CIRCATIDAL VARIABILITY IN THE BEHAVIORAL-RESPONSES OF A SANDBEACH ISOPOD EURYDICE-PULCHRA (LEACH) TO ORIENTATIONAL CUES

Spontaneous, phasic changes in the behavioural responses of freshly co llected Eurydice pulchra (Leach) (Isopoda:Crustacea) to light and wate r flow were studied in the laboratory and related to what is known of the behaviour of this isopod when free swimming. During the expected f lood tide period, isopods in laboratory experiments generally expresse d positive phototaxis, high photokinesis and negative rheotaxis. On th e shore at that time they usually move upshore in the surface waters o f the surf zone. In the laboratory, a spontaneous change in rheotaxis behaviour was recorded at the time of the expected high tide and the i sopods tended to stem water currents, presumably associated with the a voidance of stranding on the shore. During the expected ebb tide perio d, isopods studied in the laboratory showed negative phototaxis, low p hotokinesis 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. pukhra burrowed before expect ed low tide, away from a directional light source. If, however, the is opods were not able to reburrow due to the absence of sand in laborato ry experiments, a return to photopositive swimming was recorded during the expected ebb, behaviour which on the shore would return the indiv iduals to the water's edge. Rhythmic responsiveness of E. pukhra to di rectional cues appears to fine-tune the tidal swimming pattern of this isopod, inducing temporal changes in behaviour which control both ver tical and horizontal distribution on the shore.