Scientists first suspected the crabs' unusual day to night colour change after noticing differences in images of them taken at different times. Rather than directly reacting to the ambient light, the crabs combined a natural daily rhythm of colour change with reactions to the colour of the surface they occupied.
This changes their camouflage so that they are very well camouflaged against the yellow sand during the day, and dark at night - we think to blend in with shadows on the beach.
Individuals did not change colour when put into dark conditions, but they did become brighter when placed on a white versus a black substrate. Our findings show that ghost crabs have a circadian rhythm of colour change mediating camouflage, which is fine-tuned by adaptation to the background brightness.