Client reef fish prefer more blue-saturated cleaner wrasses Labroides dimidiatus.

In interspecific signalling, vivid colours have been interpreted to enable species recognition and maximise signal detection. Recently, it has been shown that vivid colours can also convey information on individual fitness, which could be advantageous for receivers. In the marine cleaning mutualism between the Indo-Pacific bluestreak cleaner wrasse (Labroides dimidiatus) and their reef fish visitors (known as clients), the colour of cleaners may provide behavioural cues, which clients could use to make decisions, as higher blue saturations are associated with better cleaning services. Here, we asked whether clients perceived differences in blue saturation when evaluating individual cleaners, and whether they used this information to choose them. To do this, we introduced the client threadfin butterflyfish (Chaetodon auriga) to three differently saturated video sequences of cleaners (minimum saturation, control and maximum saturation). We found that clients actively preferred video sequences of more saturated cleaners when played against lower saturations (i.e. spending more time in the preference area, facing more frequently and for longer periods, and entering the preference areas near the more saturated monitors more frequently), which provided more evidence that the blue colouration of cleaners may be a true signal of individual quality. By being able to distinguish these saturations, it is likely that in the wild, clients use this signal to make decisions and select the cleaners that provide a better cleaning service.