Essential oils compromise tegumental integrity in Sparicotyle chrysophrii: Insights from SEM and histological analyses.

This study investigates the morphological effects of various essential oils (EOs) on the monogenean Sparicotyle chrysophrii, a major gill parasite of gilthead sea bream (Sparus aurata), through scanning electron microscopy (SEM) and histological analyses. Adult parasites obtained from experimental infections were exposed in vitro to different EOs at concentrations ranging from 62.5 to 500 ppm. Cinnamomum zeylanicum, Origanum vulgare, Thymus vulgaris, and a commercial blend (Mix Gill) exhibited the highest antiparasitic efficacy, achieving complete mortality at 500 ppm within one hour. SEM observations revealed extensive tegumental disruption, including surface erosion, deep wrinkling, and clamp detachment, which increased in severity with EO concentration and exposure time. Histological analyses confirmed widespread tissue degeneration, characterized by vacuolization, cytoplasmic rarefaction, and loss of the neodermal and muscular organization. Even at sublethal concentrations, lesions consistent with swelling, vesicle formation, and tegumental disorganization indicated progressive degeneration. These structural alterations suggest that EOs compromise the parasite's external integrity, leading to functional collapse of the tegument and haptor. The results support a time- and concentration-dependent mechanism, likely involving disruption of membrane integrity and osmotic imbalance. Collectively, to the best of our knowledge, this is among the first studies to provide detailed ultrastructural evidence of EO-induced damage in S. chrysophrii and highlights their potential as eco-friendly antiparasitic agents for aquaculture. Further studies integrating ultrastructural and biochemical analyses are warranted to elucidate the molecular pathways underlying these effects.