Microscopic findings of host-parasite interaction between the Korean rockfish, Sebastes schlegelii, and the monogenean ectoparasite, Microcotyle sebastis.

This study was conducted to examine the pathogenesis of gill degeneration in the Korean rockfish Sebastes schlegelii, infested with the monogenean ectoparasite, Microcotyle sebastis. We collected 30 Korean rockfish from a fish farm in Tongyeong-si, Gyeongsangnam-do, and examined them with light microscopy, scanning electron microscopy (SEM), and histopathology, in March 2018. The monogenean trematode, M. sebastis, was detected in 27 Korean rockfish (90%), with the intensity of infection being 31.7 per fish. The characteristic surface ultrastructures such as tegument with transverse striations, genital atrium, genital pore, and opisthaptor with numerous clamps were observed. The worms were firmly attached to the gill lamellae using clamps from the opisthaptor, causing gill damage and degeneration. The distal part of the lamellae was ruptured by the sclerites of the clamps. The histopathological examination revealed epithelial hypertrophy, hyperplasia, and occasionally fusion of the lamellae. These ultrastructural and histopathological findings provide some understanding of the pathogenesis of gill degeneration in the Korean rockfish infested with M. sebastis.