[The chronobiology of the cercaria of Ribeiroia marini (Faust and Hoffman, 1934) parasite of Biomphalaria glabrata: effect of the photoperiod on the rhythm of emergence].

Ribeiroir marini cercariage emerge from the snail-host during the night, according to a circadian rhythm. The photoperiod is the synchronizer of this rhythm. Inversion of the photoperiod is immediately followed by inversion of the periodicity of emergence. No cercaria emerge in continuous light conditions. This inhibitory effect of light exerts its influence on the emergence but not on the production of cercariae. The rhythmic emergence is almost maintained in continous dark, but darkness cannot induce emergence at any time. It is possible that an innate production rhythm interferes with the emergence rhythm.