Co-infection of two eukaryotic pathogens within clam populations in Arcachon Bay.

The parasitic species (= ) (Perkinsea, Alveolata) infects a wide range of mollusc species and is responsible for mortality events and economic losses in the aquaculture industry and fisheries worldwide. Thus far, most studies conducted in this field have approached the problem from a "one parasite-one disease" perspective, notably with regards to commercially relevant clam species, while the impact of other species should also be considered as it could play a key role in the disease phenotype and dynamics. Co-infection of and has already been sporadically described in Manila clam populations in Europe. Here, we describe for the first time the parasitic distribution of two species, and , in individual clam organs and in five different locations across Arcachon Bay (France), using simultaneous detection by quantitative PCR (qPCR) duplex methodology. We show that single-infection largely dominated prevalence (46-84%) with high intensities of infection (7.2 to 8.5 log-nb of copies. gof wet tissue of Manila clam) depending on location, suggesting that infection is driven by the abiotic characteristics of stations and physiological states of the host. Conversely, single infections were observed in only two sampling stations, Ile aux Oiseaux and Gujan, with low prevalences 2 and 14%, respectively. Interestingly, the co-infection by both spp., ranging in prevalence from 12 to 34%, was distributed across four stations of Arcachon Bay, and was detected in one or two organs maximum. Within these co-infected organs, largely dominated the global parasitic load. Hence, the co-infection dynamics between and may rely on a facilitating role of in developing a primary infection which in turn may help infect as a reservoir for a preferred host. This ecological study demonstrates that the detection and quantification of both parasitic species, and , is essential and timely in resolving cryptic infections and their consequences on individual hosts and clam populations.