98
G.N. Somero et al.
freezing temperatures has, however, been paired with the loss of abilities
to withstand, and to acclimatize to, temperatures much above 0 °C.
Although cold adaptation and stenothermy have developed in parallel in
Antarctic notothenioids, they are not necessarily linked in any mechanistic
sense. That is, there is no a priori basis for concluding that adaptation to
low temperatures should necessarily restrict a species' ability to tolerate
high temperatures. Thus, as we examine cold adaptation and stenothermy
in Antarctic notothenioids, it is important to keep in mind that these two
most striking characteristics may be the consequence of quite different
causal factors.
Metabolic Compensation to Low Temperature
A long-standing debate in polar biology concerns whether or not the
metabolic (respiratory) rates of polar ectotherms, including fishes, are ratecompensated for function at low temperatures [8]. Do the respiratory rates
of notothenioid fishes exhibit a temperature-compensatory upwards
adjustment, such that, at the low body temperatures of these species, their
rates of oxygen consumption are similar to those of more warm-adapted
species at their higher habitat (body) temperatures? Initial studies were
interpreted as providing evidence for a substantial degree of metabolic
cold adaptation in Arctic fishes [9] and Antarctic nototheniid fishes (albeit
not in an Antarctic zoarcid) [10]. However, further study of polar and
temperate species, coupled with critical reexamination of earlier data
[8,11,12] undermined the generalization that polar fishes are highly coldadapted in their respiratory rates. As emphasized by Holeton [11] and
Clarke [8], the use of whole-organism oxygen consumption rates to study
metabolic cold adaptation is fraught with logical and experimental pitfalls.
For instance, comparisons of active and sluggish species, especially
species belonging to distantly related families, may yield more information
about the effects of "mode of life" (Zimmermann and Hubold, this Vol.)
on metabolic rate than about adaptation to temperature per se. The time
period during which a fish is allowed to acclimate to the experimental
chamber, and the dietary status and reproductive state of the fish also may
greatly affect its rate of oxygen consumption. Because of the difficulties
associated with studies of whole-organism respiration, we sought to find a
less ambiguous index of metabolic temperature compensation, one that
would allow meaningful comparisons to be made between warm- and
cold-adapted fishes belonging to widely different families and having
different modes of life.
Our approach to resolving the question about the degree to which
metabolic rates of Antarctic fishes are cold adapted involves measurement
G.N. Somero et al.
freezing temperatures has, however, been paired with the loss of abilities
to withstand, and to acclimatize to, temperatures much above 0 °C.
Although cold adaptation and stenothermy have developed in parallel in
Antarctic notothenioids, they are not necessarily linked in any mechanistic
sense. That is, there is no a priori basis for concluding that adaptation to
low temperatures should necessarily restrict a species' ability to tolerate
high temperatures. Thus, as we examine cold adaptation and stenothermy
in Antarctic notothenioids, it is important to keep in mind that these two
most striking characteristics may be the consequence of quite different
causal factors.
Metabolic Compensation to Low Temperature
A long-standing debate in polar biology concerns whether or not the
metabolic (respiratory) rates of polar ectotherms, including fishes, are ratecompensated for function at low temperatures [8]. Do the respiratory rates
of notothenioid fishes exhibit a temperature-compensatory upwards
adjustment, such that, at the low body temperatures of these species, their
rates of oxygen consumption are similar to those of more warm-adapted
species at their higher habitat (body) temperatures? Initial studies were
interpreted as providing evidence for a substantial degree of metabolic
cold adaptation in Arctic fishes [9] and Antarctic nototheniid fishes (albeit
not in an Antarctic zoarcid) [10]. However, further study of polar and
temperate species, coupled with critical reexamination of earlier data
[8,11,12] undermined the generalization that polar fishes are highly coldadapted in their respiratory rates. As emphasized by Holeton [11] and
Clarke [8], the use of whole-organism oxygen consumption rates to study
metabolic cold adaptation is fraught with logical and experimental pitfalls.
For instance, comparisons of active and sluggish species, especially
species belonging to distantly related families, may yield more information
about the effects of "mode of life" (Zimmermann and Hubold, this Vol.)
on metabolic rate than about adaptation to temperature per se. The time
period during which a fish is allowed to acclimate to the experimental
chamber, and the dietary status and reproductive state of the fish also may
greatly affect its rate of oxygen consumption. Because of the difficulties
associated with studies of whole-organism respiration, we sought to find a
less ambiguous index of metabolic temperature compensation, one that
would allow meaningful comparisons to be made between warm- and
cold-adapted fishes belonging to widely different families and having
different modes of life.
Our approach to resolving the question about the degree to which
metabolic rates of Antarctic fishes are cold adapted involves measurement