350 Comparative Animal Physiology 



at 16.6° averaged only 3.3 per cent of mammalian heat production.^ '^'^ Turtles 

 in water are at the same temperature as the water (data from Baldwin'^ 

 excepted), and an intact turtle cools or warms faster than a pithed one, 

 because of distribution of heat by the blood.^-'" Frogs have less mass than 

 turtles, and, when dead, frogs cool or warm faster than when alive. 



Thermal Reception and Orientation. Aquatic poikilotherms cannot freeze 

 so long as they remain in water. However, certain temperature ranges appear 

 to be most favorable for their growth and maintenance, and in a temperature 

 gradient they tend to select or aggregate in a temperature range which is 

 "optimal" or nearly "optimal." A behavior response implies that the organism 

 senses an environmental difference and that some sort of orientation occurs. 

 The character of the taxic responses (orientation) to thermal stimuli has been 

 discussed by von Buddenbrock-" and by Fraenkel and Gunn.*'^ 



The classic experiments of Jennings'^^ showed that ciliate protozoans tend 

 to aggregate in a region of thermal neutrality, avoiding extremes of cold and 

 heat. When a ciliate enters a region above or below thermal neutrality, it 

 backs and turns until it arrives in the "preferred" range, where specimens tend 

 to collect. This is well shown by hypotrichs such as Oxytricha, which give a 

 characteristic "shock" reaction when stimulated. There is no direct orientation, 

 but rather selection by trial and error. 



Leeches which normally suck warm blood (Hiriido inedicinalis) are positive 

 to warm objects, responding to differences of 3 degrees above the surface water 

 temperature; all other leeches fail to show this temperature response. •" 



Water temperature is important in determining the distribution of many 

 fish, although it is often difficult to decide between temperature per se and 

 oxygen and CO2 content as being crucial in natural waters. In a gradient 

 tank many fish, by a combination of locomotor variables, tend to aggregate. 

 Herring are said to show good aggregation reactions in water temperature 

 gradients of 0.5° C. and to be able to detect gradients as small as 0.2° C.^^*' 

 The temperature selected depends on the acclimatization temperature and the 

 rate of transfer from one temperature to another or the rate of rise or fall of 

 temperature, and it is not necessarily the same as the "normal" temperature; 

 hence the term "optimal temperature" has little meaning in gradient behavior. 

 This is well shown by the following data"*^ for the fish Girella: 



Acclimatization Selected 



Temperature Temperature 



10° C. 18° C. 



20 23.6 



30 24.3 



Several kinds of temperature reaction and diverse receptors sensitive to 

 tem|)erature have been described in fish. When the temperature of any of 

 several kinds of fish is raised rapidly to some specific value (27.6° for catfish, 

 26.7° for Ihindulus), a sudden flutter-type reaction occurs; this reaction is said 

 not to occur if the lateral line nerves are cut.'"' The "spontaneous" activity 

 of tiic neuromasts in the lateral line organ increases very much with rising 

 temperature; lience tlie fish brain must receive lateral line impulses correspond- 

 ing to temperature.'^'' In elasmobranchs there are large sensory bulbs on the 

 surface of the head, the ampullae of Lorenzini, which have much in common 



