Academic journal article The American Midland Naturalist

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

Academic journal article The American Midland Naturalist

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

Article excerpt


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.


Anesthesia is an artificially produced unconsciousness used during surgery or other medical procedures to block sensations of pain. Different types of anesthesia are used routinely in the treatment and evaluation of amphibians (Wright, 2001). Reversible anesthesia causes total or partial loss of sensation and immobility, allowing for procedures such as implantation of passive integrated transponders (PIT tags), elastomers and radio transmitters (Madison, 1997; Madison and Farrand, 1998; USGS, 2001). Typically, the choice of anesthesia has been based on what other researchers have used rather than a critical evaluation of different methods.

Benzocaine and MS-222 are two well-known anesthetics that have been successfully used to anesthetize amphibians in both field and laboratory settings (Stouffer et al., 1983; Werner, 1991; Colberg et al., 1997; Goldberg et al, 2002; Lowe, 2004). Both are powder anesthetics that are prepared with water, and animals are typically immersed in the resulting solution (USGS, 2001; Wright, 2001). Although the literature contains information on these and other anesthetics, no study, to the best of our knowledge, has compared the use of both anesthetics in a single species. In this study, we experimentally compared the effectiveness of benzocaine and MS-222 in Arizona tiger salamanders (Ambystoma ligrinum nebulosum).

Ambystoma tigrinum nebulosum is a facultatively paedomorphic species in which some larvae metamorphose while others retain their larval morphology to become paedomorphic adults (Semlilsch, 1985; Whiteman, 1994; Denoël et al., 2002). Facultatively paedomorphic species provide an interesting comparison for anesthesia, because paedomorphic adults have gills and a more permeable epidermis than metamorphic adults (Whiteman, 1994), potentially facilitating the uptake of anesthesia. Thus, we predicted that both anesthestics would operate more quickly and effectively on paedomorphic adults than on metamorphic adults.


Fifteen metamorphic tiger salamanders were collected from Kaichlen Pond and ten paedomorphic tiger salamanders were collected from Marshall Pass Pond #1, both in the Gunnison Basin of south-central Colorado, during July 2003. The experiment consisted of the two anesthetic treatments, using identical concentrations (0.02%). Although this concentration is below that recommended for MS-222 to induce anesthesia in terrestrial amphibians (0.1%; Wright, 2001), we chose it because it was recommended for larval (paedomorphic) forms using MS-222, and for terrestrial (metamorphic) forms using benzocaine (Wright, 2001). …

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