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 O2 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 P(crit): 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.