Color change in the Sargassum crab, Portunus sayi: response to diel illumination cycle and background albedo

Abstract

Floating mats of Sargassum macroalgae provide a model system for studying multiple aspects of animal coloration. The endemic crab Portunus sayi has heterogeneous yellow and brown patterning, which matches its algal background. We show that by fluctuating the chromatophores underneath its transparent carapace, the crab can alter its coloration within hours in response to diel variability in the ambient light field and to changes in background reflectance. Held in a naturalistic illumination and temperature regime, P. sayi displayed a distinct diel cycle of coloration, being pale at night and darker during the day. Individuals under constant illumination showed a modified cycle, retaining their nocturnal shading but becoming significantly paler during day time. On monochromatic black, grey, and white surfaces, crabs showed an ability to change coloration in response to their backgrounds, as integrated reflectance (ΣR) of crabs generally followed background albedo. This study expands on earlier work which revealed that P. sayi utilizes a distinct camouflage strategy from other cryptic Sargassum crabs to achieve background color matching in the view of predators. Dynamic color change in this species may play roles including photoprotection and enhancing camouflage in a unique marine environment.