Optic flow and sea-land orientation in the sandhopper Talitrus saltator

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.

Landscape vision and zonal orientation in the Equatorial sandhopper Talorchestia martensii

We investigate the role of the landscape in the zonal recovery of the Equatorial sandhopper Talorchestia martensii Weber. It is known that this species uses the sun and the magnetic compasses to return to the belt of damp sand of the beach following the shortest route (the sea-land axis). However, the sun is not always easy to use at Equatorial latitudes because of astronomical reasons (its zenithal culmination and its hourly azimuthal speed) at least during the central time of the day (around noon) and close to the equinox. Our experiments were performed in Kenya, during the equinoctial period. We tested adult individuals, belonging to Malindi (Kenya) population, in a confined environment with and without the vision of the landscape of their home beach and with the vision of the prominent landscape of a different-orientated shore (Temple Point). Releases were carried out with either natural or zeroed magnetic field. Results clearly show the importance of the landscape as an orienting factor mainly during the central hours of the day when it seems to assume a greater importance than magnetic cues.