In Vitro Antiprotozoal Activity of Extract against a Ciliate Causing High Mortalities in Turbot Aquaculture.

is an histophagous parasite that infects flatfish, namely turbot (), and cause significant losses in aquaculture units. The available measures for control have limited efficiency, and some cause harm to fish. Hence, sustainable and natural control strategies are urgently needed. This study evaluated the in vitro bioactivity of the ethanol extract of calyces on population growth rate (PGR), oxidative stress biomarkers (glutathione--transferases (GST), glutathione reductase (GR), glutathione peroxidase (GPx), total glutathione (TG) and catalase (CAT), neurotoxicity (acetylcholinesterase, AChE), activity and gene expression of proteases as major virulence factors. extract inhibited parasite PGR (IC = 1.57 mg mL), and caused significant changes in the activity of antioxidant enzymes (LOEC = 0.22 mg mL), especially GPx, TG, and CAT. The activity of proteases was also severely inhibited (IC = 0.76 mg mL), and gene expression of catepsin 90 and leishmanolysin proteases was downregulated. Organic acids and phenolic phytochemicals in hibiscus extract are potentially responsible for the antiprotozoal bioactivity herein determined. Therefore, extract can be a promising disease-control alternative against the ciliate proliferation, cellular defense mechanisms and pathogenicity. Still, its applicability in aquaculture settings, and potential effects on farmed fish, should be further elucidated.