Behavioural colour change in the hermit crab Pagurus bernhardus:: reduced crypticity when the threat of predation is high

One means of avoiding detection by predators is to employ cryptic colouration. in some cases, individuals can rapidly adjust their colouration, enabling them to remain cryptic in a heterogeneous environment or to vary colouration in response to predation threat. This is often achieved physiologically but in some cases animals are able to adjust their external appearance by behavioural means. Hermit crabs could potentially enhance their crypticity by occupying gastropod shells that match the substrate. However, changing shells is risky and the benefits of crypsis must be balanced against the imminent threat of predation. Here we show that naked hermit crabs initially enter the shell that shows the greatest contrast with the substrate, but may subsequently move into a shell that offers a greater degree of background matching. However, in the presence of a predator cue they tend to remain in the shell that offers low crypticity. This indicates that rapid colour change may be effected by behavioural mechanisms but this is restricted by the need to make a behavioural decision on the basis of a range of information sources.