Rock pool fish use a combination of colour change and substrate choice to improve camouflage

Camouflage can be achieved by both morphological (e.g. colour, brightness and pattern change) and behavioural (e.g. substrate preference) means. Much of the research on behavioural background matching has been conducted on species with fixed coloration and body patterns, while less is known about the role background choice plays in species capable of rapid (within minutes or seconds) colour change. One candidate species is the rock goby, Gobius paganellus, a common rock pool fish capable of rapid changes in colour and brightness when placed on different backgrounds. However, their ability to match different backgrounds is not unbounded, with some colours and brightness being easier to match than others, thus raising the possibility that gobies may use behavioural background matching to make up for their limited ability to match certain backgrounds. We used digital image analysis and a model of predator vision to investigate the ability of rock gobies to match chromatic (beige and greenish-grey) and achromatic (varying brightness) backgrounds. We then conducted choice experiments to determine whether gobies exhibited a behavioural preference for the backgrounds they were best at matching. Gobies rapidly changed their colour and brightness when placed on the different backgrounds. However, the level of camouflage differed between backgrounds: fish were better at matching beige than greenish-grey, and darker than lighter backgrounds. When given the choice, gobies displayed a behavioural preference for the backgrounds they were best at matching. Our findings therefore show that rock gobies, and probably other animals, use a combination of morphological and behavioural means to achieve camouflage and in doing so mitigate limitations in either approach alone.