Color change in the Sargassum crab, Portunus sayi: response to diel illumination cycle and background albedo
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.
We wish to thank the staff of Keys Marine Lab in providing facilities, vessel support, and collection assistance, as well as an anonymous reviewer for suggestions to improve an earlier version of this manuscript.
This work was funded by the Office of Naval Research Multi-University Research Initiative (N000140911054), and by a pre-doctoral award from the University of Connecticut Department of Marine Sciences.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
Human/animal rights statement
All applicable national, state, and University of Connecticut ethical standards regarding the use of animals were observed. Only invertebrates were used in this study. All care was taken to ensure humane treatment of animals.
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