Respiratory properties of blood in flatback turtles (Natator depressus)

Oxygen equilibrium curves and other respiratory-related variables were determined on blood from the flatback turtle (Natator depressus) and, for comparison, on some samples from the loggerhead turtle (Caretta caretta). The oxygen carrying capacity of the flatback turtle, 4.9-8.7 mmol l(-1) (n = 49), is at the high end of the range in diving reptiles. Oxygen affinity (P(50)) was similar in both species at 5% CO(2), ranging from 37 to 55 mmHg (43 mmHg +/- 5.3 SD, n = 24, 25 degrees C, pH 7.17) in flatbacks and 43-49 mmHg in loggerheads (46 mmHg +/- 2.0 SD, n = 7, 25 degrees C, pH 7.13), whereas at 2% CO(2), flatbacks had a higher oxygen affinity. The curves differed in sigmoidicity, with Hill n coefficients of 2.8 and 1.9 in flatbacks and loggerheads, respectively. The Bohr effect was small in both the species, consistent with results from other sea turtles. Lactate levels were high, perhaps because the samples were taken from turtles coming ashore to lay eggs. Flatbacks are rarely found in waters deeper than 45 m. It is suggested that they have a respiratory physiology particularly suited to sustain prolonged shallow dives.