高知県野見湾の養殖カンパチにおけるカプサラ科単生類2種の季節的寄生状況および遺伝的多様性について

Abstract

Nomi Bay, in Kochi Prefecture, Japan, is famous as the cradle of greater amberjack (Serio/a dumerili) culture in Japan. Two species of capsalid monogeneans, Benedenia seriolae and Neobenedenia girellae, which are causative agents of skin fluke disease, are a serious problem for fish culture in the bay. They affect the fishes' growth, contribute to secondary infections, and in some cases cause mortality. In this study, we examined seasonal prevalence and the genetic diversity of these two skin flukes on cultured greater amberjack in Nomi Bay. In 2015, the average number of flukes per fish from 6 July to 2 December was 0.7-25.5 for 0-year-old fish, and from 6 July to 29 December was 10.7-85.3 for 1-year-old fish. These skin flukes were almost all B. seriolae. In 2016, the average number of flukes per fish was 1-21. 7 from 5 February to 26 December for 0-year-old fish, and 3-62.3 from 21 January to 26 December for 1-year-old fish. The flukes collected from 0-year-old fish between 30 September and 18 November and from 1-year-old fish between 13 September and 14 October were all N. girellae. This change in dominant parasite species was probably due to an increase in water temperature to over 30 C in August. Ten haplotypes of B. seriolae were identified, based on mitochondrial cytochrome c oxidase subunit 1(COI) gene sequences from 25 randomly selected specimens from each sampling date. Two of the 10 were reported in our previous study. All 10 haplotypes were grouped with the Japanese clade of the COI gene sequence phylogenetic tree. Nine N. girellae specimens had 100% COI gene sequence similarity with specimens collected from greater amberjack juveniles cultured in Dadonghai, China, suggesting that N. girellae may be introduced in spring every year through the import of juveniles.