Gill Histopathology Associated with Lernanthropus kroyeri and Diplectanum aequans Infestation in European Seabass (Dicentrarchus labrax)

Gill-associated parasites pose a considerable threat to Mediterranean aquaculture, especially in intensively cultivated species such as European seabass (Dicentrarchus labrax). This seasonally conducted study comprehensively evaluated the histopathological effects of two prevalent ectoparasites, Lernanthropus kroyeri and Diplectanum aequans, on the gill tissues of infested European seabass (D. labrax) cultivated in marine cage systems, utilizing macroscopic, microscopic and histopathological methods, while determining the prevalence and mean intensity of each parasite species. Parasitological analyses indicated that L. kroyeri was the predominant parasite, with the maximum prevalence observed in summer (53.5%), whereas D. aequans demonstrated maximum prevalence in fall (73%). Differences in parasite prevalence among seasons were analyzed using Pearson's chi-square test. The low frequencies results were confirmed using the Fisher–Freeman–Halton exact test. L. kroyeri was primarily found on the proximal branchial lamellae, whereas D. aequans was chiefly located on the secondary lamellae. Histopathological investigations revealed significant gill injury marked by epithelial hyperplasia, lamellar fusion, sub-epithelial edema, mucus cell proliferation and localized necrosis. In extreme instances, total degeneration of secondary lamellae and structural disruption of primary filaments were noted, indicating considerable impairment of respiratory function. Moreover, hemorrhagic lesions and increased mucus secretion suggested a weakened mucosal barrier, potentially rendering fish susceptible to secondary infections. In conclusion, these parasites collectively aggravate gill injury and hinder gas exchange. These findings suggest that health monitoring in cage-cultured European seabass should be seasonally adjusted, with targeted screening for L. kroyeri in summer and D. aequans in fall, together with gill-based lesion scoring to support earlier intervention before severe impairment of respiratory function develops.