Camouflage and Pattern Change in the Common Shore Crab Carcinus maenas

Abstract

In nature, one of the most common and effective adaptions for reducing the threat of predation is to decrease the likelihood of detection through camouflage. The shore crab (Carcinus maenas) faces several predators in the wild and is also widely distributed across several habitats differing in substrate. Previous literature has recorded the diversity in shore crab pattern particularly amongst juveniles, linking the variation in pattern to differences in habitat substrate. A possible explanation for these phenotype - environment associations is morphological pattern change. However, the plasticity of pattern variation in shore crabs has received little attention. Building on previous studies’ findings, that the shore crab is capable of changing brightness, for the first time, this study assesses whether shore crabs are also capable of changing carapace pattern under experimental conditions, to improve camouflage over a period of 12 weeks on two artificial backgrounds. My findings show that indeed, shore crabs show plasticity in carapace pattern and brightness, and that, through the eyes of avian and fish vision, this resulted in improved camouflage for individuals on the uniform artificial background. Results for individuals on the patterned artificial background were less conclusive. The second part of this thesis focused on explaining the significance of this plasticity and phenotypic variation in the shore crabs natural habitats. The study quantified two strategies of camouflage; background matching and disruptive colouration. This was to establish differences in strategy between shore crabs from rockpool habitats and shore crabs from mudflat habitats. I further tested for differences between juveniles and adults amongst these habitats. My findings found clear differences between habitats. Using an avian predator model for vision, results for individuals from rockpool habitats highlighted significantly higher edge disruption than shore crabs from mudflats and conversely, shore crabs from mudflat habitats were found to have a significantly better match between carapace pattern and background pattern than rockpool crabs. In addition to this, our findings indicated differences between adults and juveniles. These findings provide support for differences in camouflage strategy between habitats and suggest that the effectiveness of the strategy may change as crabs mature. Overall, the findings provide evidence for improved camouflage through pattern and luminance change in shore crabs under experimental conditions, and differences in camouflage strategy between crabs collected from homogeneous habitats and those from heterogeneous habitats.