Sky radiance and spectral gradient are orienting cues for the sandhopper Talitrus saltator (Crustacea, Amphipoda)

The sandhopper Talitrus saltator relies on both the sun and the moon compasses to return to the belt of damp sand on the beach in which it lives buried during the day. In addition to the sun, the gradient of radiance and the spectral distribution across the sky could provide directional information that T. saltator can potentially use to orient itself during the day even when the sun is not visible (e.g. cloudy sky). The scope of this work was (1) to determine the intensity levels of sky radiance that the sandhoppers use in their zonal recovery and (2) to investigate whether this species relies on the celestial spectral gradient in its zonal recovery. Sandhoppers were tested in the laboratory under artificial radiance or spectral gradients. Our results show that under an artificial sky simulating the natural radiance gradient on a cloudless day, sandhoppers orientated toward the correct seaward direction of their home beach; however, individuals lost their ability to use the intensity gradient as an orientation cue when the radiance was attenuated by at least 40%. Sandhoppers were also able to head in the correct seaward direction of their home beach at any time of the day by using the spectral gradient as their only source of visual orientation reference.