Morphological and Molecular Characteristics of Anisakis typica Larvae in Two Species of Threadfin Bream, Nemipterus hexodon and N. japonicus, from the Gulf of Thailand

The third stage larvae (L3) of Anisakis typica were detected in 2 species of threadfin bream, Nemipterus hexodon and N. japonicus, from the Gulf of Thailand, and were morphologically and molecularly characterized. Total 100 threadfin breams, 50 Nemipterus hexodon and 50 N. japonicus, were examined with naked eyes after the opening of abdominal cavity with scissors. Almost all infected larvae remained alive and active even the fish were transported for 1–2 days. Anisakid larvae were exclusively distributed in the body cavity and rarely in the liver. The prevalence of A. typica L3 were 68.0% and 60.0% in N. hexodon and N. japonicus and their infection intensities were 3.5 and 4.2 per fish infected each. Morphological and morphometric analysis were performed by viewing specimens under both a light microscope and a scanning electron microscope. Interestingly, the protruded mucron of Anisakis typica under SEM showed a distinct cylindrical shape that differed from the cone shape of A. simplex. The protruded mucron could be used to identify A. typica L3 larvae in the future. A comparison of the ITS1-5.8S-ITS2 rDNA nucleotide sequences of these species revealed high blast scores with A. typica. Conclusively, it was confirmed that A. typica L3 are prevalent in threadfin breams from the Gulf of Thailand, and their morphological and molecular characters are something different from those of other anisakid larvae, including A. simplex and A. pegreffii.