Because anesthesia of amphibian larvae is currently problematic, the aim of this study was to test if a fish anesthetic containing ∼500 g/L isoeugenol (Aqui-S) could anesthetize tadpoles. Three concentrations of Aqui-S were tested (of commercial solution 10, 20, and 50 μl/L) with 10 tadpoles of the southern brown tree frog (Litoria ewingii) per concentration. Tadpoles were anesthetized for 15 min in water at 18–20°C. Anesthetic criteria included four time points and two heart rates: time to loss of response to stimulation, loss of righting reflex, and return of spontaneous movement and of righting reflex after removal from Aqui-S, and heart rate after induction and after 15 min of anesthesia. No tadpole showed signs of irritation or excitement, no tadpole died, and all returned to normal behavior. The times for induction of anesthesia (time to loss of righting reflex) decreased significantly (P < 0.001) with increasing concentration: means of 27.5, 16.8, and 5.7 min for 10, 20, and 50 μl/L, respectively. Heart rate after 15 min of anesthesia was significantly lower (P = 0.006) for concentrations of 20 and 50 μl/L compared to 10 μl/L. At the lowest concentration (10 μl/L), anesthesia was light. Recovery times were significantly delayed (P < 0.001) for 50 μl/L, approximately double those for the lower concentrations. Spontaneous movements began at 56.9, 51.4, and 119 min for 10, 20, and 50 μl/L Aqui-S, respectively. Although Aqui-S did anesthetize tadpoles at all three concentrations, with 20 μl/L being the most suitable, additional longer-term toxicity testing is required prior to its adoption as an anesthetic agent for this species.
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