SUMMARY
An adaptation strategy whereby O2 partial pressure, PO2, in the tissues is maintained within a low,narrow range of 1–3 kPa, largely independent of the inspired PO2, has been reported in water- and air-breathing poikilotherms and in homeotherms. Based on the postulate that this basic cellular mechanism has been established since the early stages of evolution, it has been hypothesized that it could be the consequence of an early adaptation strategy to maintain cellular oxygenation within the same low and primitive range. To test this hypothesis we studied the basic mechanisms of oxygen regulation in podocopid ostracods, minute crustaceans that have existed on earth for at least 500 million years. Podocopids lack any regulatory mechanism for adapting their ventilation to cope with changes in water oxygenation, and instead adjust their tissue oxygenation status by migrating through the O2 gradient to sediment layers where the PO2 of the water is 3–5 kPa. Experimental manipulation of the O2 profile induced their vertical migration to follow this precise water PO2 and demonstrates the existence of a regulation strategy. This strategy must be associated with the lower PO2 values within the animal's carapace valves, showing that podocopids can actively regulate their tissue PO2 at constant but even lower values than the water. In conclusion, the low tissue PO2strategy could have existed in early crustaceans and, by extension, in early animals.
