The secret appetite of cleaner wrasses. (Fish).

The little helpers known as cleaner fish, which nibble parasites off larger reef fish, actually prefer to nibble their clients.

Earlier experiments had already caught cleaner fish apparently cheating, taking nips of flesh and skin-covering mucus from their customers, says Alexandra Grutter of the University of Queensland in Brisbane, Australia. However, she says, it wasn't dear whether the cleaner fish actually had a taste for their clients or were just hungry enough to nip the customer when the search for parasites proved arduous.

Grutter and Redouan Bshary of Cambridge University in England administered a taste test. They trained the cleaner wrasses (Labroides dimidiatus) to eat off underwater trays and then offered an array of parasites and fish-mucus samples.

The cleaners ate significantly more mucus than parasites, the researchers report. What's more, parrotfish mucus proved more appealing than snapper mucus. The taste test feeds the speculation that cleaning developed from opportunistic nipping at other fish, Grutter says. No wonder client fish periodically dart threateningly at their cleaners, she says.--S.M.