Color changes of fiddler crab between seasons and under stressful conditions: Patterns of changes in lightness differ between carapace and claw

Abstract

In animals, coloration on the body surface could have various functions, such as thermoregulation, camouflage, and intraspecific communication. Because individuals often experience different ecological and social contexts/environments, changes of coloration due to these contexts/environments would be adaptive. The functions of coloration, in addition, may differ between different body regions. In this study, I compared the changes in coloration (lightness) between the carapace and the large claw of male fiddler crabs, Austruca lactea, to estimate the function of coloration in each body part in this species. I conducted monthly field surveys to assess seasonal changes of coloration of the crabs' carapace and large claw. I also carried out a field experiment to determine short-term changes in coloration under stressful conditions. Monthly surveys revealed that claw color was always lighter than the carapace color. Both of these were lightest during the breeding period. Interestingly, carapace lightness positively correlated with carapace width in the peak of the breeding season (July), while it was negatively correlated with carapace width in the non-breeding season (May and September). Claw lightness also differed significantly between months, although claw lightness was less variable than carapace lightness. In the field experiment, carapace coloration rapidly (within ten minutes) became darker under stressful conditions, but claw coloration changed very little, even after crabs were exposed to stressful conditions for 30 min. These results suggest that the function of coloration differs between carapace and claw. Carapace coloration may have functions related to thermoregulation and/or camouflage, and may therefore rapidly respond to fluctuating contexts/environments. Carapace coloration may also have a nuptial function for courtship that is related to body size. Claw coloration could be used to convey relatively more stable information, and may play a role in species recognition for mate choice, and resource holding potential for territorial fighting. Multiple factors, including physical and social factors, may affect each other, and consequently influence coloration of each body part in this species.