Intraspecific color diversity and camouflage associated with ontogeny in an insular land crab

Many species exhibit high intraspecific color variation between sexes or ontogenetic phases due to sexual selection or sex-/age-specific differential predation. In crabs, the function and adaptive value of carapace coloration have been investigated mainly in aquatic and/or intertidal species, and it is poorly understood in terrestrial species (Gecarcinidae), which are exposed to different selective pressures. Using digital photography and image analysis, we tested if the coloration of the insular land crab Johngarthia lagostoma in Trindade Island (Brazil) varies according to individuals’ size, sex and ecological processes related to the differential occupation of the available habitats. Three color types were observed (black, purple and yellow), with black crabs being exclusive and predominant in the smaller size classes (carapace width < 30 mm). After this size threshold, yellow crabs dominate throughout ontogeny, while purple individuals are less frequent. Crabs of the three color types occur in both sexes, and the frequency of each type, as well as their brightness and color metrics, was similar between males and females. Black and purple crabs occupy mainly hill areas, but yellow crabs predominate throughout the island. Camouflage by background matching seems to be particularly important for small black crabs at recruitment (sand of the beaches) and resident areas (hills vegetation and soil), where individuals exhibit higher color matching. However, although yellow and purple crabs conceal better against hill and beach backgrounds, respectively, their coloration is probably under neutral selection and has no function for camouflage, since large J. lagostoma crabs are rarely predated in nature. Significance statement Intraspecific color variability is common in many animal species and can be linked to different ecological processes. In crabs, the function of body coloration for camouflage has been studied mainly in aquatic species that are exposed to high-predation pressure. However, land crabs also exhibit a remarkable intraspecific color variation, but are exposed to reduced predation pressure especially on oceanic islands, and therefore, the function of coloration remains unexplained. We used digital photography and image analysis to assess the general appearance of an insular land crab species. Our results indicate that small individuals, which are under high predation and cannibalism risk, are black and match background color, while large crabs are purple or yellow, but their coloration probably has no camouflage function.