The Japanese threadfin bream N. japonicus (Bloch) (Perciformes: Nemipteridae) is an important marine food fish in Asia. However, our present knowledge of the occurrence of its nematode parasites is still limited. In the present study, the species composition and infection rate of ascaridoid nematodes in N. japonicus from the South China Sea, were studied for the first time. Five ascaridoid species, namely Anisakis typica (L3), Hysterothylacium amoyense (L3), Hysterothylacium sp. IV-A (L3), adult of H. thalassini and Raphidascaris lophii (L3), were identified using integrative taxonomy. Hysterothylacium amoyense was the most prevalent species (prevalence 47.2%, mean intensity 14.9 ± 17.1). Hysterothylacium thalassini and R. lophii were reported in the Japanese threadfin bream for the first time. Two different genotypes of A. typica (overall prevalence of 3.4%; mean intensity 1.7 ± 0.9) were found in the South China Sea for the first time. The unique restriction polymorphism patterns of three species of Hysterothylacium are provided for rapid diagnosis. Our present results indicate that RFLP analysis of ITS region, using the restriction enzymes HhaI and RsaI, represents a simple and practical method for large-scale surveys of Hysterothylacium for seafood industry.

The complete mitochondrial sequence of the Japanese threadfin bream, Nemipterus japonicus has been determined. The circle genome is 16,995 bp in size, and consists of 13 protein-coding genes, 2 rRNA genes, 22 tRNA genes and a control region. The gene order and composition of N. japonicus was similar to that of most other teleosts. The base composition of H-strand is 28.11% (A), 28.02% (T), 16.64 % (G) and 27.24 % (C), with an AT content of 56.12%. All genes are encoded on the heavy strand with the exception of ND6 and eight tRNA genes. The mitochondrial genome of N. japonicus presented will be in favor of resolving phylogenetic relationships within the family Nemipteridae and the Perciformes.