Sparkle-Clean Sharks: thresher sharks practice good hygiene in the Philippines

As marine biologists continue to study the physiology and behaviors of sharks, we are learning more and more about these fascinating creatures and what we can do to protect them if it's not too late. Case in point:

Thresher sharks have been greatly depleted in number - caught for their fins and meat - which is disappointing over and above the obvious loss in population because it is a very unique species. The thresher shark's namesake elongated tail fin has been shown to be used as a hunting tool, slashing and stunning its prey. Diver encounters with thresher sharks have always been scarce as they often swim in deep, open waters and don't typically congregate in large groups.

However, off the northern tip of Cebu in the Philippines, a dive tourism business is growing based on the thresher shark. Apparently, a concentration of thresher sharks frequent a seamount there, which increases the opportunity for divers to see these beautiful sharks in the wild.

Researchers from U.K.'s Bangor University have been studying the behavior of these sharks in the Philippines and have recently published their findings in the journal PLoS One. According to the research team, the thresher sharks are frequenting the deep seamount to get a good cleaning. Residing at the seamount are a number of cleaner wrasses, a fish well-known for cleaning larger reef fish by picking away parasites and dead skin or scales. It seems that thresher sharks have caught on to the benefits of this hygienic service and will cruise close to the seamount, slowing down to half their normal speed and allowing the cleaner wrasses to give them a thorough once over.

The researchers have filmed this behavior, amassing over 1,000 hours of footage. They are quick to point out that mankind can have a disruptive effect on this symbiotic relationship - the seamount already shows evidence of dynamite fishing, a local commercial technique that literally destroys reef communities.

According to lead researcher Simon Oliver, “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.”

Fellow researcher Dr. John Turner said, regarding their studies, “The work uniquely describes why some oceanic sharks come into coastal waters to perform an important life function which is easily disturbed by man.”

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.