Eugenol exposure during gravid and non-gravid periods in the fish Cnesterodon decemmaculatus: effects on anesthesia, acetylcholinesterase activity, and parasitic response

Abstract

Eugenol is a widely used plant-based anesthetic in aquaculture. However, its anticholinesterase activity may affect reproductive physiology, particularly in gravid females. This study examined the effects of eugenol on the anesthetic response, acetylcholinesterase (AChE) activity, and parasite abundance in wild-caught adult females of the Neotropical fish Cnesterodon decemmaculatus. Ninety females were exposed to five eugenol concentrations (10, 20, 30, 40, and 50 mgL⁻1), and a control group without anesthetic. Anesthesia induction and recovery times were recorded, and AChE activity was estimated in gravid females. Induction time decreased with increasing eugenol concentration up to 40 mgL⁻1, with no further reduction at 50 mgL⁻1. Recovery times did not differ significantly among treatments. AChE activity was significantly inhibited, and both abortion and mortality were observed in gravid females exposed to 10 mgL−1. Additionally, a reduction in the mean abundance of the Eschistostomatidae family was detected only after the 40 mgL⁻1 treatment, in all treated specimens. These findings highlight the physiological vulnerability of gravid females to eugenol exposure and suggest a potential antiparasitic effect at specific concentrations. The results emphasise the importance of considering reproductive status when assessing anesthetic safety in ovoviviparous fish.