How Do You Clean A Shark's Skin?: small reef fish know how

Sharks have amazing skin - sleek and smooth when felt in one direction, as rough as sandpaper when felt in another. Shark skin is built on denticles, rather than scales, which is similar to the compounds that also make up the predator's teeth. In many species of pelagic, or open ocean, sharks, the skin is tightly woven and very hydrodynamic, making it impervious to many but not all of the ocean parasites that are floating about looking for a host and a free ride.

For those persistent parasites, like copepods, which are able to latch onto a passing shark, some sharks, according to researchers at Bangor University, North Wales, U.K., will make use of one of nature's symbiotic relationships that is also utilized by many reef fish to rid themselves of annoying parasites.

In reef communities, from tropical to cold water, there are small fish that will remove parasites and other material from the skin of larger fish. By doing so, the smaller fish is provided a food source and the larger fish receives a hygienic cleaning. Symbiosis evolves because the larger fish, which could easily consume the smaller fish, realizes the benefit from the smaller fish's actions and allows the cleaning fish to go about its business, even swimming into the larger fish's mouth and through its gills to provide a thorough cleaning.

Having studied several species of sharks in the Philippines, Bangor University researcher Simon Oliver says that several sharks, like the endangered thresher shark, will sometimes move into coastal areas not to feed but for the express purpose of getting a parasite-removal cleaning.

"Parasites are extraordinarily successful organisms and would propagate if the sharks had no way of getting rid of them. So these cleaning services are essential to the life history of these animals," said Oliver.

In tropical environments, the cleaner wrasse, or bluestreak wrasse, is known for staying in one area, establishing a "cleaning station" where large fish will hover patiently while the cleaner wrasse goes about its work - an oceanic car wash of a sort. In colder climates, fish like the senorita, another type of small wrasse, perform the same function. Oliver suspects that, while not all species of shark are able to literally stop and hover while getting a grooming, he did see evidence of sharks coming in close and getting a beneficial once-over by resident cleaner fish.

Oliver also observed that the sharks would momentarily set aside the tendency of some species to resist close contact with other species of sharks.

"It's like a lion at a waterhole with an antelope. Its thirst takes precedence over the natural order of things. The grey reef shark could easily take a bite out of a thresher, or a ray, but doesn't, which shows the necessity for these cleaners" said Oliver.

Not all sharks will swim to sea mountains or coastal reefs to take part in this kind of dermatological hygiene. Makos and white sharks can be observed with long, hair-like streamers along the edges of their fins - the telltale sign of established parasites. But it is another fascinating aspect of behavior in the ocean environment that even pelagic animals, like some species of shark, have come to recognize and take advantage of what the reef fish community has to offer other than being a source of prey.

Read more about shark cleaning in the BBC News.