Cleaners show off before biting clients. (Fish Fraud).

Some of the reef fish that make their living by nibbling parasites off other fish may be luring clients into seams by offering free massages.

Widespread fish called cleaner wrasses (Labroides dimidiatus) normally graze on the parasites and diseased tissue of other species that stop by for grooming, explains Redouan Bshary of the University of Cambridge in England. However, some of the cleaner wrasses cheat now and then by taking a bite of healthy flesh out of a client.

Now Bshary proposes new twists to this seam. Client fish seem to keep track of the reputation of specific cleaner wrasses, preferentially visiting those previously seen with an unruffled customer, he reports in the Oct. 22 Proceedings of the Royal Society of London B. Also, some devious cleaners take advantage of this so-called image scoring by clients, says Bshary. They give extra-special treatment to one client, even stroking it with their fins in a fish version of a massage. When the next client settles in, perhaps lured by the apparent four-star service, the cleaner wrasse bites.

Other work has shown that people also decide whether to cooperate based on the image of their potential partners. "This is the first study of image-scoring in animals other than humans," Bshary says.

"Some cleaners have found a way to exploit image scorers," he says. "Contrary to the predictions of models, image scoring does not lead to pure cooperation" between cleaners and clients.

Bshary observed the cleaner wrasses in a sheltered bay opening into the Gulf of Aqaba at Mersa Bareika, Egypt. Some cleaner wrasses nipped their clients, which gave a visible jerk, about five times more often than typical cleaner wrasses did. The biting cleaners mostly nipped nonpredatory species, not fish that could bite them back.

Bshary monitored this fishy business for an hour at each of 28 cleaning stations. He reports that a potential client was significantly more likely to stop for a cleaning if the previous cleaning had just ended well than if it ended in an aggressive chase or spat. He scored as positive those endings in which the previous client swam away serenely in the 5 seconds before a new cleaning session started.

Bshary also noticed that about half of the biting cleaners' interactions with their littler clients consisted of fin contact only. The cleaner fish, riding on the back of the client, brushes its pelvic and pectoral fins against the client and "provides a massage," says Bshary. The fish generally do this at the end of an apparently aggressive encounter, as if making peace after a fight. The next cleaning after the massage, Bshary found, more often than not ended with biting.

Manfred Milinski of the Max-Planck Institute of Limnology in Plon, Germany, cautions that the ease for calling these behaviors image scoring is "not a watertight proof" because Bshary merely observed the behaviors and hasn't performed experiments that directly test for image scoring. However, he predicts that more research would confirm the hypothesis.