High dissolved oxygen extends dive duration and suggests physical gill use in a vertebrate

Dissolved oxygen (DO) dramatically impacts the habitat use of many aquatic animals, particularly for air-breathing animals that rely on 'physical gills' for respiration while submerged. Invertebrates that use bubbles as physical gills directly uptake DO from the water for respiration. However, no vertebrate animals have yet been documented using physical gills. Semi-aquatic Anolis lizards dive underwater when threatened, rebreathing a bubble of air to stay submerged for extended periods. If semi-aquatic anoles use rebreathing bubbles as physical gills, we would expect that higher DO levels in water could prolong dive duration. We manipulated DO in trial arenas to test whether higher DO content would increase dive duration in a representative semi-aquatic anole, Anolis aquaticus. Because DO and water flow are typically correlated in natural environments, we also separately tested the effect of flow rate on dive durations. We found that dive durations were significantly longer at the highest DO (with no flow) and at the lowest flow rates (with constant DO). Complementary field data on Anolis aquaticus perch sites suggest that lizards are more likely to be found at perches near regions of streams with relatively low flow but adequate DO. This study provides early evidence that a vertebrate air-breathing animal may use bubbles as a physical gill, which is the first step toward identifying physical gill use in vertebrates.