The role of the antennae in the compass-based orientation of the equatorial sandhopper Talorchestia martensii Weber (Crustacea Amphipoda)

Abstract

The equatorial sandhopper Talorchestia martensii uses the sun, the moon and the natural magnetic field as orienting compass references. In some arthropod species, the antennae are important for time compensated solar orientation. Therefore, we evaluated the role of antennae in solar, lunar and magnetic compass orientation in T. martensii. We released intact and antennaeless individuals in a Plexiglas bowl in the dark, and under the sun or moon, with and without the horizontal component of the magnetic field and recorded their orientation. For the tests under the moon, sandhoppers were released in conditions of modest azimuthal speed (10–15°/hr) and high azimuthal speed (≈ 59°/hr) of the moon. No significant difference in the orienting behaviour between antennaeless and intact individuals was observed. Therefore, the antennae do not seem to be involved in the sun or moon compass mechanisms of T. martensii. The photopositive responses showed by individuals released under high azimuthal speed of the moon and with the horizontal component of the magnetic field zeroed, indicate that the moon is probably not used as a chronometric compass reference in this species because of the variations in its azimuthal speed. This suggests a mechanism of photomenotaxis and therefore, a difference from sandhoppers from temperate latitudes that use a moon compass.

KEY WORDS:

• amphipods
• antennae
• equatorial orientation
• moon compass
• sun compass

HIGHLIGHTS

• The equatorial Talorchestia martensii is challenged by rapid azimuthal variation in celestial bodies.

• We show that magnetic orientation compensates celestial orientation when azimuthal variation is high.

• Ablating the antennae of T. martensii reveals that these appendages are not required for either solar or lunar orientation.

• We hypothesize that chronometric compass systems are used only when the azimuth variation is modest (<15°/hr).

ACKNOWLEDGEMENTS

For their kind assistance during our stay in Djibouti, we wish to thank Mr G. Rizzo (Honorary Consul of Italy in Djibouti), Drs Madian Mohamed Said, Carlo Astini, and Miriam Martinelli. We wish also thank Mr Seif (campement d’Obouki, Obock). At the end of the experimental session, the sandhoppers were released on their beach of origin.

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the authors.

Additional information

Funding

This research was funded mainly by personal funds and only in part by the University of Florence (funds ex-60% assigned to A. Ugolini).