106
G.N. Somero et al.
Conclusions
The biochemical and physiological characteristics of Antarctic
notothenioid fishes reflect adaptation to the low and extremely stable
temperatures found in the Antarctic Ocean. Antifreeze proteins help to
protect these fishes from freezing (Cheng and DeVries, this Vol.), and
other biochemical adaptations facilitate metabolic function at near-freezing
temperatures. The extent to which the respiration rates of Antarctic fishes
are cold-adapted remains unclear. Indeed, as Clarke [8] has cogently
argued, it seems unlikely that metabolic compensation to temperature can
be studied meaningfully at the level of whole-organism respiration.
However, our data on activities of LDH and CS in brain indicate that
partial compensation to temperature exists in ATP-producing pathways in
this organ. The finding that compensation, while substantial, is still only
partial raises the interesting question as to why compensation in ATP
production is not complete. Future studies must examine the energy costs
of maintaining neural function, for instance, transmembrane ion gradients,
in nervous tissue at different temperatures [17].
The stenothermy of Antarctic fishes noted at the whole organism level
[5] is also evident in many of the biochemical characteristics of these
species. The mitochondria of T bernacchii have the lowest tolerance of
high temperature reported for any organism. The temperature at which the
Acceptor Control Ratio decreases rapidly, and the extremely low
Arrhenius Break Temperature for mitochondrial respiration both reflect
what appears to be a general characteristic of the thermal responses of the
physiological systems of Antarctic notothenioids: a pronounced ability to
function well at extremely low temperatures, but the inability to function
at, and to acclimate to, temperatures much above 0 °C.
The lack of induction of heat shock proteins in T bernacchii may be
another illustration of the loss of function at high temperatures that has
occurred during the evolution of the notothenioids. These fishes provide
striking examples of what might be termed the "blind cave fish
phenomenon," that is, the loss of traits that have ceased to be important for
the organism in its new environment. The absence of hemoglobin and
myoglobin in members of the Channichthyidae may be indicative of what
has happened to other biochemical systems in the notothenioids, for which
low temperatures or stable temperatures have eliminated the raison d'etre
for the protein(s) encoded by the gene(s). The loss of ability to express
heat shock proteins seems consistent with evolution for many millions of
years in an environment having only minimal variation in temperature.
Thus, evolution of the notothenioids has involved concurrent acquisition
of biochemical adaptations essential for survival at low temperatures and
G.N. Somero et al.
Conclusions
The biochemical and physiological characteristics of Antarctic
notothenioid fishes reflect adaptation to the low and extremely stable
temperatures found in the Antarctic Ocean. Antifreeze proteins help to
protect these fishes from freezing (Cheng and DeVries, this Vol.), and
other biochemical adaptations facilitate metabolic function at near-freezing
temperatures. The extent to which the respiration rates of Antarctic fishes
are cold-adapted remains unclear. Indeed, as Clarke [8] has cogently
argued, it seems unlikely that metabolic compensation to temperature can
be studied meaningfully at the level of whole-organism respiration.
However, our data on activities of LDH and CS in brain indicate that
partial compensation to temperature exists in ATP-producing pathways in
this organ. The finding that compensation, while substantial, is still only
partial raises the interesting question as to why compensation in ATP
production is not complete. Future studies must examine the energy costs
of maintaining neural function, for instance, transmembrane ion gradients,
in nervous tissue at different temperatures [17].
The stenothermy of Antarctic fishes noted at the whole organism level
[5] is also evident in many of the biochemical characteristics of these
species. The mitochondria of T bernacchii have the lowest tolerance of
high temperature reported for any organism. The temperature at which the
Acceptor Control Ratio decreases rapidly, and the extremely low
Arrhenius Break Temperature for mitochondrial respiration both reflect
what appears to be a general characteristic of the thermal responses of the
physiological systems of Antarctic notothenioids: a pronounced ability to
function well at extremely low temperatures, but the inability to function
at, and to acclimate to, temperatures much above 0 °C.
The lack of induction of heat shock proteins in T bernacchii may be
another illustration of the loss of function at high temperatures that has
occurred during the evolution of the notothenioids. These fishes provide
striking examples of what might be termed the "blind cave fish
phenomenon," that is, the loss of traits that have ceased to be important for
the organism in its new environment. The absence of hemoglobin and
myoglobin in members of the Channichthyidae may be indicative of what
has happened to other biochemical systems in the notothenioids, for which
low temperatures or stable temperatures have eliminated the raison d'etre
for the protein(s) encoded by the gene(s). The loss of ability to express
heat shock proteins seems consistent with evolution for many millions of
years in an environment having only minimal variation in temperature.
Thus, evolution of the notothenioids has involved concurrent acquisition
of biochemical adaptations essential for survival at low temperatures and