Camouflage and Individual Variation in Shore Crabs (Carcinus maenas) from Different Habitats

Abstract

Camouflage is widespread throughout the natural world and conceals animals from predators in a vast range of habitats. Because successful camouflage usually involves matching aspects of the background environment, species and populations should evolve appearances tuned to their local habitat, termed phenotype-environment associations. However, although this has been studied in various species, little work has objectively quantified the appearances of camouflaged animals from different habitats, or related this to factors such as ontogeny and individual variation. Here, we tested for phenotype-environment associations in the common shore crab (Carcinus maenas), a species highly variable in appearance and found in a wide range of habitats. We used field surveys and digital image analysis of the colors and patterns of crabs found in four locations around Cornwall in the UK to quantify how individuals vary with habitat (predominantly rockpool, mussel bed, and mudflat). We find that individuals from sites comprising different backgrounds show substantial differences in several aspects of color and pattern, and that this is also dependent on life stage (adult or juvenile). Furthermore, the level of individual variation is dependent on site and life stage, with juvenile crabs often more variable than adults, and individuals from more homogenous habitats less diverse. Ours is the most comprehensive study to date exploring phenotype-environment associations for camouflage and individual variation in a species, and we discuss the implications of our results in terms of the mechanisms and selection pressures that may drive this.

Color Change, Phenotypic Plasticity, and Camouflage

“…Second, shore crabs are a common and widely distributed species found in many habitats, potentially requiring different appearances to provide camouflage in each. Consistent with this, crabs from rocky, mudflat, and mussel bed type habitats show differences in both color and pattern (Todd et al, 2006(Todd et al, , 2012Stevens et al, 2014b; Figure 3). However, it is worth noting that despite a number of studies investigating phenotypeenvironment associations that are likely for camouflage (across a range of animal taxa), only one study of sand fleas has tested camouflage directly (e.g., phenotype-environment matching; Stevens et al, 2015).…”

“…First, there is increasing evidence that juveniles can undergo some changes in brightness that may improve camouflage (Powell, 1962b;Stevens et al, 2014a). In addition, they very likely undergo substantial changes in appearance through phenotypic plasticity as they molt (Todd et al, 2006;Stevens et al, 2014b;Jensen and Egnotovich, 2015; Figure 2). Indeed, signaling patterns in fiddler crabs can undergo substantial changes between molts (Detto et al, 2008).…”

“…to disruptive coloration (Hogarth, 1978;Stevens et al, 2014b; Figure 4). The level of diversity is linked to the habitat type/location where they live.…”

“…One of the advantages of phenotypic plasticity is that it can enable animals to change their appearance depending on the habitat where they live, or even to resemble specific background types/microhabitats (Keeble and Gamble, 1899;Gamble and Keeble, 1900;Rosenblum, 2006;Todd et al, 2006;Stevens et al, 2014bStevens et al, , 2015Hultgren and Mittelstaedt, 2015;Jensen and Egnotovich, 2015;Russell and Dierssen, 2015;Duarte and Flores, 2016). This may be particularly valuable in species with high dispersal and planktonic larval stages, such as in many crustaceans, because there may be uncertainty as to where juveniles will settle and what the visual environment is like.…”

“…This may be particularly valuable in species with high dispersal and planktonic larval stages, such as in many crustaceans, because there may be uncertainty as to where juveniles will settle and what the visual environment is like. Another feature of many of these species is that they have high levels of intraspecific diversity in color and pattern (Figures 4, 5) even at the same locality (Keeble and Gamble, 1899;Gamble and Keeble, 1900;Todd et al, 2006;Stevens et al, 2014b;CarvalhoBatista et al, 2015;Hultgren and Mittelstaedt, 2015;Jensen and Egnotovich, 2015;Duarte and Flores, 2016). The reasons for this are as yet largely unclear.…”