Myxosporeans are well-known parasites infecting food fishes in fresh and marine water around the globe. Grass carp (Ctenopharyngodon idella), a freshwater food fish commonly cultured in India with has significant economic importance. Herein, the study focuses on the description of a new myxosporean species, Myxobolus grassi sp. nov. from the gills as primary site and liver as secondary site of infection in grass carp. Both organs (gill and liver) were infected concurrently in the host and the prevalence of grass carp infection was 4.05% in gill filaments and liver, respectively. Identification of species was based on the morphological and morphometric features of the myxospore as well as 18S rDNA sequence data. A smear from gill and liver exhibited hundreds of morphologically similar myxospores. BLAST search revealed 98% sequence similarity and 0.03 genetic distance with M. catlae (KM029967) infecting gill lamellae of mrigal carp (Cirrhinus cirrhosus) from India and 98-84% sequence similarity with other myxobolids in India, China, Japan, Malaysia, Turkey and Hungary. Phylogenetically, it clustered with other myxobolids infecting gills and related organs (i.e., vital organ) of Indian cyprinid carp species. On the basis of myxospore morphology and 18S sequence, we propose M. grassi sp. nov.

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