Factors Driving Background Choice in Scorpionfish

For a successful hunt, marine ambush predators such as scorpionfish need to be well camouflaged to deceive their prey. When the natural environment is heterogeneous, one strategy to maintain camouflage is choosing backgrounds to achieve better crypsis. We tested if two Mediterranean scorpionfish species, Scorpaena maderensis and S. porcus, select backgrounds according to this strategy. By using visual modelling on data extracted from standardised photography of natural substrates, we show that scorpionfish should be able to visually distinguish between different substrates using achromatic rather than chromatic contrast information. We then conducted a behavioural experiment where scorpionfish could choose between backgrounds that were similar or different (lighter and darker) to their own average body luminance, as measured in previous studies. Unexpectedly, scorpionfish did not choose backgrounds of luminance similar to their own but instead settled preferentially on darker backgrounds. To investigate these results further, we characterised the colouration of scorpionfish's distinct pattern components after their choice using calibrated image analysis. We show that the darker parts of the fish pattern best matched the dark background for both species, and that S. porcus increased internal pattern contrast when choosing the darker background. We therefore propose that the preference for dark backgrounds enhances camouflage via disruptive colouration. The choice for specific backgrounds, in addition to their ability to rapidly change colour and intensify disruptive colouration, can potentially allow scorpionfish to camouflage in a broad range of microhabitats.