Morphological Characterization and Molecular Phylogenetic Analysis of Kudoa iwatai from Large Yellow Croaker (Larimichthys crocea) as a New Host in China

Simple Summary Larimichthys crocea is the most important economic marine cultured fish in China. Kudoa parasites are critical pathogens that infect a wide range of marine and freshwater fish. Compared to the hundred marine Kudoa species recognized in wild and cultured fish worldwide, records of Kudoa are relatively few in China. In this report, large yellow croakers were found to be infected with Kudoa for the first time in China. Morphological observations and molecular techniques were combined for the final identification of Kudoa iwatai. Additionally, the morphological characterization and phylogenetic status of Kudoa iwatai have been described in detail. This study enriches knowledge about Kudoa iwatai and provides a direction for early disease prevention and monitoring of large yellow croakers. Kudoa (Myxosporea: Multivalvulida) parasites are critical pathogens in marine and freshwater fish associated with significant economic losses and reduced market prices caused by post-mortem myoliquefaction or numerous cysts on muscles. In the present study, large yellow croakers infected by Kudoa were found during fish disease surveillance in China in November 2020 and used for morphological observation and characterization using light DIC microscopy and electron microscopy. Numerous creamy-white oval plasmodia were observed in muscles and on the surface of brain cartilage, gill arches, and serosal surfaces. The spores were considerably longer and thicker than previously reported Kudoa, with protruding polar filaments (PFs) in the mature spores, fingertip-shaped apical projections (APs), and polar capsules. Phylogenetic analyses with SSU rDNA, LSU rDNA, and mitochondrial DNA showed that the Kudoa-infected sample (LcK-2020) had the highest similarity to Kudoa iwatai reported in Japan. Based on the morphological characterization and phylogenetic analysis, it could be concluded that the sample LcK-2020 was infected by Kudoa iwatai, which would be the first report of Kudoa iwatai infection in large yellow croaker in China.