Metabolism, gas exchange, and acid‐base balance of giant salamanders

Abstract

The giant salamanders are aquatic and paedomorphic urodeles including the genera Andrias and Cryptobranchus (Cryptobranchidae), Amphiuma (Amphiumidae), Siren (Sirenidae), and Necturus (Proteidae, of which only N. maculosus is considered 'a giant'). Species in the genera Cryptobranchus and Necturus are considered aquatic salamanders well adapted for breathing water, poorly adapted for breathing air, and with limited abilities to compensate acid-base disturbances. As such, they are water-breathing animals with a somewhat fish-like respiratory and acid-base physiology, whose habitat selection is limited to waters that do not typically become hypoxic or hypercarbic (although this assertion has been questioned for N. maculosus). Siren and Amphiuma species, by contrast, are dependent upon air-breathing, have excellent lungs, inefficient (Siren) or no (Amphiuma) gills, and are obligate air-breathers with an acid-base status more similar to that of terrestrial tetrapods. As such, they can be considered to be air-breathing animals that live in water. Their response to the aquatic hypercarbia that they often encounter is to maintain intracellular pH (pH(i) ) and abandon extracellular pH regulation, a process that has been referred to as preferential pH(i) regulation. The acid-base status of some present-day tropical air-breathing fishes, and of Siren and Amphiuma, suggests that the acid-base transition from a low PCO(2) -low [] system typical of water-breathing fishes to the high PCO(2) -high [] systems of terrestrial tetrapods may have been completed before emergence onto land, and likely occurred in habitats that were typically both hypoxic and hypercarbic.

How body size and development biased the direction of evolution in early amphibians

“…Gigantism Among amphibians, only cryptobranchid and amphiumid salamanders reach body lengths beyond 1 m, and only sirenids come close to this measure (Bonett et al 2009;Ultsch 2012). By comparison, the Triassic temnospondyls evolved true giants: these superficially crocodile-like predators inhabited streams and lakes and reached an almost world-wide distribution in the warm-temperate Pangean climate.…”

“…The absence of dermal scales, which were almost universally present in Paleozoic tetrapods, suggests that skin breathing might have played a more important role in these large temnospondyls -but there is no direct evidence for this. Despite its large size, the giant salamander Cryptobranchus is known to breathe in the water almost exclusively via its skin (Ultsch 2012). Hence, increased size does not necessarily eliminate the possibility of cutaneous respiration.…”

Dermal bone in early tetrapods: a palaeophysiological hypothesis of adaptation for terrestrial acidosis

“…plagiosaurids, colosteids and baphetids), These forms were predominantly bottom-dwelling taxa, with heavily ossified skeletons, and dermal sculpture may have added to this ballast weight. Alternatively, these aquatic forms may have specialized for hypercapnic or hypoxic aquatic environments, as indicated by their flattened body forms, and their integumental features represented an adaptation for carbonic and metabolic acid buffering in this type of environment (see also [35]). …”

“…A third hypothesis, that the ability to tolerate high levels of CO 2 evolved before the transition to land (see also [34,35]), will be discussed later.…”

Preferential intracellular pH regulation: hypotheses and perspectives

“…This pattern of acid-base regulation possibly allowed these transitioning vertebrates, which likely possessed acid-base characteristics similar to those of present-day water breathers (high pH e , low arterial P CO2 and low plasma [HCO 3 − ]) and still relied on the gills for exchange of acid-base ion equivalents (Ultsch, 1996(Ultsch, , 2012Witzmann, 2015), to protect pH i during terrestrial excursions. Gradually, coupled pH regulation may have replaced preferential pH i regulation as the strategy to maintain whole-body pH as (1) these terrestrial excursions became longer, (2) the animals' blood acid-base status became more similar to that of terrestrial air breathers (higher arterial P CO2 , higher plasma [HCO 3 − ] and lower pH e ) and (3) the kidneys replaced the gills as the dominant organ for acid-base regulation.…”