LAUSR

Abstract Ocimum gratissimum L. (known for its relaxant/sedative effects and antimicrobial properties) and Hesperozygis ringens (Benth.) Epling (known for its anti-parasitic properties) are two plants belonging to the family Lamiaceae. Currently, studies have demonstrated that the essential oils (EO) of these two plants present anesthetic and sedative activity in fish, as well as antimicrobial and antiparasitic properties against different fish pathogens. Larvicidal activity against insect larvae (fish predators) was also reported for both EOs. This study evaluated the toxicity of the O. gratissimum (OG) and H. ringens (HR) EOs in the freshwater zooplankton Daphnia pulex and their activity against different fish pathogens (bacteria and a monogenean parasite) isolated from silver catfish ( Rhamdia quelen ). The in vivo antioxidant activity of OG EO also was assessed in different tissues of this species. In addition, results from studies related to the application of both EOs in aquaculture were reviewed and summarized. The acute toxicity test in D. pulex determined EOs lethal concentration (LC50s – 24 h) to be 37.8 mg L −1 for OG EO and 61.5 mg L −1 for HR EO. Both EOs showed moderate activity against the bacteria Aeromonas hydrophila and Aeromonas veronii (MIC 400–800 μg mL −1 ) and weak activity against Citrobacter freundii and Raoltella ornithinolytica . The EOs also showed significant antiparasitic activity against the monogenean parasite Gyrodactylus sp. OG EO added to the water prevented the lipid peroxidation and increased the antioxidant status of the evaluated fish tissues. These plant-based products appear as sustainable and effective alternatives for conventional drugs and/or chemicals for disease control and prophylaxis in aquaculture.