The moon orientation of the equatorial sandhopper Talorchestia martensii Weber

Abstract

The difficulties to use the moon as a compass cue are well known: in the same lunar month, the moon never rises at the same hour, it does not show the same shape, and it is not always visible at night. At the equator, the use of the moon as an orienting cue is even more difficult than in the temperate latitudes. In addition to the difficulties listed above, it should be added (1) the relevant variation in its hourly azimuthal speed when the moon approaches the zenith, (2) the zenithal culmination (i.e., no angle on the horizontal plane), and (3) its changes in the culmination (from North to South and vice versa). Here, I present some experiments carried out using the equatorial sandhopper Talorchestia martensii during the zenithal culmination of the moon to clarify its use as an orienting cue taking into account the already demonstrated use of the magnetic field in the orientation of this species. Experiments were carried out in confined environment, with the magnetic sNorth deflected to East, in nights of zenithal culmination of the full moon. The results indicate that the moon is used together with the magnetic field by T. martensii when the azimuthal variation of the moon is ≤10°/h and its zenithal distance is >10°. However, when the moon’s azimuthal variation is >10°/h and its zenithal distance is ≤10°, the moon is no longer used as an orientating cue. The sole compass reference is now the magnetic field. Therefore, equatorial sandhoppers use the same relationship between orienting mechanisms to overcome the difficulties with astronomical orientation to the sun or the moon.

Significant statement

At the equator, the use of the moon as an orienting cue is difficult. Sandhoppers use the moon and the magnetic field only when the moon is far from the zenith; otherwise, they use the magnetic compass alone.