PRITCIIARD, HUNTER, and LASKER: EXERCISE AND BIOCHEMICAL CHANGES 



dent locomotor activity of tlie red muscle cannot 

 be ignored. At some speed slower than any 

 used in the present experiment jack mackerel 

 may depend only on red muscle for propulsion 

 and on lipids for fuel. At what velocity red 

 muscle begins to play a major role or how sig- 

 nificant this speed may be in the life of the an- 

 imal are questions that remain to be answered. 

 The most tenable explanation for these data is 

 that both muscle systems were used at the sub- 

 threshold speed but we are unable to choose 

 which system played the more significant role. 

 Jack mackerel appear to be specialized in 

 body form and swimming capabilities for high- 

 speed continuous swimming (Hunter, 1971). 

 Thus the physiological characteristics we have 

 described, namely use of glycolysis in white mus- 

 cle for swimming, high liver glycogen levels, 

 and tolerance of high muscle lactate levels may 

 represent specializations for high-speed swim- 

 ming and may not be representative of the gen- 

 eral pattern in fishes. On the other hand, 

 Tmchiirits may share these characteristics with 

 other fishes of similar habits, for example other 

 carangids and the scombroid fishes. It seems 

 possible that evolution may have favored the 

 development of these physiological character- 

 istics because severe velocity limits may be set 

 by aerobic lipid metabolism. 



ACKNOWLEDGMENTS 



We thank Messrs. David Holts, William Rom- 

 mel, and Andrew Kuljis for their technical as- 

 sistance during this study. 



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