CHAPTER XV 



GEOGRAPHIC DIVISIONS OF THE PELAGIC 

 COMMUNITIES OF THE SEA 



Unlike many of the biotic divisions of the land, the animal com- 

 munities of the sea are not separated into irregularly placed subdivi- 

 sions but rather extend as broad zones. Although physiographic divisions 

 can be recognized, these separate the animal communities of the lesser 

 seas, gulfs, and bays from the oceans proper instead of coinciding 

 with the geographic divisions between the oceans. In these smaller 

 bodies of water such as the Mediterranean, Baltic, or Black seas, there 

 are distinct peculiarities in temperature, especially in gradation with 

 depth, in salt content, and in the accumulation of chemicals in the 

 water, which condition the development of recognizable animal com- 

 munities. These are the more distinct when such land-locked seas 

 are connected with the ocean proper only through narrow and shallow 

 straits such as the straits of Gibraltar. Within the oceans similar differ- 

 ences may occur in the littoral regions, especially near the mouths of 

 great rivers such as the Amazon or the Congo. The geographic dis- 

 tribution of the animal communities of the oceanic pelagial are, in 

 the final analysis, determined primarily by temperature. In littoral 

 areas the relationships are frequently complicated by depth, type of 

 bottom, presence of fresh water, and similar factors. 



The major animal communities of the sea are the most extensive of 

 life zones; they may be compared to the climatic zones of the con- 

 tinents, but far surpass them in area and in homogeneity of environ- 

 mental factors and of associated biota. The distinctions to be made 

 are extremely simple and consist most plainly in a subdivision of the 

 animal communities of the ocean into those of warm and of cold 

 waters, very roughly corresponding with the tropical and subtropical 

 areas on the one hand and the cooler waters on the other. 



The boundaries between these vast areas are not determined by 

 latitude, and run entirely independent of the conventional tropics of 

 Capricorn and Cancer. They are much more closely correlated with the 

 isotherms of the surface waters; in fact, Meisenheimer's divisions based 

 primarily on the limits of the tropical pteropods coincide in the north 

 Atlantic with the 15° isotherm and in the south Atlantic with that of 



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