Behavioral responses of a sand-beach amphipod to light and pressure
Light-oriented responses of the sand-beach amphipod Synchelidium micropleon (Barnard) in an optical arrangement that simulated the natural underwater angular light distribution were compared with previous measurements upon stimulation with a highly directional light source. Since positive and negative phototaxis occurred in both situations, phototaxis is not a laboratory artifact and will occur in nature. During rising tide amphipods move up the beach at the leading edge of the wave uprush zone. Maintenance of this position during the day results from an interaction of behavioral responses to light and hydrostatic pressure. The sequence of events is that a wave passing over the buried amphipods causes an increase in pressure, which evokes a negative geotaxis, and the animals exit from the sand. The increase in light intensity upon leaving the sand evokes an initial ascent into the wave due to positive phototaxis. This response reverses to negative within a few seconds, and the animals reenter the sand before they are swept back down the beach. The magnitudes of the increases in light intensity and pressure regulate the time until the reversal. This phototactic sequence can reoccur if upon entering the sand the animals are exposed to light levels below the lower threshold for phototaxis for at least 1 s. Reburrowing in the sand to a depth of about 7 mm will reset responsiveness. In this state, the animals are ready to ride the next wave up the beach and remain at the leading edge of the wave uprush. © 1986.
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