Hypoxia tolerance and anaerobic capacity in Danio and Devario

It has been long suggested that hypoxia tolerant species should have a great capacity to generate energy through anaerobic pathways to maintain energy balance when oxygen is limited; however, this assertion has not been rigorously tested. In the present study, I characterized hypoxia tolerance in 12 groups representing 10 species from the genera Danio and Devario (with three strains of D. rerio) and examined whether there is a phylogenetically independent relationship between variation in hypoxia tolerance and anaerobic capacity as judged by enzyme activity and anaerobic substrate concentrations present in various tissues. Hypoxia tolerance was assessed using two measures: time to loss of equilibrium (LOE) and the oxygen tension that yields 50% LOE in a group of fish over 8 hr (TLE₅₀). Time to LOE to low oxygen was very sensitive to changes in water PO₂, with no LOE seen over 8 hr in some species at 16 torr (2.1 kPa) and complete LOE within 30 min at 8 torr (1.1 kPa). At 12 torr (1.6 kPa) however, there was significant variation in time to LOE among all the species investigated. In three species (Danio rerio, Danio albolineatus and Danio choprai) time to LOE at 12 torr showed the same pattern of hypoxia tolerance as TLE₅₀. Despite the variation in hypoxia tolerance seen among the species under study, there was very little variation in the critical oxygen tension (Pcrit), which is the environmental PO₂ at which fish transition from an oxyregulating strategy to an oxyconforming strategy. Routine Ṁ₀₂ varied between the species, but the variation was primarily explained by body size and not hypoxia tolerance. Anaerobic energy capacity was estimated by measuring maximal enzyme activities of pyruvate kinase (PK), lactate dehydrogenase (LDH) and creatine phosphokinase (CPK), and concentrations of glycogen and glucose in muscle, liver and brain, plus creatine phosphate (CrP) and ATP in muscle. Through comparative analysis, I showed that the variation in hypoxia tolerance seen among species was related to some aspects of anaerobic energy metabolism, but not in a consistent fashion, indicating that other factors contribute to describing the variation in hypoxia tolerance.Science, Faculty ofZoology, Department ofGraduat