An Evaluation of MS-222 and Benzocaine as Anesthetics for Metamorphic and Paedomorphic Tiger Salamanders (Ambystoma tigrinum nebulosum)

Abstract

Anesthesia is often used in amphibian studies, yet little information is available regarding the effectiveness of different anesthetics in the same species and across different life stages. We tested two popular anesthetics, benzocaine and MS-222 (tricaine methanesulfonate), in metamorphic (terrestrial adults) and paedomorphic (aquatic adults) Arizona tiger salamanders (Ambystoma tigrinum nebulosum). Benzocaine induced anesthesia more quickly than MS-222 in the concentration used here (0.02%) and was less variable in induction time. Metamorphic adults had higher and more variable induction times than paedomorphic adults. Recovery time was longer and more variable for animals subject to benzocaine rather than MS-222, but did not vary with morph. Our results suggest that benzocaine has higher per gram effectiveness than MS-222 for this species, which likely underlies the increased suggested dosages in the literature for the latter anesthetic. Such increased effectiveness at low concentrations, along with the monetary and time costs associated with MS-222, suggest that benzocaine is a more efficient and less costly anesthetic than MS-222. Because paedomorphic adults were more susceptible to anesthesia than metamorphic adults, paedomorphs may also be more affected by other aqueous chemicals, including pollutants.