Quantitative analysis of carapace pattern polymorphism in the grapsid crab Hemigrapsus penicillatus (De Haan, 1835) (Decapoda, Varunidae)

Many shore crabs exhibit juvenile colour pattern polymorphism on their carapace, which helps avoid predation through camouflage. However, colour pattern polymorphism has been described only in a limited number of species, and its quantitative descriptions are rare. Here, we studied the Asian grapsid crab Hemigrapsus penicillatus to describe its polymorphic dichromatic carapace patterns. We specifically examined how dichromatic patterns and the degree of pattern polymorphism change as size increases and whether sexual/size differences exist in carapace patterns. Our results showed that H. penicillatus exhibits polymorphic forms in both the juvenile and adult stages. Additionally, compared with males, females were more likely to exhibit dichromatic carapace patterns and had a larger light-coloured area on their carapace. Estimation of the likelihood of the carapace being light or dark coloured suggested that certain areas on the carapace are less variable than other areas. We discuss potential adaptive explanations for our results.