The Effects of Temperature on Metabolism of the American Eel Anguilla rostrata (LeSueur): Compensation in the Summer and Torpor in the Winter

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Abstract
At acclimation temperatures between 10 and 20 C (typical of native summer temperatures), American eels feed regularly and display the compensatory adjustments in oxygen consumption rates which are characteristic of many ectothermic species (i.e., ). However, eels acclimated to temperatures typical of winter months (i.e., ≤ 5 C) for periods in excess of 5 wk cease feeding and display a dramatic decrease in oxygen consumption rates ( between 5- and 10 C-acclimated eels = 4.10) indicative of a torpid metabolic state. Studies of oxygen consumption rates after acute transfer of eels from higher to lower temperatures indicate that (1) compensation to 10 and 15 C requires a minimum of 16-20 h, and (2) that respiratory rates typical of the torpid state occur within the first 1-2-h posttransfer. The low temperature torpid state is accompanied by no significant increases in enzyme activities above levels of 15 C-acclimated fed eels, and several examples of decreases were noted. Although a 6-mo fast at 10 or 15 C results in substantial decreases in lipid stores, enzyme activities, and hepatosomatic index, eels fasted at 5 C have metabolic reserves and enzyme activities nearly identical with those of the 15 C-acclimated fed eels. Possible molecular mechanisms of torpor and the ecological implications of torpor and fasting are discussed.