Trematode cercariae inhabit predictable environments and respond to trigger cues with genetically fixed releaser responses when foraging for the upstream host. The effect of light and gravity on the transmission of Echinostoma caproni cercariae to Biomphalaria glabrata was investigated experimentally. Transmission chambers were constructed of clear polyvinyl chloride pipe. Snails were constrained within the chamber to prevent movement, while permitting the cercariae to swim freely. A trial consisted of 2 infected B. glabrata shedding E. caproni cercariae placed at the center of the chamber, with 5 uninfected B. glabrata placed 10 cm on either side (or above and below) of the shedding snails as sentinels. There was no significant difference in the prevalence of infection sentinel snails in either experiment (light vs. dark or top vs. bottom); however, mean intensity was significantly higher in sentinel snails in the dark portion of the chamber (42.5 vs. 10.4; P = 0.001) and the top of the transmission chamber (66.1 vs. 38.0; P = 0.0003). There was a high correlation between the number of metacercariae collected from sentinel snails and the total number of infective units (metacercariae unsuccessful cercariae): r = 0.992 (light vs. dark) and r = 0.957 (top vs. bottom), respectively, at cercariae densities estimated from 22 to 3,304/L. The results suggest that cercariae of E. caproni exhibit negative photo- and geotaxis in searching for a second intermediate host. Stereotypical releaser responses to environmental trigger cues (light and gravity) allow E. caproni cercariae to exploit flexible strategies for completing the life cycle consistent with the broad range second intermediate and definitive hosts used by E. caproni cercariae and adults, respectively.
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