Reversible effects of tricaine methanesulfonate (MS-222) in behavior and cardiac activity in the Amazonian Flag Tetra Hyphessobrycon heterorhabdus

Abstract Tricaine methanesulfonate (MS-222) is one of the most widely used anesthetics in aquaculture management procedures and the ornamental fish trade; however, at excessive concentrations, it may induce undesirable physiological responses in fish. Thus, we evaluated the safety and reversibility of MS-222–induced anesthesia in the Amazonian flag tetra, Hyphessobrycon heterorhabdus . Fish were subjected to immersion baths at concentrations of 50, 75, 100, and 125 mg·L⁻ 1 for 5 min, followed by recovery in clean water. Behavioral effects and heart rate were assessed through electrocardiographic (ECG) recordings. Fish exhibited a longer latency to loss of postural reflex at 50 and 75 mg·L⁻ 1 (180–200 s), in contrast to 100 and 125 mg·L⁻ 1 (60–100 s). However, upon transfer to clean water, animals treated with 50 and 75 mg·L⁻ 1 recovered the postural reflex rapidly (100–120 s), whereas recovery was slower at 125 mg·L⁻ 1 (240–280 s). ECG tracings demonstrated a concentration-dependent reduction in heart rate, accompanied by increased duration of the R–R, P–Q, and S–T intervals, as well as the QRS complex. However, anesthesia did not alter the amplitude of the cardiac electrical impulse in the fish. After recovery, ECG components returned to baseline parameters similar to those of the control group, with no subsequent irregularities in cardiac rhythm. Therefore, it is concluded that the most effective concentrations of MS-222 for H. heterorhabdus range from 75 to 100 mg·L⁻ 1 , considering response time and amount of substance used, thereby providing a safe window for anesthetic induction in this species.