Ultrastructural and molecular characterization of Haplosporidium montforti n. sp., parasite of the European abalone Haliotis tuberculata

Abstract

A new member of the parasitic phylum Haplosporidia, which was found infecting the connective tissue, gill, digestive gland, and foot muscle of Haliotis tuberculata imported from Ireland and experimentally grown in Galicia (NW Spain), is described. Scanning electron microscopy, transmission electron microscopy, and molecular characterization of the small subunit ribosomal RNA (SSU rRNA) gene were carried out to confirm the description of this species. The ultrastructural morphology of the spores and their surrounding ornaments attached to the spore wall was described from light, scanning, and transmission electron microscopy observations. Systemic infection with uninucleated and multinucleated plasmodia containing spherical nuclei was observed among several sporocysts containing the different spore maturation stages. The spores were spherical to slightly ellipsoidal (2.42 ± 0.5 × 2.31 ± 0.6 μm). The apical zone of the spore wall was modified into a complex opercular system covering a circular orifice that measured about 0.5 μm in diameter. The operculum was connected to the spore wall by a hinge. The spore wall was about 110 nm thick, with 4 filaments (20–28 μm long). The filaments were composed of the same material that formed the wall. The cross-sections through the base of these filaments showed T-like and X-like sections. Internally, the uninucleated endosporoplasm contained typical haplosporidian structures, such as, haplosporosomes, a spherulosome, and mitochondria with vesicular cristae. The SSU rRNA gene sequence was different from previously reported haplosporidian SSU rRNA gene sequences, corroborating morphological data that this was an undescribed species. Based on differences from previously described haplosporidians in ultrastructural characteristics of the spore and SSU rRNA gene sequence, we describe the abalone haplosporidian as Haplosporidium montforti n. sp.