Through predators’ eyes: phenotype–environment associations in shore crab coloration at different spatial scales

Improved camouflage through ontogenetic colour change confers reduced detection risk in shore crabs

“…We show that ontogenetic changes in coloration can facilitate improvement in camouflage and thus alter predation risk in shore crabs. Importantly, our results are in direct accordance with findings in the field (Figure , Supporting Information Figure S3), where crabs are also more green, increasingly uniform and darker with age (Nokelainen, Hubbard et al, ; Stevens et al, ). Thus, our study shows how mechanisms of colour change and adaptive value of camouflage underly how the phenotypes of wild animals change with age/size.…”

“…In contrast, we found that only crabs reared on the “mudflat” background improved their match over several weeks. Earlier work has repeatedly reported that wild adults are more uniform, green and darker in appearance than juveniles (Crothers, ; Hogarth, ; McGaw, Kaiser, Naylor, & Hughes, ; Nokelainen, Hubbard et al, ; Reid et al, ; Stevens et al, ; Styrishave et al, ; Todd et al, ). Low chromatic variability in adult crabs could also be partly a result of physiological constraints as larger crabs must invest more on reproductive structures and carapace strength rather than to maintenance of chromatic variability in protective coloration (Anderson et al, ).…”

“…Ontogenetic colour change in the green shore crab ( Carcinus maenas ) in the field. The data are derived from large‐scale field monitoring study by Nokelainen, Hubbard et al (). The figure shows the change in carapace colour over time obtained from avian vision model cone catch data.…”

“…Our results also showed that the crabs developed more uniform patterning (see also Figure S2). It is not well known what maintains the high colour variation in juvenile crabs, but it may be related to the need to match variable background habitats at spatial scales (Nokelainen, Hubbard et al, ) that are relevant when individuals are small, and/or breaking predator search image formation (Bond & Kamil, ; Duarte et al, ; Karpestam et al, ; Punzalan et al, ). It is plausible that juvenile crabs may also rely on other types of camouflage, such as disruptive coloration (Todd et al, ), and this may be habitat‐specific, with crabs from rock pools favouring disruption and crabs from mudflats tending towards background matching.…”

“…Shore crabs are located in a wide range of habitat and substrate types around the shore, each with different appearances, including estuaries, mudflats, sandy beaches, shingle, pebbles, mussel beds and rocky coastline (Brian, Fernandes, Ladle, & Todd, ; Crothers, ; Edwards, ; Stevens et al, ; Todd et al, ). The collection methods largely follow established protocols (Nokelainen, Hubbard, Lown, Wood, & Stevens, ; Stevens et al, ). Briefly, the crabs were collected by hand during low tide alongside the beach from approximately 50 m length, and thus, our sampling included crabs from different substrates.…”