Anesthetic effects of AQUI-S 20E® (eugenol) on the afferent neural activity of the oyster toadfish (Opsanus tau)

Anesthesia is used to sedate aquatic animals during transportation or to immobilize them for surgery. However, most studies have focused on the behavioral effects of induction and recovery, without addressing the effect of anesthetic on neural activity. This study investigated the neural response of anterior lateral line afferent fibers in the oyster toadfish, Opsanus tau, during exposure to incremental increases of AQUI-S 20E (0.001–0.006%), to determine if eugenol (the active ingredient of AQUI-S 20E) influences neural activity of the fish lateral line system. Ventilation rate significantly decreased following AQUI-S 20E exposure with the surgical plane of anesthesia reached at 0.003%, characterized by shallow ventilation, equilibrium loss, and no response to tactile stimuli. Spontaneous and evoked firing rates of anterior lateral line fibers also significantly decreased following exposure, although the effect was transitory as neural activity recovered in the majority of fibers (70%) within 30 min of anesthetic withdrawal. While AQUI-S 20E proved effective in inducing the surgical plane of anesthesia without compromising survival, it is not recommended for acute neural preparations due to its depression of neural activity. However, the depression of lateral line sensitivity at low concentrations could play a role in reducing the stress response during fish transport.