An undescribed species of Microsporidia Balbiani, 1882 was isolated from the muscularis mucosa of the intestinal mucosa of the reared Atlantic bluefin tuna (Thunnus thynnus). Transmission electron microscopy showed that the pyriform unikaryotic microsporidium, measuring 3.6 μm ± 0.08 in length and 1.8 μm ± 0.04 in width (inferred from TEM sections), had two layers of 14–17 coils of polar filament and a robust manubrium of the filament. No developmental stages were observed in xenoma. Phylogenetic analysis of the small subunit rRNA showed closest similarity with Kabatana spp. (88.4%), clustering the microsporidium together in a sister group with K. takedai and Kabatana sp. (JI-2008)/K. newberryi clade. While recognized Kabatana members show ovoid, rounded to pyriform spore, lack of sporophorous vesicle or xenoma, 3–10 coils of polar filament in 1–2 rows and are localized within cytoplasm of trunk muscle, the new species is markedly pyriform, xenoma forming, with many filament coils in 2 rows and parasitizing smooth intestinal muscle. Since morphological features were not typically congruent with any of Kabatana spp. so far described and molecular clustering indicated paraphyletic position within Kabatana clade, we suggest assigning described species to collective group Microsporidia, as Microsporidium milevae sp. nov., until more evidence permits potential formation of the new genus.
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