Animals exposed to a decreasing partial pressure of oxygen - hypoxia - are
traditionally categorized as either oxygen regulators or conformers. Regulators can
maintain their standard metabolic rate when exposed to hypoxia until the so-called
critical partial pressure of oxygen is reached. At further decreasing partial pressures of
oxygen, beyond the critical point, the metabolic rate will decrease and the animal will
start utilizing anaerobic metabolism with a concurrent accumulation of lactate (R in Fig.
1). Conformers, on the other hand, are not able to maintain standard metabolic rate during
hypoxia, and it will decrease linearly with decreasing partial pressure of oxygen (C in
Fig. 1). Metabolic rate versus hypoxia may also have a trace like T in Figure 1. – neither
a conformer nor a regulator, but a Typical trace that can be seen in many publications.
Reasons for this shape will be discussed below
The sluggish toadfish (Opsanus tau) is the classic example of an oxygen
conformer (Hall, 1929) and has often been quoted as such for more than 5 decades. Keys
(1930), however, questioned Hall’s results due to the experimental design, which was
based on a modified version of a flow-through respirometer described by Ege and Krogh
(1914). While oxygen partial pressure decreased from normoxia to severe hypoxia as the
flow of water through the respirometer was decreased, partial pressure of CO2 increased,
but this was ignored by Hall. Keys (1930) also considered that stratification affiliated
with this particular flow-through respirometry led to serious errors.
The sturgeon Acipenser transmontanus is another classic example of an oxygen
conformer, as reported by Burggren and Randall (1978). They found that gill stroke
volume and ventilation rate, and hence gill ventilation decreased with decreasing partial
pressure of oxygen (Fig. 3). Since oxygen extraction also decreased, oxygen consumption
decreased as well, and it was concluded that the sturgeon clearly was an oxygen
conformer. Compared to other teleosts it is unusual that gill ventilation volume decreases
with decreasing oxygen partial pressures. It is also unusual that the gill oxygen extraction
is highest at the highest gill ventilation volume – usually oxygen extraction is highest in
normoxia at which the ventilation volume is normally the lowest. Oxygen consumption
was reported to decrease to less than 5 % of the normoxic rates during hypoxia exposure,
and because there was no sign of anaerobiosis, it was concluded that this ancient fish
reduces total energy expenditure during hypoxic exposure, rather than switching from
aerobic to anaerobic metabolism – it simply shuts down metabolism.