Phototactic responses of the cercaria of Echinoparyphium recurvatum duringphases of sub-maximal and maximal infectivity

Phototactic responses of cercariae of the echinostome Echinoparyphium recur vatum (Digenea: Echinostomatidae) were investigated during phases of sub-ma ximal and maximal infectivity using a simple Light/dark choice chamber. The phototactic responses of a total of 400 cercariae were examined in 20 repl icate experiments during their sub-maxinal infectivity dispersal phase (0.5 h post-emergence from the first intermediate host Lymnaea peregra) and dur ing their maximal infectivity phase (2.5h post-emergence). Cercariae in the sub-maximal infectivity dispersal phase were found to be positively photot actic, significantly larger numbers of them occurring on the Light side of a light/dark choice chamber P < 0.001. Cercariae in the maximal infectivity phase were found to be negatively phototactic, significantly larger number s of them being found on the dark side of the choice chamber, P<0.001. This report of 'phototactic switching' demonstrated by the cercariae of E. recu rvatum from a positively phototactic dispersal phase to a negatively photot actic infection phase constitutes the first record of such behaviour in the cercaria of an echinostome.