GEOGRAPHIC DIVISIONS OF THE PELAGIAL 



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17° (Fig. 75). Ortmann 1 sets somewhat different boundaries on the 

 basis of the distribution of the fairly constant high temperatures in 

 which the warmth-limited, stenothermal tropical life can exist. His 

 boundary lines therefore bend far toward the equator along the west 

 coast of South America and of Africa on account of the cold Antarctic 

 currents and the upwelling of water from the cold ocean depths. As 

 shown in Fig. 75, the boundary lines of these two investigators differ 

 essentially in that according to Meisenheimer the tropical animal com- 



Fig. 75.— Limits of the warm water region: heavily dotted line according 

 to Ortmann, heavy broken line according to Meisenheimer; polar limits of 

 the transition zone, lightly dotted line after Ortmann, light broken line after 

 Meisenheimer. Distribution of reef corals vertically ruled, after Joubin. Up- 

 welling cold bottom water, heavy coast line. 



munities of the Atlantic and Indian oceans are united around South 

 Africa, whereas, according to Ortmann, they are entirely separated. 



Both the warm and the cold belts of the sea can be subdivided. 

 Marine animals appear to recognize an equatorial belt of water with 

 a temperature above 25° as distinct from cooler tropical waters lying 

 on either side with temperatures from 20 to 25°. Similarly there are 

 recognizably different animal communities in the cold-water areas 

 around the poles with temperatures below 10° as distinguished from 

 those of the less cold waters with temperatures between 10 and 15°. 

 Indeed, in the best-studied region, the north Atlantic, other subdivisions 

 can be made; for example, the animal communities of the cold waters 



