Anesthesia of Baryancistrus xanthellus (Siluriformes: Loricariidae): Bioassays with exposure and re-exposure cause reverse tolerance to eugenol

Baryancistrus xanthellus (Golden Nugget Pleco) is an ornamental catfish endemic to the Xingu River basin in Brazil and widely exported for the aquarium trade. This study evaluated the effectiveness of eugenol anesthesia in B. xanthellus under two scenarios: an initial exposure and a re-exposure to the same anesthetic. The experiments were conducted in two phases, two months apart, each testing incremental concentrations of eugenol (2 mg·L −1 added at two-minute intervals) until reaching surgical anesthesia. Two control groups (water only and 70 % ethanol at equivalent concentrations) were subjected to identical handling and observation procedures. Data were analyzed using Permutational Analysis of Variance (PERMANOVA), treating anesthesia stage as the dependent variable, eugenol concentration as the independent variable, and fish size plus exposure history as covariates. The results revealed reverse tolerance to eugenol: fish re-exposed to the anesthetic required significantly lower concentrations to achieve surgical anesthesia (14.50 ± 3.20 mg·L −1 ) than during the initial exposure (21.69 ± 3.38 mg·L −1 ). However, recovery times did not differ significantly between the first and second exposures. No mortality occurred within 72 hours post-anesthesia in any group, indicating low short-term toxicity. These findings highlight the need to re-evaluate anesthetic protocols for repeated procedures in ornamental catfish. We also propose new reference tables specifically tailored to describe anesthesia and recovery stages in benthic Loricariids. Further investigations into the biochemical and physiological mechanisms underlying reverse tolerance to eugenol are warranted. • Baryancistrus xanthellus showed reverse tolerance to eugenol during the second exposure. • No mortality was observed within 72 hours after anesthesia with eugenol. • Recovery time did not differ between initial exposure and re-exposure. • New anesthesia and recovery tables were developed for Loricariidae benthic fish.