Marine microsporidia: Hidden diversity, aquatic emergence and zoonotic risks.

Marine Microsporidia represent an increasingly important but historically underestimated group of aquatic intracellular parasites. Although traditionally associated with invertebrates and fishes, recent molecular and environmental studies have revealed extensive hidden diversity across marine ecosystems together with unexpected phylogenetic links between aquatic and human-associated lineages. This review synthesizes five decades of research (1975-2025) concerning the biology, evolution, taxonomy, host-pathogen interactions, ecological diversity, and zoonotic significance of marine Microsporidia. Particular emphasis is placed on members of the Enterocytozoonidae, including Enterocytozoon hepatopenaei, Nucleospora spp., and Desmozoon lepeophtherii, which represent major threats to aquaculture sustainability and may contribute to cross-environmental transmission pathways. Environmental DNA studies further indicate that marine microsporidian diversity is substantially underestimated by morphology-based approaches. However, current evidence for zoonotic transmission remains largely indirect, resting chiefly on phylogenetic relatedness, shared genotypes across hosts and environmental (eDNA) detection; direct, seafood-borne human infection has not yet been demonstrated. Current evidence suggests that aquaculture intensification, climate-driven environmental change, and expanding seafood trade may facilitate the emergence and dissemination of zoonotic microsporidian lineages. Integrating marine Microsporidia into One Health surveillance frameworks is therefore increasingly important for both ecosystem and public health management.