C. L. PROSSER 



353 



fornia, limpets (Acmaea limatula) from the high intertidal zone 

 have consistently lower heart rates at a given temperature than 

 those from the low intertidal zone (Segal et oi, 1953). Mytilus 

 show a similar vertical variation in pumping rate; 75 cm. ver- 

 tically are equivalent in pumping to 330 miles latitudinally when 

 tested at 16°C. (Rao, 1953). By transplantation of limpets and 

 Mytilus in the vertical gradient, complete acclimation was 



100 

 80 

 60 



^?30 



c 

 a. E 



E 



20 



10 



48° N. 



-O 39° N. 



34° N. 



JL 



_L 



1 



J 



6 8 10 12 14 

 Temperature (°C.) 



16 18 20 



Fig. 3. Rate of pumping of water by 50-g. specimens of Mijtilus cali- 

 fornianus as a function of temperature for populations collected at three 

 latitudes of western North America. From Rullock (1955); redrawn from 

 Rao (1953). 



demonstrated within a few weeks. Latitudinal transplantation 

 experiments have not been reported, but it is likely that many 

 of the latitudinal effects with respect to temperature are pheno- 

 typic. 



Racial differences have been invoked to explain latitudinal 

 differences in reproductive rates; poikilothermic animals from 

 cold waters generally develop more rapidly than those from 

 warm waters when tested at the same temperature, especially in 

 the lower range. A low-temperature race of Daphnia atkinsoni 

 is found in pools in England where the breeding season is re- 



