Amphibians as Laboratory Animals

Abstract

INTRODUCTION Amphibians include over 4,500 species within 3 major lineages---caecilians, salamanders, and anurans (Stebbins and Cohen 1995). These lineages are linked by several unique physiological traits. The most prominent is the indirect lifestyle of many amphibians--the aquatic gill-breathing larval stage and the aquatic or terrestrial lung or skin-breathing adult stage. A second feature of amphibians is the presence of 2 types of skin glands: mucous and granular. Mucous glands keep the skin surface moist to allow transcutaneous respiration, but they also put the animals at risk of desiccation. The granular glands secrete toxins and function primarily for defense (Zug 1993). The more than 160 species of caecilians are not diverse, all being legless and inhabiting tropical regions. This poorly understood group of amphibians is used in research relatively infrequently, and, therefore, will not be discussed further (see Wake 1994, for review of caecilian husbandry). The salamanders are represented by nearly 400 species, some of which have a four-legged aquatic larval stage, while others (primarily the Plethodontidae) pass the larval stage within eggs that are laid in moist places on land (Stebbins and Cohen 1995). Some salamanders retain the larval gills through their adult stage and depend on oxygenated water throughout their lives. While most salamanders have legs, they have become vestigial in some. The anurans, or frog and toads, are the most successful lineage of amphibians with approximately 4,000 species distributed around the world in a multitude of habitats. The larval stage of most, but not all, anurans includes a legless tadpole or polliwog. The degree of dependency on water varies--species range from purely aquatic to completely terrestrial. Frogs are not only the most diverse group of amphibians; they also are by far the most commonly used amphibians in research. While space constraints limit detailed discussion of amphibian biology and taxonomy here, the topic is thoroughly covered elsewhere (Moore 1964-1976; Frost 1985; Duellman and Trueb 1986; Duellman 1993; Zug 1993).