Some crabs that live more than a mile deep in the ocean, away from sunlight, have an exotic color vision capability that combines sensitivity to blue light and ultraviolet.
Human eyes are unable to see ultraviolet light, whose wavelength is shorter than that of blue.
It may seem puzzling evolutionary oddity that these crabs see the light blue and ultraviolet, if we only take into account the fact that sunlight does not reach those depths. But that sensitivity to blue light and ultraviolet must have a reason for being.
The team of biologist Sonke Johnsen of Duke University in Durham, North Carolina, United States, and biologist Tamara Frank of Nova Southeastern University, believes that the ability to detect the crabs can serve to distinguish friendly food them from others they would be toxic products.
In a way, the visual ability of crabs thus works as a color code reader which identifies food products.
These animals are perhaps being worth their sensitivity to ultraviolet and blue light to distinguish corals with many likely to be toxic, which emit a faint green or blue-green bioluminescence, and differentiate well the crabs eat plankton, which shines in blue light.
The discovery explains how use their eyes for some deep-sea animals and how their sensitivity to light influences their interactions with their environment.