A New Host–Parasite Association: A Glugea sp. (Microsporidia) Infecting the Commercial Fish Sardina pilchardus from an Atlantic Fishery

A microsporidian species, Glugea sp., was identified infecting the visceral cavity of commercial sardines (Sardina pilchardus) from an Atlantic fishery. Macroscopic examination revealed a white, friable mass that readily disintegrates, releasing spherical to ovoid microsporidian spores. Ultrastructural analysis shows an isofilar polar tube with 10–15 coils arranged in one to three rows. The study combined macrophotography, light microscopy, histological staining, transmission electron microscopy (TEM), and molecular analyses. Morphologically, Glugea sp. shares some features with Glugea vincentiae; however, molecular data do not support a close phylogenetic relationship between these two species. Phylogenetic analysis based on 18S rRNA sequences places the organism within a clade comprising G. plecoglossi, G. thunni, G. atherinae, G. gasterostei, G. hertwigi and G. anomala. Despite the high sequence similarity observed within this group, morphological and ultrastructural characteristics allowed differentiation of the present species, highlighting the limitations of relying solely on conserved molecular markers for species delimitation. A comprehensive morphological and molecular description of Glugea sp. is provided. Given the ecological and economic relevance of microsporidia, as well as their recognized role in animal and human disease, this new host–parasite association may have implications for fisheries and food safety, particularly considering the widespread consumption of sardines.