Since October 2016, the endangered fan mussel, Pinna nobilis, has suffered mass mortality episodes throughout the Mediterranean Sea caused by a haplosporidian parasite, Haplosporidium pinnae, which could eventually lead to the extinction of the species. In response to this crisis, rescue projects have emerged in several Mediterranean countries and tend to focus on reintroducing individuals to their natural environment. To ensure that the reintroduction of healthy P. nobilis is successful, it is imperative that the parasite is no longer present in the natural habitat and that there is no risk of a new infection. Instances in which molluscs have been infected by haplosporidian parasites, in habitats where the host species was absent for a couple of years, have been reported, suggesting that haplosporidian parasites can persist in the ecosystem for at least 2 y. We hypothesized that H. pinnae could survive without P. nobilis, using another host and without causing similar mortality. This study aimed to test if other bivalve species living in the same environment as P. nobilis could act as potential reservoirs for the parasite. The presence of the parasite was investigated using haplosporidian PCR barcoding with specific primers in 15 species of marine bivalves collected at eight sites spread along the Southern French Mediterranean coast. Out of 116 specimens, successful PCR amplifications were obtained in seven individuals of Cerastoderma glaucum but the amplified sequences were not attributed to H. pinnae, and the species remains to be identified. This study did not succeed in detecting a species of mollusc that could potentially maintain the parasite responsible for fan mussel mortality in the environment, but does provide initial results that significantly enhance our understanding of the ongoing pandemic that is currently devastating P. nobilis populations.