Natural Sciences and Mathematics
The shore crab Hemigrapsus oregonensis is highly variable in the coloration of its carapace, with morphologies ranging from dark brown to yellow-green, reddish, and white. This polymorphism may serve as an asset against predation if there exists selective pressure on the crabs to select substrates based on their phenotype. We sampled crabs from multiple sites along the Puget Sound, using image analysis to quantify the “whiteness” of the carapace and the substrate. The size and sex of the crabs was also recorded. Though there was not a significant relationship between carapace whiteness and substrate whiteness (possibly due to lack of significant difference between sites), female crabs were found to be significantly smaller and whiter than male crabs; furthermore, the total number of white crabs of either sex significantly declined as body size increased. These results suggest that white crabs, and especially male white crabs, experience differential mortality as their size increases. The sex difference in whiteness could be attributed to behavioral differences between males and females that lead to higher male mortality in white crabs.