RESPIRATORY SYSTEM In amphibians, gas exchange always occurs across a moist surface. Although cutaneous respiration is important in both larval and adult forms, as a general rule larval amphibians utilize gill structures for respiration, while adults use lungs, although there are many exceptions to this. There are three modes of respiration described in adult caecilians and anurans: pulmonic, buccopharyngeal and cutaneous. A fourth mode exists in adult urodeles, that being branchial respiration from retained gill structures seen in neotenic species such as sirens, mudpuppies, axolotls and Texas blind salamanders (Goin et al. 1978; Mitchell et al. 1988; Wright 1996, 2001c). In most amphibians, gill structure shows some variability depending on the species and their environment. The gills of larval anurans are usually smaller and simpler than those of salamander larvae. While the branchial arches of tadpoles are covered by an operculum, in many species of salamander, especially neotenic species, the gills are external. The gills of most caecilians are resorbed before birth or hatching, while the gills of anurans resorb during metamorphosis. Most terrestrial salamander species lose their gills and develop lungs like anurans; however, many of the aquatic neotenic species will retain their gills and still develop normal lungs. A few families of salamander, notably the Plethodontidae and Hynobiidae, lack lungs or have lungs that are reduced in size (Goin et al. 1978; Mitchell et al. 1988; Wright 1996, 2001c). The lungs of amphibians are simple saclike structures that lack true alveoli. As a result, most lungs are subdivided internally by delicate reticulate infoldings of the pulmonic tissue that significantly increase the surface area for gas exchange. Complete cartilaginous rings support the tracheal tissues. The trachea is variable in length depending on the species, but in general is considered short, and bifurcates quickly into main bronchi. Amphibians lack a diaphragm so they rely on coordinated movements of their axial and appendicular muscles for gas exchange in the lungs. Buccopharyngeal gas exchange occurs through the pumping action of the larynx during inspiration and expiration. During periods of reduced oxygen availability (such as hibernation) amphibians may switch to cutaneous respiration. As cutaneous respiration is not as efficient as pulmonic respiration, many amphibians have developed specialized integumentary structures, such as lateral folds, costal grooves or cutaneous “hairs”, as seen in the African hairy frog (Trichobatrachus spp.) (Mitchell et al. 1988; Wright 1996, 2001c). Skin The highly permeable skin of amphibians is a major site of gas exchange in terrestrial, semiaquatic, and aquatic species. Cutaneous respiration accounts for some gas exchange in certain species of reptiles (Fig. 6.20). Exchange of respiratory gases occurs by diffusion and is facilitated by a relatively thin layer of keratin and a rich supply of capillaries in the skin. Exchange of gases across the skin in water is limited by the same physical factors as exchange across other respiratory surfaces. Ventilation of skin, as with gills and other respiratory surfaces, is required to disrupt the boundary layer that can develop. Xenopus has been observed to remain submerged longer and to move less frequently in moving compared to still water. Most plethodontid salamanders have neither lungs nor gills and are largely terrestrial (Fig. 6.21). The majority of their gas exchange occurs through the skin. In these salamanders, in contrast to others, there is no partial separation of the oxygenated and venous blood in the heart. Many species of this diverse group, because of their mode of respiration, are limited to cool, oxygenated habitats and to nonvigorous activity. Their oxygen uptake is only one-third that of frogs under similar conditions. Plethodontids that inhabit tropical habitats where temperatures can be high, such as Bolitoglossa in tropical rainforests, are active primarily on rainy nights. Waterproof frogs sacrifice their ability to undergo cutaneous respiration in exchange for the skin resistance to water loss. Some amphibians increase their capacity for cutaneous respiration by having capillaries that penetrate into the epidermal layer of skin. This modification is carried to an extreme in Trichobatrachus robustus, the “hairy frog,” which has dense epidermal projections on its thighs and flanks. These projections increase the surface area for gaseous exchange. Hellbenders, Cryptobranchus alleganiensis, live in mountain streams in the eastern United States. These large salamanders have extensive highly vascularized folds of skin on the sides of the body, through which 90% of oxygen uptake and 97% of carbon dioxide release occurs. Lungs are used for buoyancy rather than gas exchange. The Titicaca frog, Telmatobius culeus, which inhabits deep waters in the high-elevation Lake Titicaca in the southern Andes, has reduced lungs and does not surface from the depths of the lake to breathe. The highly vascularized skin hangs in great folds from its body and legs (Fig. 6.22). If the oxygen content is very low, the frog ventilates its skin by bobbing. Other genera of frogs, salamanders, and caecilians (typhlonectines) have epidermal capillaries that facilitate gas exchange. Gas exchange in tadpoles occurs across the skin to some degree in all species. Tadpole skin is highly permeable, similar to that of adults. Gas exchange across the skin is prevalent in bufonids and some torrent-dwelling species that do not develop lungs until metamorphosis. Microhylids, some leptodactylids, and some pipids have reduced gills, thus increasing their reliance on cutaneous respiration. Recent studies show that some reptiles, once thought not to exchange gases through the skin, may use cutaneous respiration for as much as 20–30% of total gas exchange. In some aquatic species, such as Acrochordus and Sternotherus, gas exchange across the skin is especially significant for carbon dioxide (Fig. 6.20). Even in terrestrial taxa such as Lacerta and Boa, measurable amounts of gas exchange occur cutaneously. A sea snake, Pelamis platurus, frequently dives and remains submerged. During these dives, oxygen uptake equals 33% of the total, and 94% of the carbon dioxide loss is through the skin. Exchange does not occur through scales but rather through the skin at the interscalar spaces. Larval amphibians breathe primarily through gills. Adult amphibians may retain and use gills, lose gills and develop lungs, breathe with both gills and lungs, or have neither and utlize cutaneous respiration mechansims. X. laevis tadpoles and axolotls have both gills and lungs and will gulp air at the water’s surface. Axolotls flex their external gills to move fresh water over the filaments; this behavior increases when animals are housed in warm water with decreased oxygen content (Gresens, 2004). Adult plethodontids (lungless salamanders) lack both lungs and gills, and rely on cutaneous respiration. Skin, in fact, is the primary respiratory surface in most amphibians and must be kept moist. In species that use lungs for respiration, air is forced in and out of the lungs by movement of the buccopharyngeal floor (Zug, 1993). Lungs lack alveoli and are very fragile and easily ruptured (Wright, 1996) (Fig. 18.5). In many frog species, the trachea is short, and bifurcation occurs close to the glottis; this anatomic feature must be taken into account when performing endotracheal intubation. Amphibian Lungs : The lungs of amphibians are relatively simple in structure. In most anurans they connect directly with the larynx. In Urodeles the lungs consist of a pair of elongated sacs, the left being longer than the right. In some the lining is smooth, but in others alveoli are present. Often the alveoli are confined to the basal portion. The left lung in caecilians is very short, and alveoli line the entire surface of the right lung. In frogs and toads the lining is more complex since thw wall is thrown into numerous infundibular folds, which in turn are lined with alveoli. Amphibians usually develop lungs at the time of metamorphosis, but in some frogs, toads, and salamanders, they appear during larval life. Plethodontid salamanders in which they never appear, relies on cutaneous and buccopharyngeal respiration during adult life. Such neotenic salamanders as Necturus have lungs in addition to gills. .