The Fitzroy turtle, Rheodytes leukops, is a bimodally respiring freshwater chelid that can extract a significant portion of its total O2 supply from the water, chiefly via highly modified cloacal bursae (Priest 1997). The relatively high reliance of R. leukops upon aquatic respiration translates into markedly longer dives compared to primarily air-breathing turtles (Gordos and Franklin 2002, Priest and Franklin 2002); however, the diving behaviour of aquatic chelonians is dependent not only upon the animal's physiology, but also upon the interaction of the turtle with fluctuating environmental conditions. Therefore, I investigated the effect of changing biotic and abiotic factors upon the diving ecology of R. leukops.
Using pressure-sensitive time-depth recorders, the diving behaviour of R. leukops was recorded over four consecutive seasons (Austral autumn 2000 to summer 2001) within Marlborough Creek, central Queensland, Australia to investigate how seasonal and daily fluctuations in aquatic temperature and PO2 level, as well as the behavioural state of the turtle, influenced the diving ecology of R. leukops in a natural setting. As water temperature increased and aquatic PO2 level decreased from winter (16.5°C; PO2 = 126.4 mmHg) to summer levels (27.3°C; PO2 = 63.4 mmHg), R. leukops switched from a facultative to obligate air-breather presumably due to the decreased efficiency of aquatic respiration in meeting the turtle's metabolic demands in summer. On a daily scale, surfacing frequency increased significantly during the daylight hours in autumn and summer, with peak surfacing levels normally occurring around dawn and dusk. Diurnal surfacing patterns recorded for R. leukops in autumn and summer are attributed to periods of increased activity (possibly associated with foraging) during the daylight hours rather than to fluctuating water temperatures and aquatic PO2 levels given the negligible daily variation observed in the two factors below the surface of the water. No consistent diel surfacing trend was recorded for R. leukops in winter nor spring.
The effect of increasing water depth (50, 100,150 cm) and water velocity (5, 15, 30 cm s-1) upon the surfacing behaviour of R. leukops was examined under controlled conditions. Increasing water depth had no effect upon the surfacing frequency of R. leukops, despite a seven-fold increase in the time required to reach the surface for pulmonary gas exchange. Alternatively, increasing water velocity significantly impeded the surfacing frequency of R. leukops, with the turtle effectively switching from an obligate to facultative air-breather. Given that the described preferred habitat of R. leukops is fast-flowing riffle zones (Legler and Cann 1980; Tucker et al. 2001), aquatic respiration presumably allows the species to inhabit and exploit locations characterised by high water velocities.
Changes in blood gas, acid-base, and plasma ion status were examined for R. leukops during prolonged dives of up to 12 h to determine whether an O2 debt developed. Blood PO2 declined significantly with dive length; however, oxy-haemoglobin saturation levels remained greater than 30 % for all R. leukops sampled. Moreover, plasma lactate levels for R. leukops remained less than 3.0 mmol 1-1 despite repeated dives in excess of 2 h. Results from this study indicate that R. leukops utilised aquatic respiration to remain aerobic during extended dives, thus avoiding the development of a metabolic acidosis. In addition, R. leukops avoided a respiratory acidosis, presumably through the elimination of metabolically produced CO2 to the water across extra-pulmonary respiratory sites.
Finally, changes in heart rate (fH) and cloacal ventilation (fC) were investigated in R. leukops under normoxic (133.9 mmHg) and hypoxic (28.4 mmHg) conditions at 25°C to assess whether a diving bradycardia was evident. During normoxic trials, mean fH and fC remained constant irrespective of dive length (range: 0.3 - 69.1 h; mean 3.9 ± 0.8 h). Exposure to aquatic hypoxia; however, resulted in a marked decrease in submergence length (0.9 ± 0.1 h; range 0.2 - 1.6 h), with R. leukops displaying a significant negative correlation between dive length and mean fH and fC. R. leukops therefore appears to alter its strategy from maintaining aquatic O2 extraction via cloacal respiration during normoxia to decreasing its O2 consumption rate via a diving bradycardia when exposed to aquatic hypoxia.
Results from this thesis indicate that R. leukops utilises aquatic respiration as a distinct respiratory strategy to support extended submergences; however, the diving physiological ecology of R. leukops is dependent upon the interaction of the turtle with its environment.