Sharks like to keep clean

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Maintaining good hygiene is rendering sharks more vulnerable to humans, according a study published in a recent issue of the online journal PLoS ONE.

The study by Simon Oliver and coauthors investigated the interactions between the pelagic thresher sharks (Alopias pelagicus) and cleaner wrasses (the cleaner fish Labroides dimidiatus and the moon wrasse Thalassoma lunare) at a seamount (underwater mountain) in the Philippines.

Using remote underwater cameras at one of five cleaning stations in Monad Shoal in the Visayan Sea for 232 days over 16 months (between July 2005 and December 2009), the authors recorded more than 1200 hours of footage for analysis.  

As the site is well known to tourists for observing thresher sharks, the authors also recorded the frequency and duration of recreational SCUBA divers visiting the cleaning station.

The authors recorded 97 instances of cleaning wrasses interacting with sharks during the period of their study. Nineteen of these interactions were interrupted, of which about one third were caused by the arrival of SCUBA divers (the other two-thirds were caused by the arrival of a second shark or ray).

Of the more than 2700 instances of cleaner inspections, the wrasses concentrated most on the pelvic (34%), pectoral (23%) and caudal (22%) fins and the least on the dorsal fins and gills (2% each).  

The sharks were found to visit the cleaning sites most often during the morning, with the frequencies gradually declining throughout the day.  

The gradual decline in the frequency of pelagic thresher shark cleaning events from morning until evening may be driven by hungry cleaners, which provide higher quality services early on in the day.

The authors also found that the sharks, which typically cruise in the open waters of the oceans at high speeds, adapt their behaviour to make themselves more accessible to cleaner wrasses.  

The sharks typically swam in circles over the territories of the cleaner wrasses, lowering their tails at the same time to make themselves more attractive to cleaner fish.  At the same time, the swimming speed of the sharks was greatly decreased to about half their normal value.  

The decrease in the thresher shark's swim speed combined with the conspicuous lowering of the caudal fin and its systematic circling behaviour, may provide an increased opportunity for the cleaners to inspect, thereby making pelagic thresher sharks more attractive clients.

According to the authors, evidence is mounting that in addition to acting as social refuges and foraging grounds for large visiting marine predators, seamounts may also support pelagic ecology by functioning as cleaning stations for oceanic sharks and rays.

The results of the study underscore the importance of protecting these areas (the study site had already been degraded by decades of dynamite fishing, note the authors).

Conservation policy should take into account the facts that the pelagic thresher shark is classified as Vulnerable by the IUCN and that the sharks’ movement into shallower coastal waters makes them more vulnerable to human disturbance.

For more information, see the paper: Oliver SP, Hussey NE, Turner JR, Beckett AJ (2011) Oceanic sharks clean at coastal Seamount. PLoS ONE 6(3): e14755. doi:10.1371/journal.pone.0014755.