Circatidal variability in the behavioural responses of a sandbeach isopod Eurydice pulchra (Leach) to orientational cues

Abstract

Spontaneous, phasic changes in the behavioural responses of freshly collected Eurydice pulchra (Leach) (Isopoda;Crustacea) to light and water flow were studied in the laboratory and related to what is known of the behaviour of this isopod when free swimming. During the expected flood tide period, isopods in laboratory experiments generally expressed positive phototaxis, high photokinesis and negative rheotaxis. On the shore at that time they usually move upshore in the surface waters of the surf zone. In the laboratory, a spontaneous change in rheotaxis behaviour was recorded at the time of the expected high tide and the isopods tended to stem water currents, presumably associated with the avoidance of stranding on the shore. During the expected ebb tide period, isopods studied in the laboratory showed negative phototaxis, low photokinesis and a resumption of negative rheotaxis, when on the shore they migrate down in the water column and so downshore in the undertow. When held with sand in a test tank, E. pulchra burrowed before expected low tide, away from a directional light source. If, however, the isopods were not able to reburrow due to the absence of sand in laboratory experiments, a return to photopositive swimming was recorded during the expected ebb, behaviour which on the shore would return the individuals to the water's edge. Rhythmic responsiveness of E. pulchra to directional cues appears to fine-tune the tidal swimming pattern of this isopod, inducing temporal changes in behaviour which control both vertical and horizontal distribution on the shore.