Cercarial dispersal is the result of fixed action patterns in response to reliable environmental cues. We tested the effect of age on the preference of Echinostoma caproni cercariae for light or dark. Individual cercariae were isolated within 10 min of release from Biomphalaria glabrata and placed in a Carolina™ Deep-Well Slide. Half of the slide (top and bottom) was covered with electrical tape to exclude light. The entire chamber of the slide was observed on low power of a dissecting microscope so the cercaria was readily visible whenever it was in the lighted portion of the slide. The amount of time a cercaria spent in the light and the number of times it crossed from light to dark during a 5-min period at 0, 1, 2, and 4 hr were determined (n = 20). The mean amount of time cercariae spent in the light declined significantly from immediately after release (127.7 sec) as compared to 1 hr (68.4 sec), 2 hr (51.6 sec), and 4 hr (10.6 sec) postemergence. The same pattern was seen in the average number of times cercariae crossed from light to dark in a 5-min period: 10.7, 7.2, 6.95, and 1.5, respectively. Cercariae showed no preference for light or dark immediately upon release (P = 0.119), nor was there a correlation between the amount of time spent in the light and the number of crossings at this time period. Cercariae spent a significantly greater amount of time in the dark with age (1–4 hr), and the number of crossings at each of these time periods was highly correlated with the time spent in the light. These findings suggest that light is not an important cue for E. caproni cercariae immediately upon release; however, they develop a strong preference for darker habitats, or an aversion to light, as they age.
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Vol. 79 • No. 1