SHORT COMMUNICATION LOW ARTERIAL POl IN RESTING CRUSTACEANS IS INDEPENDENT OF BLOOD OXYGEN-AFFINITY

In the literature about the respiratory physiology of water-breathers, the arterial partial pressure of oxygen, P&o2, is reported to range from about 2 to 10-13 kPa (Shelton etal. 1986; McMahon and Wilkens, 1983). In fish, Shelton etal. (1986) reported that animals with low blood O2-affinity exhibit high P a ^ and vice versa. But in laboratory conditions, the excitation state of the animals may explain much of this variability: high excitation is associated with high PaOl values and low excitation with low PaOl values (see McMahon, 1985, for relevant discussion on crustaceans). In conditions where animal excitability was reduced to a minimum, we reported that low PaOl values correspond to an apparent set point in crayfish Astacus leptodactylus (Massabuau and Burtin, 1984), fish Silurus glanis (Forgue etal. 1989) and mussel Anodonta cygnea (Massabuau etal. 1991). Indeed, when the inspired POl (Pio2) varies from 3 to 40kPa, Pa^ remains in the range of 1-3 kPa. In resting crabs Carcinus maenas contrary to all previously reported data (see McMahon and Wilkens, 1983) and Eriocheir sinensis, we also reported that FaOj in normoxic conditions is kept in a low range around 1-3 kPa. This is independent of the blood pigment concentration (Forgue etal. 1992). Except for A. cygnea, which has no respiratory pigment, the above species exhibit mid values of blood O2-affinity, with P50 (the values of Po? at which 50 % of the respiratory pigment is oxygenated) around 0.7-1 kPa at physiological pH (7.8-8.0) and 15°C. The present study examined the effects of variation in O2affinity on the setting of Pa^. We present data obtained, at rest and during normoxia, in crustaceans chosen for their distinctly different blood O2-afnnities with P50 varying from 0.2 to 2.0 kPa. Seven species of freshwater (fw) and/or seawater (sw) crustaceans were examined from January to October 1991: Procambarus clarkii (fw), Astacus leptodactylus (fw), Eriocheir sinensis (fw and sw; studied in sw), Carcinus maenas