Molecular Identification of the Nematode <i>Eustrongylides ignotus</i> in Mandarin Fish, <i>Siniperca chuatsi</i>

ABSTRACT Mandarin fish ( Siniperca chuatsi ) is a high‐value economic freshwater fish in China; however, the frequent occurrence of diseases threatens its production. To this end, we explored the impact of a nematode parasite newly found in the abdomen of mandarin fish collected from Pudong, Shanghai, China. This study was focused on two dimensions: (1) to use molecular biology methods to identify the taxonomic position of the nematode while also observing its morphological characteristics and (2) to assess the impact of nematode parasitism on the immune system of mandarin fish. This nematode exhibits a pink body colour and primarily resides in the swim bladder (an average 8–10 nematodes per fish). Additionally, 1–3 nematodes were observed in the abdomen, liver and peritoneum, while none were detected in the digestive tract. Initial molecular identification using universal nematode primers successfully amplified a specific band (378 bp), and BLASTn analysis revealed that the sequence showed a high similarity (99.20%) to Eustrongylides ignotus (OQ885489.1) and Eustrongylides sp. (MN066569.1, PP236907.1). The nematode was categorized under the genus Eustrongylides . In the second‐round identification, specific ITS primers for Eustrongylides produced an amplification of 845 bp, which matched (100%) the sequence of MK340917.1 ( E. ignotus ). Therefore, we identified the nematode as E. ignotus . The trunk of the nematode is uniform in thickness, while the head and tail taper gradually. The head features several spiny papillae and papillary structures. The nematodes caused cellular vacuolation, nuclear displacement and inflammatory responses in the liver. The expressions of IL‐1β , IL‐8 , TNF‐α and NF‐ĸB were up‐regulated in the liver of infected fish, with the NF‐ĸB displaying the most significant increase (484.37‐fold). Only TNF‐α was detected as significantly up‐regulated in the swim bladder. This study provided essential data for molecular identification and diagnosis of nematode infections and offers initial insights into the immune response of mandarin fish.