Life at stable low oxygen levels: adaptations of animals to oceanic oxygen minimum layers.

Zones of minimum oxygen level are found at intermediate depths in most of the world's oceans and, although the oxygen partial pressure in some of these 'oxygen minimum layers' is only a fraction of a kilopascal, populations of pelagic metazoans exist there. These oxygen minimum layers are areas of the water column and the associated benthos with stable conditions of continuously low oxygen level and low temperature at intermediate depths (400-1000 m depth) over vast areas. Off California, where PO2 at the oxygen minimum is 0.8 kPa, there are abundant populations of animals both in the water column and on the bottom. Farther to the south in the eastern tropical Pacific, oxygen partial pressures of less than approximately 0.4 kPa result in very low biomasses and diversity of animals at minimum layer depths. At the minimum oxygen levels found off California, most animals which inhabit the minimum zones appear to support their routine metabolic demands via aerobic metabolism. They do this by being very effective at removing oxygen from water. Among the adaptations of pelagic crustaceans to these conditions are: (1) enhanced ventilatory abilities, (2) enhanced percentage removal of O2 from the ventilatory stream, (3) large gill surface areas, (4) short diffusion distances from the water to the blood, and (5) hemocyanin respiratory proteins with a very high affinity for O2, high cooperativity and large Bohr effects. The lower O2 consumption rates of many deeper-living species are also functionally adaptive in that they facilitate aerobic survival at low PO2. However, they are not adaptations to the minimum layer, since similarly low rates are found in the same and comparable species living at the same depths in regions without well-developed minima, and these animals are unable to survive at the low PO2 values of the minima. While anaerobic metabolism may be important for metabolic rates above the routine level for most animals in the minimum layer, there is little evidence for the use of sustained anaerobiosis in the species studied. In summary, given the stable presence of very low O2 levels in the minima, the primary adaptations of animals living within them are those that support aerobic metabolism by giving the animals remarkable abilities to extract O2 from water. These abilities are notably better than those of animals adapted to unstable hypoxic environments, such as intertidal mudflats, while the latter animals rely to a much greater extent on anaerobiosis and perhaps on metabolic suppression to survive periods of anoxia.