Phototaxis of a sand-beach amphipod: Physiology and tidal rhythms

Abstract

1. The response spectrum for phototaxis of the sand-beach amphipodSynchelidium sp. shows relatively uniform responsiveness from 460 to 600 nm (Fig. 1). 2. Animals on rising tides are more negatively phototactic and less sensitive to light than on falling tides (Fig. 2). 3. When suddenly stimulated with light on rising tides, they show an initial positive phototaxis, which quickly reverses to negative. Similar stimulation on falling tides evokes either continuous positive phototaxis or a positive response which reverses to negative after a much longer time than on rising tides. 4. Freshly collected animals have tidal rhythms in the general sign of phototaxis (more positive on falling tides) (Fig. 4) and the reversal in phototactic sign from positive to negative upon sudden stimulation with light (Fig. 3). These rhythms are endogenous and tidal, since they persist under constant conditions, and the timing is dependent upon natural tidal times. In addition, a circadian rhythm in sensitivity is suggested, in which they have a lower intensity threshold for phototaxis at night than during the day (Fig. 6). 5. It is proposed that these photoresponses are functionally significant during migration up the beach in the swash zone on rising tides and down the beach on falling tides.