Hypoxic Boundary Layers Surrounding Skinbreathing Aquatic Amphibians: Occurrence, Consequences and Organismal Responses

Abstract

We have used oxygen microelectrodes to characterize the relationship between the partial pressure of oxygen Poo2 and the distance from the skin in a variety of amphibians immersed in unstirred normoxic water. A substantial hypoxic boundary layer surrounded the skin in nearly every case, with the Poo2 at the skin-medium interface typically varying between 2 and 4 kPa at both 10 and 20°C. The degree of hypoxia varied little among the anatomical sites examined. We hypothesized that the formation of hypoxic boundary layers should abbreviate voluntary diving in amphibians that use cutaneous oxygen uptake from water to supplement oxygen stores in the body during dives. To test this, we compared voluntary submergence times of amphibians diving in stirred and unstirred normoxic water. The dives of frogs (Xenopus laevis) and salamanders (Siren lacertina) in stirred water averaged 2.3–2.5 times longer than dives in unstirred water. Diving Xenopus also underwent more voluntary movements (other than swimming to the surface to breathe air) in unstirred water than in moving water. A closed extracorporeal loop containing an oxygen electrode was used to record the Poo2 of lung gas during forced dives in Xenopus to determine the time required to deplete pulmonary oxygen stores to a level normally associated with the end of voluntary dives (Poo2=4kPa). In normoxic water flowing at 0.54cms−1 or faster, the decline in pulmonary Poo2, to this level required on average 18.7 min; in unstirred normoxic water, the average time was 13.8 min. This difference is tantamount to a 36 % extension of submergence time in flowing water. These findings and those of companion studies suggest that hypoxic boundary layers have a major and pervasive influence on the respiratory status of skinbreathing amphibians in water.