RESPIRATION 



163 



over wide variations in 2 tension, showing the adequacy of diffusion for 

 supplying 2 requirements. The oxygen consumption of some animals 

 continues to rise at tensions above normal atmospheric pressure, e.g. 

 Actinia, Limulus, suggesting that the tissues are unsaturated with 2 under 

 normal conditions. 



Relative Independence of External 2 Tensions. In contrast are those 

 animals in which the respiratory rate remains fairly steady over a wide 

 range of gaseous tensions. Such ability is relative, and critical tensions 



w F 



o 



\ 



c; 

 .o 



6 



<o 

 c; 

 o 



c; 



>< 



o 



C.C l 



wo 



inm* 1 



10 100 



Wet weight 



Fig. 4.13. Oxygen Consumption of Tropical and Arctic 

 Crustaceans at their Normal Habitat Temperatures 



The tropical forms extrapolated down to C C on the basis of Krogh's standard 

 temperature curve would fall along the dotted line, corresponding to a rate 30-40 times 

 lower than at 30°C. Arctic forms show appreciable adaptation in that their metabolic 

 rates are only 4-10 times lower than tropical forms. (Double log plot, regression lines 

 correspond to a weight/rate exponent of 0-85.) (From Scholander et ah (142).) 



below which independence ceases differ greatly from one animal to 

 another. Some examples of critical 2 tensions for various animals at a 

 temperature of 20°C are as follows (mm Hg) — 



Arbacia eggs 20 



Loligo pealei 45 



Palaemonetes vulgaris 80 



Fundulus heteroclitus 16 



Tetraodon macula tus 100 



Stenotomus chrysops 30 



Even fairly closely related animals show great differences in this respect, 

 Callinectes, for example, being unable to regulate whereas Palaeomonetes 



