Visual shell preference in the white-banded hermit crab supports chromatic instead of monochromatic sensation

Shell selection in hermit crabs is a demanding task that integrates multiple sensory cues, including visual information. Because the shell provides cryptic shelter from predation, the accuracy of visual assessment would yield higher fitness. Classically, many hermit crab species have been considered monochromatic based on the finding of only one visual pigment in surveyed species and shell preferences in a few well-studied species based on differences in shell and background brightness. However, it is unclear whether a tropical hermit crab uses the same monochromatic strategy, particularly given recent discoveries of multiple visual pigments in Decapoda species. Here, we tested shell selection in the white-banded hermit crab, Calcinus seurati: the crabs were tested for their preference between blue and green shells against either a blue or green background because most crustaceans carry visual pigments with maximum absorbances in the blue-green range (480-540 nm). The contact frequencies were then expected to be higher for shells with larger Weber contrast scores according to the reported monochromatic decisions. In contrast, the white-banded crabs exhibited a consistent preference for the green shell regardless of Weber contrast values. The result invites a reappraisal of the possibility of colour vision during shell assessment in hermit crabs.