710 COX 



shock resistance of either postlarval or juvenile spot, Leiostomus 

 xanthurus. Since their intent was to simulate the effect of passage 

 through a power-plant cooling system, latent mortality 96 hr after a 

 20-min exposure to test temperature was used as the measure of the 

 effect of temperature acclimation to constant and fluctuating 

 temperatures, and the maximum variation of the cycles tested was 

 ±4°C. Hoss et al. (1975) reported that the heat resistance of 

 postlarval pinfish was unaffected by acclimation to 8 to 12° C cycles 

 but was increased by 7 to 13° C cycles. Otto (1974) also found that 

 acclimation to cyclic regimes resulted in CTMax values consistently 

 above the base temperature, but it was always less than the CTMax 

 for fish acclimated to the constant temperature equivalent to the 

 peak of the cycle. The generalization may be affected by the 

 methods used. The critical thermal maximum or minimum or CTM 

 (Cowles and Bogert, 1944; Lowe and Vance, 1955; Lowe and Heath, 

 1969; Hutchison, 1961) and resistance time to death (Fry, Hart, and 

 Walker, 1946; Brett, 1952) are methods more commonly used to 

 measure the effect of temperature compensation. The results of the 

 differing methods used in these studies may not be directly 

 comparable, however, and the extent of the cyclic temperature range 

 may also be important. 



Mechanisms have been proposed to explain increased acclimation 

 in fluctuating regimes. Brett (1944) suggested a series of incremental 

 gains in temperature tolerance during each rise in daily temperature 

 which was not entirely lost when the waters cooled at night. If 

 summation of acclimation responses to the high side of the 

 temperature fluctuation occurs over a sufficient period of time, the 

 eventual gain in heat tolerance should approximate the constant 

 temperature corresponding to the maximum of the cycle. The 

 acclimation to the cycling temperature regimes occurring for more 

 than 2 weeks during both phases of this study may have been 

 insufficient to produce a response equal to the maximum tempera- 

 ture of the cycle. This possibility suggests an unusually slow 

 summation of responses, some other mechanism modifying or acting 

 independently of summation responses, or an inherent "equilibrium" 

 acclimation less than the maximum. 



The simple summation mechanism is not adequate to explain all 

 observations. Hutchison and Ferrance (1970) observed that frogs 

 acclimate to the maximum of a daily temperature cycle after 

 exposure to a single 24-hr (15 to 25°C) cycle. Ninety-six hours of 

 acclimation to a constant 25° C was required to achieve a comparable 

 response. The acclimation responses of invertebrates to various 

 constant and fluctuating temperature regimes are considerably less 



