In the present study, Neorhadinorhynchus nudus (Harada, 1938) is reported from the frigate tuna Auxis thazard (Lacepéde) (Perciformes: Scombridae), in the South China Sea for the first time. The detailed morphology of N. nudus was studied using light and scanning electron microscopy based on the newly collected material. The results showed some morphometric variability between our specimens and previous studies, including the number of hooks per longitudinal row and the size of copulatory bursa and eggs. Our SEM observations also revealed all proboscis hooks emerged from elevated round rims on proboscis surface. In addition, N. nudus was firstly characterised using molecular methods by sequencing and analysing the ribosomal ITS and mitochondrial cox1 regions. There is no nucleotide divergence found in the ITS sequences, but a low level of nucleotide variability detected in the cox1 regions (the level of intraspecific nucleotide variability being 0.75% to 2.54%). The DNA sequence data obtained herein will indeed be a useful reference for rapid and accurate species identification of Neorhadinorhynchus.

Tunas of the genus Auxis are cosmopolitan species and the smallest members of the tribe Thunnini, the true tunas. In the present study, COI sequences of mtDNA were employed to examine the evolutionary history and phylogenetic relationship between A. thazard and A. rochei. A total of 29 COI sequences were retrieved from NCBI. Historic demographic analyses of sequence data showed that A. thazard has undergone sudden population expansion in the past while population size of A. rochei has been remain constant for long period. Non-significant value of Tajimas\'s D (P = 0.22400) and Fu\'s FS (P = 0.21400) test fail to reject the null hypothesis of neutral evolution for A. rochei. Phylogenetic analyses of nucleotide sequences demonstrated separate clusters for both species and are strongly supported by 98% bootstrap value. The results of the present study suggest the recent founding of A. thazard in world ocean while A. rochei represents the ancestral species.