Physiological adaptations to low oxygen levels
Adaptations to low oxygen can be not only behavioural but also physiological. Fishes
that live in frequently hypoxic habitats may have more haemoglobin in their red blood
cells, and more of those cells in their blood, and therefore a higher blood capacity to
take up and transport oxygen. Their body tissues may contain more myoglobin, a
molecule that can bind up oxygen and therefore act as an oxygen store But their
main adaptation is anaerobic metabolism, a set of biochemical pathways that do not
require oxygen to yield energy. This type of metabolism is not very efficient and can
lead to the accumulation of relatively toxic by-products, such as lactic acid, and
therefore when oxygen is present anaerobic metabolism is put aside in favour of its
more efficient aerobic counterpart. But when oxygen is rare and metabolic demand is
low, as in a cold water fish for example, anaerobic metabolism can contribute to
survival for days, weeks, or even months. For example, through the use of anaerobic
metabolism, goldfish can survive for up to 9 days at 4 oC in only 0.5 ppm of oxygen.
Similarly, from February to April there is virtually no oxygen at the bottom of
northern lakes, and yet crucian carp, Carassius carassius, survive there because of
their anaerobic metabolism and the cold winter temperatures that lower their energy
requirements.37 Finally, drought is another ecological condition that selects for
anoxia tolerance via anaerobic metabolism. For example, the killifish Austrofundulus
limnaeus lives in ephemeral ponds in Venezuela and the eggs it produces can enter
diapause and survive for up to 60 days in the complete absence of oxygen