Cleaner blues: Condition-dependent colour and cleaner fish service quality.

While vivid colours in sexual signals can provide information on individual quality, vivid colours in interspecific signals have been interpreted mostly as indicating species identity and maximizing signal detection. Here we investigate if colour differences in an interspecific signal could also indicate relevant aspects of individual quality because, similarly to sexual signalling, in interspecific communication it could sometimes be advantageous to assess individual quality. For example when interacting with cleaner species, clients should benefit from assessing which individual cleaners provide better service. Since oxidative stress commonly influence condition-dependent colour signals, we oxidized the diet of cleaner wrasses (Labroides dimidiatus) in laboratory conditions to test if this affects their vivid blue skin colour and the quality of their cleaning service. Compared to controls, experimental cleaners decreased blue colour saturation after the oxidized diet treatment, decreased the quality of their cleaning service by performing less tactile stimulation and, although clients in the laboratory were de-parasitized, also tended to decrease the touching the client with the mouth. We used visual modelling, based on the spectral sensitivity of vision in various client species, to show that some client species can perceive these changes in cleaner blue colour saturation, while other client species not. We suggest that, similarly to sexual signals, some vivid colours used in interspecific communication may convey information on aspects of individual quality that are relevant to heterospecifics.