Procaryotic infections in the mussel Mytilus galloprovinciallis and in its parasite the turbellarian Urastoma cyprinae

Mussels Mytilus galloprovincialis from the Thau lagoon (Mediterranean coast of France) were regularly sampled to determine the prevalence and intensity of parasitic infections. Microscopically, hepatopancreatic tubules of the mussel appeared infected by a rickettsia-like organism (RLO). Each RLO were surrounded by 2 unit membranes, and colonies composed of several bacteria were enclosed within a vacuolar membrane of the host cell. In addition, examination by transmission electron microscopy revealed that the RLO was infected by phage particles. Histological investigations of the turbellarian Urastoma cyprinae parasitizing the mussels have shown that this ectoparasite was also infected by 2 types of procaryotes, a chlamydia-like organism (CLO) and a mollicute-like organism (MLO). The CLO displayed characteristic developmental stages of the Chlamydiales and was secondarily infected by electron-dense particles presumed to be phage particles. The MLO exhibited some morphological characteristics of the mollicutes, in that the microorganisms were bounded by a single membrane sharing a trilaminar structure. Neither of these microorganisms have previously been reported in Platyhelminthes.