Thresher sharks visit cleaning stations to rid themselves of nasty parasites, say researchers.
Scientists filmed sharks off the coast of the Philippines visiting a tropical seamount – or undersea mountain.
This is a habitat for cleaner fish, which nibble off parasites and dead skin.
The sharks repeatedly visited the station and swam slowly around, giving the fish time to delouse them.
The findings were recently published in the journal PLoS One.
This is the first time the behaviour has been seen in this species and the researchers say it shows how vital these shallow reef habitats are for the large, threatened predators.
The lead researcher, Simon Oliver from Bangor University in the UK, has been studying thresher sharks for more than five years and founded the Thresher Shark Research and Conservation Project.
For the PLoS One study , he filmed over 1,000 hours of footage of the sharks at a seamount off the northern tip of Cebu in the Philippines.
“They visit the site very regularly,” Mr Oliver told BBC News. “A huge dive tourism site has evolved around them.”
The sharks’ behaviour suggests that they go there specifically to be cleaned.
“They pose, lowering their tails to make themselves more attractive to the cleaners,” he explained.
“And they systematically circle for about 45 minutes at speeds lower than one metre per second.” This is about half the speed at which the sharks usually swim.
These reefs, which are habitats for cleaner wrasses – as the industrious little fish are known – are probably vital for the sharks’ health.
“It’s like us going to our local GP if we had a head full of lice,” said Mr Oliver. “If we weren’t able to get them treated, they could cause infections and other complications.
“Our findings underscore the importance of protecting areas like seamounts which play an important part in [the sharks’] life strategy to maintain health and hygiene.”
Mr Oliver pointed out that site of this research had already been badly damaged by dynamite fishing.
Bangor University marine biologist, Dr John Turner, who also took part in the research, said: “The work uniquely describes why some oceanic sharks come into coastal waters to perform an important life function which is easily disturbed by man.”