The response of the wash-zone amphipod, Urohaustorius sp. (Amphipoda: Talitridae), to light, gravity, slope, and magnetic fields

Abstract

Littoral talitrid amphipods that traverse the intertidal zone have been the subject of many behavioural tests. These tests have emphasised the orientation cues required for the maintenance of these talitrids along specific zones on the land-sea axis (y-axis) of the littoral zone. Comparatively, little is known of the orientation cues required by amphipods living along the wash zone of beaches. Urohaustorius sp., family Urohaustoriidae (U. gunni Barnard and Drummond, 1982, but material unavailable for verification), a sightless wash zone amphipod from the North Coast of New South Wales, Australia was tested for its reaction to light, gravity, slope, and Earth-strength field intensity. The results for magnetic field testing indicated that individuals of Urohaustorius sp. were oriented in a meaningful fashion under an Earth-strength field intensity and light. Increasing the field intensity produced a slight misdirection effect and orientation in dark conditions was limited. Individuals appeared to use a polar compass as opposed to an inclination compass. Reaction to slope became stronger with an increase in gradient and was weaker when exposed to a compensated magnetic field. A strong negative phototaxis overrode any magnetotactic response. Gravity was determined to be the strongest orientation cue. Testing in a compensated magnetic field, however, produced no geotactic response when the amphipods were not given a direction for light. The results of this study indicate that there is a complex interplay between the orientation cues used by individuals of Urohaustorius sp. to assist in retaining its position in the splash zone on a beach. Future research into the behaviour of amphipods must consider the effect of Earth's magnetic field.