important for the conservation of this endangered species, and we are making progress."

The best explanation for the silkies' behaviour is that the sharks are using their giant kin like loofahs. Shark skin is covered in thousands of tooth-like scales known as dermal denticles, and these are extremely coarse when rubbed against the grain. Coming in for a "rub-a-dub-dub" could help the smaller sharks rid themselves of parasites and dead skin, much like how orcas use pebble-bottomed coves.

The behaviour has been seen in the area before: wildlife photographer Thomas P. Peschak had a similar encounter while on assignment for National Geographic Magazine. And elsewhere in the world, other fish have been known to hit the "shark spa" when in need of a good skin peel:

A post shared by Erick Higuera (@erick.higuera) on Aug 11, 2017 at 10:53am PDT

"It does not seem to bother the whale sharks at all," says the GWSP team. 

That's not too surprising: at an impressive ten centimetres (four inches) thick, the skin of whale sharks is some of the toughest in the animal kingdom. (Their denticles can easily graze human skin!)

Why would a bus-sized animal need such tough armour? Whale sharks are docile filter-feeders, and they do have some natural predators (namely killer whales and great whites). In the face of danger, the sharks will sometimes barrel roll towards a potential threat, keeping that thick top-surface skin in the line of fire:

Learn more about whale-shark skin in our previous coverage here!



Top header image: Shutterstock