Morphomolecular characterization and statistical assessment of environmental associations with Diplectanum infestation in cultured European sea bass and bath treatment efficacy.

Monogenean infections are one of the most economically important parasitic constraints in marine aquaculture; however, knowledge remains limited regarding their genetic diversity, epidemiology, and sustainable management strategies. This study investigated the prevalence, morphomolecular characterization, environmental drivers, and eco-friendly control of naturally occurring monogenean infections in European sea bass (Dicentrarchus labrax) from two marine farms along the Egyptian Mediterranean coast between February and May 2024. Infected fish exhibited respiratory distress, skin darkening, anorexia, and progressive emaciation under heavy infestation. Examination of gill smears identified an elongate-oval monogenean exhibiting morphological features consistent with D. laubieri. Phylogenetic analysis of partial 18 S rRNA gene sequences placed the specimen within the Diplectanum clade, corroborating its taxonomic affiliation (GenBank accession no. PZ253293.1). Parasitological indices varied significantly between farms, with a higher prevalence recorded at Farm 2 (75.00 ± 6.87%) than at Farm 1 (65.56 ± 7.51%) (p < 0.05). Seasonally, the highest prevalence occurred in February, followed by a decline in April and a slight increase in May. Water quality analysis indicated a progressive rise in temperature on both farms, reaching 25.40 ± 0.16 °C in May. In contrast, Farm 2 was characterized by higher salinity, pH, and total ammonia levels, together with lower dissolved oxygen concentrations. Correlation analysis demonstrated a strong negative association between parasite prevalence and water temperature (r = - 0.736, p < 0.05), whereas no significant correlations were observed with salinity, pH, dissolved oxygen, or total ammonia. In trials of treating the affected fish and based on toxicity testing, the safe exposure threshold for Y. schidigera extract was established at 150-200 µg/L, while fenbendazole exhibited a safe exposure limit of 20 µg/L. The experimental cohabitation successfully reproduced the infection, achieving a prevalence of 90% by day 14. Comparative therapeutic efficacy trials demonstrated the superior performance of Y. schidigera relative to fenbendazole under the present study conditions, with parasite reduction rates of 80.4% and 68.5%, respectively, accompanied by a lower incidence of mortality. Collectively, these findings highlight the combined influence of environmental conditions on monogenean infection dynamics and support phytogenic-based Y. schidigera as a promising eco-friendly alternative for parasite control in marine aquaculture systems.