Morphological, Histopathological, Ecological and In Vitro Antiparasitic Assessment of a Copepod Parasite, Ergasilus celestis Roberts, 1970 Infesting the Gills of the Eel Fish, Anguilla anguilla in DamieĴa Region, Egypt

This study provides an integrated morphological, histopathological, ecological, and experimental evaluation of the parasitic copepod Ergasilus celestis infecting the European eel Anguilla anguilla from Lake El-Manzala, Damietta, Egypt. The objectives were to redescribe adult female morphology, assess gill damage caused by infestation, analyze ecological factors affecting parasite prevalence and intensity, and evaluate in vitro antiparasitic activity of selected plant extracts. Ecological assessments involved examining 170 host fishes collected seasonally from October 2023 to September 2024. In vitro bioassays assessed the effects of Curcumin and Myrrh extracts on parasite survival. The results revealed pronounced morphological adaptations in the female parasite related to attachment and reproduction. Infestation induced severe histopathological damage, including epithelial hyperplasia, tissue erosion, and distortion of gill lamellae. Ecological analyses demonstrated a very highly significant seasonal pattern, with peak prevalence and intensity during spring, higher susceptibility in female hosts, and strong correlations between infestation intensity and environmental parameters (P < 0.001). In vitro assays showed a concentration-dependent reduction in parasite survival, with Curcumin exhibiting stronger antiparasitic activity than Myrrh. These findings demonstrate the influence of host biology, environmental factors, and plant extracts on E. celestis infestation, offering insights for effective parasite management and sustainable control in marine ecosystems.