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Pantry or predator? A ribbonlike Labroides dimidiatus sidles alongside a potential customer.

Dangerous Do-si-do

Dancing on the job just might save your life--at least for fish that earn their meals by picking parasites off other fish. The danger is clear: The client fish may at any moment decide the cleaner is worth more as a meal than a groom. Now, a researcher suggests that the dance of the cleaner fish plays a role in keeping the perilous relationship calm.

Biologists have proposed a number of ways that this uneasy partnership between predator and potential prey could remain happy for both parties. Some suggested that when the client fish strikes its "come-hither" pose, with an open mouth or flaring gills, it doubles as an assurance that the client isn't looking for a meal, just a good nit-picking. Other observers suggested that the wagging dance cleaner fish sometimes make, in which the fish taps the client with its rear fins and tail while bobbing up and down, might be a way for the cleaner to predict or manage the client's behavior.

To test some of these possibilities in the lab, marine biologist Alexandra Grutter of the University of Queensland in Brisbane, Australia, videotaped 144 hours of client and cleaner interactions in test aquaria. She looked at Labroides dimidiatus, one species of cleaner fish on the Great Barrier Reef. Some of the 56 client fish were well-fed, and therefore unlikely to threaten the cleaner; other clients were hungry. In addition, some clients in each group carried a heavy load of parasites, while others were less laden with creepy-crawlies.

In the 22 June issue of Current Biology, Grutter reports that the client fish with the most parasites posed most often, regardless of whether they were sated or feeling peckish. This suggests that posing doesn't mean a client is low-risk, she says. Indeed, she caught the demise of three of the 48 cleaner fish on film. But one behavior did seem to track with the hunger level of the clients: Cleaner fish performed their waggling dance at the side of the client more often when the client was hungry. She says this may somehow help the cleaner avoid injury from risky clients.

The work does seem to suggest cleaners can sense when their client is hungry, says evolutionary biologist Lee Dugatkin of the University of Louisville in Kentucky. But the study leaves him hanging on a crucial question: How does the dancing prevent conflict? There's a lot more to be understood about the story of cooperation between cleaner and client, he says.

Related sites
More information on cleaner fish
Alexandra Grutter's home page