Surrounded by the trappings of modern life, we rarely depend on the sun and moon for guidance, but for sandhoppers scurrying across beaches the correct identification of a celestial body could be a matter of life and death. ‘The ecological problem that sandhoppers have to solve is to stay or return to the damp zone of sand’, says Alberto Ugolini from the Università di Firenze, Italy. Explaining that sandhoppers remain near damp sand for protection from high temperatures and dehydration, he adds, ‘They rely on a wide range of orienting factors such as local cues and general cues, like the sun and moon for correct orientation.’ However, for the minute crustaceans to navigate using celestial objects, they have to be able to successfully distinguish between the sun and the moon. Explaining that sandhoppers are already known to use an internal clock to compensate for the moon's shifted cycle

relative to the sun, Ugolini and his colleauges Giuditta Galanti and Luca Mercatelli decided to find out whether sandhoppers use the natural gradient of light in the sky to help them distinguish between the sun and the moon (p. 2814).

Collecting the crustaceans from a nearby beach and returning them to the laboratory, the team shifted the animals' body clocks by altering their sunrise to 18:00 h and sunset to 06:00 h. Having allowed the animals to get over their ‘jet lag’ and adjust to their new time zone, Ugolini and Galanti transported the animals outside on moonlit nights and during the day to test in which direction they oriented. If the crustaceans could correctly distinguish between their view of the sun and the moon, they would align themselves to the correct day- and night-time orientations. However, if the sandhoppers were relying on their internal clock to tell them whether the object in the sky was the sun or the moon, they would mistake the sun for the moon during daylight and align themselves in the direction that they select when viewing the moon. However, the jet-lagged crustaceans coped well. They recognised the sun and positioned themselves correctly, even though their body clock was telling them to expect to see the moon.

Having confirmed that the sandhoppers could distinguish between the sun and the moon, the trio tested the effect of the natural skylight gradient during the day on the crustaceans' ability to identify the sun. Working with Mercatelli, a physicist from the Istituto Nazionale di Ottica CNR, Italy, the team simulated day and night skies in a Plexiglas dome where they could control the light gradient across the artificial sky and the intensity of the sun and moon simulations. Testing the sandhoppers during the day and night, the team simulated skylight gradients in daytime skies with a sun and recorded the sandhoppers' orientations. As they had expected, the sandhoppers positioned themselves correctly during the day when they saw the daytime sky. And when the team showed the crustaceans a daytime sky during their physiological night, the crustaceans still positioned themselves in their daytime orientation. Finally, the team removed the light simulating the sun and retested the sandhoppers' reactions during the day and night to daytime skylight gradients alone, and the crustaceans still oriented themselves toward their daytime direction, even during the night.

So sandhoppers use the skylight gradient to correctly identify the sun, and Ugolini is now keen to understand the mechanism of this crustacean's internal physiological clock, which allows it to compensate for the shifting cycle of the moon relative to the sun's diurnal clock.


The skylight gradient of luminance helps sandhoppers in sun and moon identification
J. Exp. Biol.