Photoperiodic cercarial emergence patterns of the digeneans Echinoparyphium recurvatum and Plagiorchis sp. from a mixed infection in Lymnaea peregra

Photoperiodic emergence patterns of the cercariae of two digeneans, Echinop aryphium recurvatum (Echinostomatidae) and Plagiorchis sp. (Plagiorchiidae) , were studied experimentally using a photoperiodic regime of 12 h light : 12 h dark at a constant 18 degrees C. In single species infections of the f irst intermediate host snail Lymnaea peregra, the cercariae of E, recurvatu m emerged exclusively during the light phase and those of Plagiorchis sp. e merged exclusively during the dark phase. Ln double infections, each specie s retained its own discrete photoperiodic emergence pattern. This result de monstrates the absence of interference between the mechanisms responsible f or the photoperiodic emergence pattern of each species. A degree of interfe rence between the two parasites in double infections of L, peregra was evid ent. The presence of E, recurvatum significantly reduced the number of Plag iorchis sp, cercariae emerging in a 24 h period and also delayed the mean p eak emergence time of Plagiorchis sp, cercariae by 2 h. This result is cons istent with the well. documented antagonistic effect that the redial stages of echinostomes are known to have against the sporocysts of other digenean s within the same host mollusc.