POIKILOTHERMIC ADAPTATIONS 



B (S D. (3) An accumulation of products of one step can repress or 

 can stimulate an earlier step in a sequence. The actions (2) and (3) 

 may be on enzyme- forming RNA or even on the DNA template. In 

 nature the capacityof a particular enzyme- forming system to change 

 can form the basis for selection under temperature stress. 



The biological significance of the chemical changes in tempera- 

 ture acclimation is uncertain. Many enzymes show enhanced activity, 

 some are tmaltered, and a few decreased inaction after cold accli- 

 mation. No calorimetric measurements of total energy liberation 

 have been made, and determination of P/O ratios for liver mitochon- 

 dria have led to equivocal results. The lipid changes are in the dir- 

 ection of lower melting points in the cold. A fish or a salamander at 

 a low temperature is never as active as at a high temperature. 



Behavior and nervous changes 



In a temperature gradient a fish "selects" a temperature where 

 the frequency of spontaneous movements is least; this selection is 

 determined by sensory input from cutaneous thermoreceptors and 

 is upset by lesions of the forebrain (Sullivan, 1954; Fisher, 1958). 

 The "selected" temperature is higher than a low temperature of ac- 

 climation and lower than a high acclimation level (Sullivan and Fish- 

 er, 1953, 1954), and shifts according to acclimation (Fry and Hart, 

 1948a). When maximum swimming speed is measured at different 

 temperatures, the optimal temperature rises (Fry and Hart, 1948a), 

 and the temperature at which active swimming stops is higher 

 (Roots, 1961) as the acclimation temperature rises. 



The O consumption measured in maximum swimming activity 

 rises with temperature more rapidly over a low temperature range 

 and then more slowly than does the standard or rest metabolism 

 (Figure 6). The difference between the two curves (active and stand- 

 ard) for fully acclimated fish is considered a measure of extra en- 

 ergy available for swimming, the "scope of activity" of Fry. This 

 difference curve or scope of activity rises to a maximum in lake 

 trout (Salvelinus) at a temperature close to that of maximum cruising 

 speed (Gibson and Fry, 1954), and it has been suggested that the 

 maximum motor activity is determined by the energy available to 



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