Mechanisms and evolution of hypoxia tolerance in fish

The ability of an organism to acquire O-2 from its environment is key to survival and can play an important role in dictating a species' ecological distribution. This study is the first, to our knowledge, to show a tight, phylogenetically independent correlation between hypoxia tolerance, traits involved in dictating O-2 extraction capacity and the distribution of a group of closely related fish species, sculpins from the family Cottidae, along the nearshore marine environment. Sculpins with higher hypoxia tolerance, measured as low critical O-2 tensions (P-crit), inhabit the O-2 variable intertidal zones, while species with lower hypoxia tolerance inhabit the more O-2 stable subtidal zone or freshwater. Hypoxia tolerance is phylogenetically independently associated with an enhanced O-2 extraction capacity, with three principal components accounting for 75 per cent of the variation in Pcrit: routine O-2 consumption rate; mass-specific gill surface area; and whole blood haemoglobin (Hb)-O-2-binding affinity (P-50). Variation in whole blood Hb-O-2 P-50 is strongly correlated with the intrinsic O-2-binding properties of the purified Hb while the differences in the concentration of the allosteric Hb modulators, ATP and GTP, provide a Hb system with substantial plasticity for survival in a highly O-2 variable environment.