Abstract / Introduction / Summary:
Specimens of Argulus coregoni Thorell, 1864 were collected from the body surface of ayu Plecoglossus altivelis altivelis (Temminck and Schlegel, 1846) at 135–145 m elevations in the middle reaches of the Mimi River at Shinjo in Mihama, Fukui Prefecture, central Japan, on 10–13 September 2025. Prevalence and intensity of infection were 28.6% (10 infected/35 examined) and one or two parasites per infected fish, respectively. The specimens of A. coregoni are briefly described and characterized by five or six plumose setae near the posterior margin of the first leg coxa and 57–64 supporting rods each in the marginal membranes of the first maxilla. The second leg coxa of the male bears two protrusions adorned with small spines and a digitiform projection on the ventro- and dorsoposterior margins, respectively, whereas that of the female has not such protuberances but one plumose seta on the posterior margin. This collection of A. coregoni represents its second record from ayu in Fukui Prefecture, where it was previously reported from this fish species in the Asuwa River.
