Difference in skylight intensity is a new celestial cue for sandhopper orientation (Amphipoda, Talitridae)

We investigated the use of the skylight intensity gradient as a new celestial compass cue in sandhoppers (Talitrus saltator). Experiments were conducted at different latitudes (temperate and equatorial) with different zenithal distances of the sun under a translucent (opaline) Plexiglas dome. We also used a blue filter placed on the translucent dome to remove the spectral sky gradient, and a screen for the sun to remove the skylight intensity gradient. Therefore, in our experimental conditions, the spectral gradient and skylight polarization were impossible to use as directional information. Sandhoppers used the difference in skylight intensity in their directional choice when zenithal distance was greater than 10°. Therefore, the anisotropic radiance distribution (i.e. the difference in skylight intensity between the hemidome containing the sun and the opposite one) is a real celestial factor in animal spatial orientation. However, it is probably more imprecise than the solar orientation and could not be used by all animals.