Intraspecific Genetic Variation of in Indian Mackerel Caught from the Gulf of Thailand, Samut Songkhram Province.

nematodes infecting Indian mackerel () were initially discovered in Thailand in our preliminary investigation. Nevertheless, the species of collected has not been determined nor has its genetic variation been researched. Thus, this study aimed to molecularly identify the species of specimens using the internal transcribed spacer (ITS) region of ribosomal DNA sequences. In addition, the intraspecific genetic variation was also determined using mitochondrial cytochrome oxidase subunit II (COII) gene sequences. The phylogenetic relationships of the ITS region classified all samples into ; however, the genetic variation between them could not be distinguished. By contrast, the phylogenetic tree analysis of the COII region identified all samples as . , with 17 different haplotypes by 66 polymorphic sites and five of the substitutions resulted in amino acid change. Additionally, the distribution pattern of the COII region can be separated into two groups between South America and Asian countries. All our haplotypes belong to Asian countries. Compared with the two genetic markers used in this investigation, COII appears to be a better candidate for studying genetic variation sensitive to environmental changes and intermediate or definitive host behavioral changes.