Cercariae, like miracidia, are non-parasitic larval stages implicated in the life cycle of all trematodes for the host-to-host parasite transmission. Almost all cercariae are free-living in the external environment. With a few exceptions (cercariae of Halipegus occidualis (Halipegidae) can live several months, Shostak & Esch, 1990a), cercariae have a short active life during which they do not feed, living on accumulated reserves. Most cercariae encyst as metacercariae in second intermediate hosts which are prey of the definitive host; in certain species, the interruption of the active life is achieved by an encystment in the external environment (or a simple immobile waiting strategy in a few species). In some two-host life cycles, the cercariae develop into adults after penetration (this is the case for various species causing human schistosomiasis). Some cercariae do not leave the mollusc which must then be ingested by the definitive host.

No abstract

Marteilia refringens is a major pathogen of the European flat oyster, Ostrea edulis Linnaeus. Since its description, the life-cycle of this protozoan parasite has eluded discovery. Attempts to infect oysters experimentally have been unsuccessful and led to the hypothesis of a complex life-cycle involving several hosts. Knowledge of this life-cycle is of central importance in order to manage oyster disease. However, the exploration of M. refringens life-cycle has been previously limited by the detection tools available and the tremendous number of species to be screened in enzootic areas. In this study, these two restrictions were circumvented by the use of both molecular detection tools and a mesocosm with low biodiversity. Screening of the entire fauna of the pond for M. refringens DNA was systematically undertaken using PCR. Here, we show that the copepod Paracartia (Acartia) grani is a host of M. refringens. Not only was DNA of M. refringens consistently detected in P. grani but also the presence of the parasite in the ovarian tissues was demonstrated using in situ hybridization. Finally, successful experimental transmissions provided evidence that P. grani can be infected from infected flat oysters.