CLEANER FISH WEAR 'UNIFORMS' TO ADVERTISE, AVOID DANGER

Like police and nurses, cleaner fish on coral reefs wear uniforms to signal their "professions"—a tactic that also helps the fish avoid being eaten by their clients, a new study says.

Several species of small reef fish are known to invite larger fish to
stop by "cleaning stations," where the cleaners groom their customers
and pick them free of parasites.

The clients swim away spic-and-span, and the cleaners get an easy meal: "a classic example" of a mutually beneficial relationship, the researchers write.

However, scientists have long wondered how bigger, fish-eating clients find cleaners and apparently recognize that the smaller fish are off the menu.

Karen Cheney and colleagues decided to test the theory that the cleaners' colors and body patterns are what set the fish apart.

Her team found that cleaner fish—such as gobies and wrasses—are more likely to sport a dark side stripe accentuated by patches of blue and yellow.

"We believe that they do exhibit a 'cleaner uniform' in order to make them conspicuous and easy to distinguish on a coral reef," Cheney, a biologist at the University of Queensland in Australia, said by email.

Cheney and colleagues observed the behavior of several species of wild fish known to visit the cleaners at a site in the Great Barrier Reef.

The team then added fake fish, which had been painted with a range of colors and patterns, to the reef.

The researchers found that fish painted with blue colors and striped body patterns enticed more clients to pull up to a cleaning station.

The team also used a well-known model for how fish see colors to examine how three types of client fish—barracuda, damselfish, and surgeonfish—were likely to respond to various hues.

Though each fish species has a different kind of visual system, for all of them blue would contrast most against the colors of coral reefs.

Yellow would best stand out against blue water backdrops and dark lateral stripes, the authors wrote.

This would make a blue-and-yellow striped fish very obvious to clients as they passed by a reef.

Though no one knows for sure, Cheney said her new study implies that the fish's cleaning behavior evolved before the uniform.

The blue-and-yellow patterns possibly became more common over time as fish flashing those colors attracted more clients.