First detection of the invasive Haplosporidian and Mycobacteria parasites hosting the endangered bivalve Pinna nobilis in Thermaikos Gulf, North Greece.

Mycobacterium sp. and Haplosporidium pinnae constitute invasive parasite species of bivalves, reported for the first time in the present study in the Aegean Sea and Thermaikos Gulf, respectively. During the last years, the endangered fan mussel (Pinna nobilis) experienced several mortality events in the Mediterranean Sea that caused deaths to 90% or more of their populations and have been attributed to infections by these pathogens. In Greece, two mass mortality events have been recently reported, namely in the Gulf of Kalloni and in Limnos island. In the present study we investigated the presence of both pathogens in P. nobilis from these marine areas as well as from Thermaikos Gulf using both histopathological microscopy and molecular markers. The detected parasite DNA was further quantified in the three populations utilizing a real time qPCR. Histopathological results indicated the presence of a Mycobacterium species alongside with the existence of the Haplosporidian parasite, which was identified in all mortality events in the Mediterranean Sea. The parasite was present in different phases mostly on the digestive gland epithelium. Phylogenetic analysis confirmed the taxonomy of the Haplosporidian parasite as the recently described Haplosporidium pinnae, whereas it failed to identify the Mycobacteria parasite at species level. While Mycobacterium sp. was detected in all examined specimens, H. pinnae was not detected in all diseased fan mussels. Interestingly, monitoring of P. nobilis population from Thermaikos Gulf, an estuary of extremely high importance for bivalve production, revealed the presence of both pathogens in a few specimens in higher quantity but with no symptoms of the disease. Besides, all the specimens from Thermaikos Gulf had inflammatory responses similarly to moribund specimens from mortality events.