POIKILOTHERMIC AD/^PTATIONS 



identifying changes in enzyme activity is to observe effects of in- 

 hibtors; yet these are not nearly so specific as desired. Another 

 method is to purify enzymes, but extraction precedures are often un- 

 certain as to recovery or loss of activity. A common method is to 

 supply an excess of a specific substrate so that the enzyme acting on 

 it is made limiting; this provides a useful comparison between sys- 

 tems treated differently (as by temperature) , but it does not tell 

 much about limiting steps in vivo . Tracing labelled substrates is in- 

 formative and has not often been used in acclimation biochemistry, 

 although it has indicated a general similarity of metabolic paths in 

 fish and mammals (Brown, 1960; Brown and Tappel, 1959; Martin 

 and Tarr, 1961). Useful information can be obtained from kinetic 

 studies of both intact and dissected systems. 



A serious problem, especially with poikilotherms, is the identi- 

 fication of appropriate environmental variables. Three factors, tem- 

 perature, nutrition, and photoperiod, interact in an inextricable way. 

 Many fish and amphibia eat little in the cold, and it has been common 

 practice to observe acclimation in starved animals. Unfortunately a 

 fish starved at 25 C is not comparable in its food reserves to one 

 starved at 5 G. Also if each is fed ad libitum , the absorption of food 

 may be so slow in the cold that the nutritional state is different from 

 that of one fed at 25 C. We have evidence that the metabolic differ- 

 ences are greater in starved than in fed goldfish kept at low and high 

 temperatures. Various methods, such as feeding followed by cross 

 acclimation so that the total time spent at the two temperatures is 

 the same for both groups, have been used in an effort to approach 

 nutritional equivalence, but no method is fully satisfactory. 



Photoperiod has marked metabolic effect in fish and amphibians. 

 Ekberg (1961) found a greater difference between enzymes from cold 

 and warm acclimated fish on a 17-hour than on a 7-hour photoperiod; 

 he also found a marked seasonal difference in the metabolic response 

 of goldfish gills. Roberts (1961) observed a photoperiod effect 

 on Carassius carassius at 20 C but not at lower temperatures. 

 Hoar (1955; Hoar and Robertson, 1959) observed seasonal dif- 

 ferences in temperature tolerance and in oxygen consumption 

 by goldfish even when acclimated at the same temperature; 

 these seasonal effects reflect photoperiod and may be associated 

 with enhanced thyroid activity on short photoperiod. Frogs show 

 marked seasonal differences in many of their physiological proper- 



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