POIKILOTHERMIC ADAPTATIONS 



Tolerance of sudden temperature stress, called "resistance 

 acclimation" by Precht, is also modified. This is shown by shifts of 

 both high lethal and low lethal temperatures according to acclimation 

 (Fry et al., 1946; Fry, 1947). The curves describing rise or fall of 

 the two lethal temperatures as a function of acclimation need not be 

 parallel, and the area enclosed by both curves, the tolerance zone, 

 is species specific. The relation of temperature tolerance or resist- 

 ance acclimation to capacity acclimation is not known, and further 

 knowledge of heat and cold death might indicate which processes are 

 altered. Stress tests provide a useful tool for analysis of acclima- 

 tion. 



Poikilothermic vertebrates differ from homeotherms in that 

 they tend by compensation to maintain relatively similar activity 

 when body temperature changes, whereas the homeotherm maintains 

 constant temperature. There is no "comfort?' or thermoneutral zone 

 for the poikilotherm as long as chill or heat coma are avoided. Effi- 

 ciency of feeding and general body activity increase in a non- linear 

 fashion up to some "optimal" temperature which may be only a few 

 degrees below the lethal point. Furthermore, there is no evidence in 

 poikilotherms for a sequence comparable to Selye's stress syndrome 

 of mammals. 



Whether or not hormones are involved in the enzymatic changes 

 of metabolic acclimation in poikilotherms is not known. Evidence 

 concerning thyroid participation in adaptation of fish is conflicting 

 (Hoar, 1959). A slight increase in height of thyroid epithelium at ele- 

 vated but not at reduced temperatures was reported for the minnow 

 (Phoxinus) (Harrington and Matty, 1954), and in trout the thyroid 

 shows signs of increased activity in the cold (Olivereau, 1955b). 

 However, no histological change was found in thyroids of catfish, 

 carp, tench, eel, Mugil, or Scyllium after acclimation in cold (7 C- 

 14° C) or warm (20 C-23.5 C) (Olivereau, 19 55a,b,c). Thiourea 

 treatment is said to eliminate metabolic differences between cold- 

 and warm- acclimated crucian carp (Garassiug (Suhrman, 19 55), but 

 thiourea increases the differences in Leuciscus (Auerbach, 1957), 

 Resistance to cold in long-day goldfish increases when thyroid hor- 

 mone is injected (Hoar, 1959). Thiourea increases cold resistance 

 of goldfish and decreases that of the crucian carp ( Carassius ) 

 (Precht, 1958). Iodine uptake by the thyroid is slightly increased by 



