276 



POPULATIONS OF THE SEA 



Divisions 

 per 24 hours 



0.7 - 



0.6 

 0.5 



0.4 - 



0.3 



0.2 - 



0.1 - 



4 %o 



Fig. 4. Hymefionwuas carterae: growth rate at different salinities (Braarud. 

 1951). 



essentially similar features in their response to salinity: Low 

 growth rates at 10 %o and a quick rise to 15-20 %c, with a slow 

 decline when salinity rises to 40 %o. 



For clones of the same species originating in widely separated 

 localities, there is one observation, again of Peridiniiim trochoidenm. 

 The salinity-growth relationship has been studied in clones from 

 the Oslofjord, where the salinity during its abundant occurrence 

 in summer is about 20 %o, and from Golfo di Napoli, where salinity 

 is as high as 37 %o. These clones gave practically identical curves 

 with an optimum of salinity as low as 20 %c- These last results indi- 

 cate a uniform salinity response in populations inhabiting waters 

 with widely different salinity conditions, thus giving no hint of 

 geographical races in this species, which has a worldwide distribu- 

 tion. 



