Pallas' spadefoot Pelobates vespertinus (Pelobatidae, Amphibia) tolerates extreme hypoxia

Amphibians are believed to be sensitive to hypoxia, and in terms of this ability, they are between mammals with poor tolerance to hypoxia and some turtles and fish with good tolerance to variable oxygen availability. However, we found that the Pallas' spadefoot, Pelobates vespertinus (Pallas, 1771), a Eurasian anuran burrowing amphibian species, is capable of surviving in harsh hypoxia during wintering. This species can tolerate over long-term vital oxygen concentration of almost 10 times lower than the normal atmospheric air concentration (about 2%). The lethal oxygen concentration was about 1% (from 0.6 to 1.8% for different individuals), i.e. 20 times lower than the atmospheric concentration. The Pallas' spadefoots overwinters in soils for more than half a year, having small sizes of storage organs (fat bodies and liver) and low content of lipids and glycogen in them, which are consumed at a low rate in normoxia at 3 degrees C. Under hypoxic conditions, lipid consumption stops, and glycogen consumption increases, indicating a shift from aerobic to anaerobic metabolism. The cumulative effect of the reduced metabolic rate due to low wintering temperatures and the activation of glycolysis allows the Pallas's spadefoot to be considered as one of the most hypoxia-resistant burrowing amphibian species studied.