Coinfection of large yellow croaker Larimichthys crocea by Trypanosoma sp. (Euglenozoa: Kinetoplastea) and Ceratomyxa xiangshanensis n. sp. (Cnidaria: Myxosporea) in offshore net cage systems in the East China Sea

The large yellow croaker, Larimichthys crocea, is an indigenous fish in the East China Sea and a highly ranked marine fish species, primarily reared in the coastal waters of the East China Sea. Recently, infections with hemoflagellates (causing trypanosomiasis) in this fish species were detected on the southeast coast of China; however, the pathogenicity has not been established. Here, an outbreak of mass mortality in L. crocea farmed in Zhejiang province is reported with coinfection of hemoflagellates and myxosporeans. For hemoflagellates, blood smears reveal elongated trypomastigotes measured 20.5 ± 3.4 μm × 1.9 ± 0.4 μm, and an anterior free flagellum is 11.9 ± 2.6 μm in length. Molecular analysis of the small subunit ribosomal sequence showed that the isolated hemoflagellate is conspecific with the Trypanosoma sp. previously reported in the Fujian province, which suggests a neglected transmission event along with transboundary fish delivery. Besides, combining morphological disparities with no more than 98.09 % molecular identity, the biliary myxosporean is regarded as a novel species, Ceratomyxa xiangshanensis n. sp. Histological examination of infected fish tissues showed trypanosome-associated pigment aggregation in melanoma-macrophage centers of the spleen and kidney. The subcutaneous tissue of the head is observed with moderate inflammatory infiltration in the dermis. We suggest that the year-round feeding operation and seasonal fish delivery may favor native circulation and transboundary transmission of these endoparasites. This work expands our knowledge of the parasite fauna harbored in reared L. crocea and frames the importance of routine surveillance for emerging parasitic diseases.