POIKILOTHERMIC ADAPTATIONS 



the fish (Fry, 1947). The change after acclimation of the temperature 

 at which swimming is maximum is, according to this view, due to 

 compensatory metabolic alterations such as have been described a- 

 bove. Certainly no animal can move more rapidly than energy can 

 be made available to it. Fisher (1958) has questioned whether the 

 maximum metabolism per se determines the cruising speed or 

 whether the limit may be imposed in the nervous system. It is possi- 

 ble to increase oxygen consumption beyond that at maximum cruis- 

 ing speed by electrical stimulation (Basu, 1959). Some fish (trout) 

 show two temperature optima for cruising, and these can be altered 

 by brain lesions; swimming rate is affected by light intensity (Fish- 

 er, 1958). It appears, therefore,that available energy is not the only 

 limiting factor for activity. 



In addition to changes in the temperature preferendum and tem- 

 perature of maximum cruising speed with acclimation, or the tem- 

 perature of sudden reduction in swimming, other measurements in- 

 dicate adaptive alterations in the central nervous system. Conduction 

 in peripheral nerves isblockedbycold,and the critical temperature 

 for cold block declines with cold acclimation (Roots, 1961). Spinal 



reflex movement of the fins of goldfish was blocked at 10 C, 5 C, 

 o o o ' o 



and 1 C respectively for fish acclimated to 35 C, 25 Cj and 15 



o o 



C; the reflex persisted at below 1 C in fish acclimated to 5 C. 



Roots has conditioned fish to avoid either a light or dark end of a 



divided aquarium, and also to interrupt their breathing rhythm when 



given a visual stimulus. The cold- blocking temperatures of these 



conditioned reflexes is higher than for simple reflexes, e. g., block 



o o 



occurs at 15 C for 25 C- acclimated fish. Thus a hierarchy of tem- 

 perature sensitivity isfound, with midbrain functions most sensitive, 

 spinal functions less so, and peripheral nerve least sensitive to cold 

 (Roots, 1961). 



Similarly in two species of skate (Raja ) sensitivity of nerve and 

 muscle to heat decreases in the following series: myoneural junc- 

 tion, nerve conduction, striated muscle contraction, and heart and 

 gut muscle activity (Battle, 19 26). It is concluded that important a- 

 daptive changes occur in the nervous system during temperature ac- 

 climation. Changes in nervous function reflect chemical alterations 

 of excitable membranes and subtle changes in interneuronic inter- 

 action which are totally unknown. In the absence of the insulative 



25 



