Camouflage and individual variation in shore crabs (Carcinus maenas) from different habitats - PubMed (original) (raw)
Camouflage and individual variation in shore crabs (Carcinus maenas) from different habitats
Martin Stevens et al. PLoS One. 2014.
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.
Conflict of interest statement
Competing Interests: The authors have declared that no competing interests exist.
Figures
Figure 1. Examples of the camouflage of shore crabs with different colors and patterns.
These crabs were all photographed at one of the study sites (Falmouth) and show how well different phenotypes can match different substrate patches.
Figure 2. Variation in the colors and patterns of crabs.
Shore crabs are highly variable in appearance, as demonstrated by the diversity of crabs in this image with the majority of them here collected from just one site (Falmouth). Crabs in the two right columns are mainly juveniles of sizes below approximately 10–15 mm carapace width, whereas crabs on the left three columns are individuals greater than 15 mm carapace width. Note that the crabs in this image show the range and extremity of forms that occur, rather than being representative of the most common phenotypes at any given site.
Figure 3. Correlation plots of the six different metrics of appearance with increasing crab size.
All correlations were significant showing that crabs change in appearance as they grow larger, presumably through successive molts. For most variables the amount of spread in the data decreases with increasing crab size, showing that smaller crabs have high individual variation and larger crabs are more uniform.
Figure 4. Proportions of different substrate types present at the four study sites.
A. Falmouth is characterized by a range of substrates but predominately rock, gravel/stones, and green/brown seaweed, the latter of which increases greatly lower down the shore. B. Godrevy is a site dominated by mussel beds and rock with little change with shore height. C. Helford is a mudflat area with rockpools and gravel at high shore heights being replaced by flat mudflats lower down the shore. Note that even though the high tide zone is dominated by rock and gravel there is a great deal of muddy sediment here making the appearance at all three zones muddy. D. St Mawes is a rockpool site dominated by rocks and gravel substrate, with some green/brown algae especially lower down the shore. Although the number of substrate types present is low, subjectively the appearance of the substrate is very diverse in color and contrast. Images show examples of the backgrounds at each site.
Figure 5. Difference in the appearance of adult and juvenile crabs from the four study sites.
Each panel shows crab phenotypes that to human eyes were broadly most representative of the common appearances found at each site. In each case, adult crabs are in the left columns and juveniles on the right. Broadly, crabs from St Mawes were the most diverse (hence the extra numbers of crabs shown here), in addition to juvenile crabs from Falmouth. Both these crab types show high diversity in color and pattern. In contrast, crabs from Helford were least diverse and were more uniform (less patterned) and blue-green in color.
Figure 6. Differences in the appearance of adult and juvenile crabs from four different sites.
Crabs show a wide range of significant differences among sites and life stages. Brightness is the overall reflectance of the carapace, saturation can be considered as the amount of a given color type compared to white light or color richness, and hue is the color type and is based on a ratio of color channel values (see main text), with lower values meaning that crabs are more blue-green in color. Proportion energy relates to how much one marking size dominates the crab patterns (larger values mean one or a few markings are prevalent), total energy equates to the contrast of the patterns (higher values mean more contrasting markings), and marking size is the predominant marking size found on the crabs. Small thumbnail images of crabs correspond to example individuals from the entire dataset (across all sites and life stages) with the maximum (top images), minimum (bottom images), and approximately average (middle images) values for each appearance metric.
Figure 7. Differences in the brightness of crabs among sites at different heights of the shore.
As well as showing significant differences in brightness (overall carapace reflectance) among sites, there were clear effects of shore height at Falmouth and St Mawes. At the former, crabs are brighter lower down the shore, whereas at the latter there is the opposite relationship. Crabs from Godrevy and Helford show little relationship between brightness and shore height.
Figure 8. Example granularity spectra used to calculate aspects of crab patterns.
There were large differences in the patterns of crabs with some showing markings dominated by a small number of pattern sizes of very high contrast (spectra with strong peaks), and others with more uniform appearances with a range of low contrast patterns (relatively flat spectra of low amplitude). There were strong associations of pattern attributes with different sites.
Figure 9. Differences in the individual diversity of crab appearances among sites and ages.
Data were analyzed in a multidimensional phenotypic space (MDPS) incorporating all six measurements of color and pattern (see main text). Crabs from St Mawes and juveniles from Falmouth show very high levels of individual variation, whereas crabs from other sites were less variable and showed little difference between adults and juveniles.
References
- Maan ME, Cummings ME (2008) Female preferences for aposematic signal components in a polymorphic poison frog. Evolution 62:2334–2345. -PubMed
- McLean CA, Stuart-Fox D (2014) Geographic variation in animal colour polymorphisms and its role in speciation. Biol Rev 89:860–873. -PubMed
- Stevens M (2013) Sensory Ecology, Behaviour, and Evolution. Oxford: Oxford University Press.
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