Infection of the cockle Cerastoderma edule in the Baie des Veys (France) by the microsporidian parasite Steinhausia sp

pathology, epidemiology, and phylogenyof mussel egg disease due to the microsporidianSteinhausia mytilovum(Field, 1924)in the Mediterranean mussel (Mytilus galloprovincialis)

Microsporidia are well known fungal pathogens of aquatic animals. However, the taxonomy of microsporidia is generally poorly resolved, which has consequently constrained our understanding of their pathology and epidemiology in marine animals. To date, microsporidia have been reported in both bivalves and gastropods, and microsporidia from mollusks have been classified in different genera. 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. Here we investigate the microsporidian parasite Steinhausia mytilovum associated with the mussel, Mytilus galloprovincialis, in natural beds and farms along coast of southern Italy. A survey of M. galloprovincialis was conducted in 13 mussel farms and one natural bed between 2009 and 2020. We found the presence of S. mytilovum in 10 of the investigated farms, with a prevalence ranging between 14-100% of female mussels, depending on the population and season in which they were sampled. 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. 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. In 7% of cases no reaction was observed. 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. There are five taxonomic clades of microsporidians as identified by SSU ribosomal gene sequence data. Maximum likelihood analysis assigned S. mytilovum within the Clade IV, defined as the Class Terresporidia, with closest genetic relationship (83.6% identity) to an undetermined invertebrate ovarian microsporidian. The constant presence, prevalence, and severity of S. mytilovum in coastline populations of M. galloprovincialis populations in southern Italy may indirectly reflect immunocompetence at both individual and population levels.

High prevalences of disseminated neoplasia in the Baltic tellin Limecola balthica in the Wadden Sea

The Baltic tellin Limecola balthica is one of the most common bivalves in intertidal areas in the Northern Hemisphere. Over the last 2 decades, the species has been suffering from a decrease in adult survival in the European Wadden Sea. While several factors such as global warming and fisheries have been suggested to influence the population dynamics of this bivalve mollusc, the potential role of diseases has never been investigated. In this study, we investigated whether disseminated neoplasia, a common proliferative disorder in bivalve molluscs, could play a potential role in the recent population decline of Baltic tellins in the Wadden Sea. We conducted a field survey in the Dutch Wadden Sea to (1) determine whether the disease occurs in Baltic tellins in the Wadden Sea and (2) quantify the occurrence and severity of the disease via histology. Disseminated neoplasia occurred in L. balthica at each of the 10 sampled locations with very high prevalences (21-89%) compared to those reported elsewhere for this species. The highest severity category was found in 8 to 87% of affected individuals, with severity generally increasing with prevalence. Disseminated neoplasia usually increases mortality among affected individuals and may also be associated with important sub-lethal effects, especially regarding gametogenesis. Thus, we suggest that disseminated neoplasia may play a key role in the population dynamics of the Baltic tellin, the extent of which remains to be investigated in future studies.

Detection of a parasitic amoeba (Order Dactylopodida) in the female gonads of oysters in Brazil

The impacts of oocyte parasites on the reproductive success of molluscs are largely unknown. In this study, we evaluated the presence of gonad parasites in 6 species of marine bivalve molluscs native to southern Brazil. Cultured bivalves included the mangrove oyster Crassostrea gasar (sometimes called C. brasiliana), the brown mussel Perna perna, the lion's paw scallop Nodipecten nodosus and the wing pearl oyster Pteria hirundo. Another species of mangrove oyster, C. rhizophorae, and the carib pointed venus clam Anomalocardia brasiliana (syn. A. flexuosa) were collected from the wild. Molluscs were collected in winter 2009 and summer 2010 for histopathological and molecular evaluation. An unknown ovarian parasite (UOP) was observed in histopathological sections of female gonads of C. gasar and C. rhizophorae. The UOP possessed features suggestive of amoebae, including an irregular outer membrane, frothy cytoplasm, a nucleus with a prominent central nucleolus and a closely associated basophilic parasome. PCR analysis was negative for Marteilioides chungmuensis, Perkinsus spp. and Paramoeba perurans. However, real-time PCR successfully amplified DNA from oyster gonads when using universal Paramoeba spp. primers. Also, conventional PCR amplified DNA using primers specific for Perkinsela amoebae-like organisms (syn. Perkinsiella), which are considered as endosymbionts of Parameoba spp., previously thought to be the parasome. Our results suggest that this UOP is a species of amoeba belonging to 1 of the 2 families of the order Dactylopodida, possibly related to Paramoeba spp. This study represents the first report of this type of organism in oysters. We found that C. gasar and C. rhizophorae were the most susceptible molluscs to these UOPs.

The nature of Thelohania solenopsae (Microsporidia) cysts in abdomens of red imported fire ants, Solenopsis invicta

Sixty four percent of Solenopsis invicta workers infected with Thelohania solenopsis contained 1-6 "cysts" ranging from 70 to 260 microm in diameter. Light and electron microscope analyses showed that cysts are hypertrophied adipocytes transformed by the parasites, each cyst presumably forming from a single cell. In the first step of the pathogenesis, Nosema-like spores functioning in autoinfection are produced; a diplokaryotic sequence leading to their formation causes fat body hypertrophy. When meiosis occurs, it switches parasite development to production of octospores and/or megaspores. Adipocytes become 2-4xlarger than normal in conjunction with intensive parasite multiplication and octospore maturation. Infected cells eventually lose their cellular organization and are converted into reservoirs for spores. There were no manifestations of cellular immunity, such as encapsulation or nodule formation. Similarly, there were no signs of specialized host-parasite interaction that might be interpreted as xenoma-like complexes. The role of the cysts in the parasite's life cycle is unclear. They may represent a defensive reaction of the host sacrificing the infected cells to segregate the infection. Alternatively, the cyst may help protect spores from environmental hazards and provide a concentrated infectious dose to aid horizontal transmission of the microsporidium. We propose to refer to hypertrophied adipocytes filled with T. solenospsae spores as "sporocytosacs", not "cysts."