Physiological responses to prolonged aquatic hypoxia in the Queensland lungfish Neoceratodus forsteri.

The effects of moderate and severe hypoxia on air breathing frequency and respiratory properties of the blood of the Queensland (Australian) lungfish Neoceratodus forsteri were measured in fish exposed to these conditions for 14-22 days at 20 degrees C. Haemoglobin oxygen affinity increased after exposure to moderate hypoxia (PW(O(2)) = 60 mmHg), but did not increase further after exposure to severe hypoxia (PW(O(2)) = 40 mmHg). The P(50) of whole blood (20 degrees C, P(CO(2)) = 16.0 mmHg) fell from 22.0 +/- 1.5 mmHg in normoxic conditions to 19.0 +/- 1.0 mmHg in hypoxic conditions. Under both moderate and severe hypoxia, haematocrit, haemoglobin, blood lactate, and erythrocyte phosphate concentrations did not differ from normoxic values. The observed increase in haemoglobin oxygen affinity in response to aquatic hypoxia is typical of compensatory responses seen in obligate water breathers, but smaller. This suggests that the capacity of lungfish to respond to hypoxia by breathing air removes the necessity for further left-shifting of the oxygen equilibrium curve.