galloprovincialis populations in southern Italy may indirectly reflect immunocompetence at both individual and population levels. The constant presence, prevalence, and severity of S. mytilovum within the Clade IV, defined as the Class Terresporidia, with closest genetic relationship (83.6% identity) to an undetermined invertebrate ovarian microsporidian. There are five taxonomic clades of microsporidians as identified by SSU ribosomal gene sequence data. Ultrastructural observations revealed a mitochondrial re-organization to interact with all the phases of parasite development the mitochondria were arranged outside the parasitophorous vesicle (PV) or directly interacting with the spore inside vesicle. Stenhausia mytilovum elicited an infiltrative (24.8%) or a strong capsular inflammatory response (43.4%) at gonadal follicles and surrounding vesicular connective tissue, in some cases accompanied by gonadal atresia (24.8%), leading to loss of gonadal architecture.

The parasite developed in the oocytes within a sporophorous vesicle (SV) where it produced 1–3 spores per cell, both in the cytoplasm and in the nucleus. mytilovum in 10 of the investigated farms, with a prevalence ranging between 14 and 100% of female mussels, depending on the population and season in which they were sampled. galloprovincialis was conducted in 13 mussel farms and one natural bed between 20.

Here we investigate the microsporidian parasite Steinhausia mytilovum associated with the mussel, Mytilus galloprovincialis, in natural beds and farms along coast of southern Italy.

Despite ongoing work to better describe these genera, including detailed microscopic and ultrastructural images, so far we lack information on microsporidian phylogeny and pathogenicity of species within these genera. To date, microsporidia have been reported in both bivalves and gastropods, and microsporidia from mollusks have been classified in different genera. However, the taxonomy of microsporidia is generally poorly resolved, which has consequently constrained our understanding of their pathology and epidemiology in marine animals. Microsporidia are well known fungal pathogens of aquatic animals.

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