CHAPTER XIX 

 COMMUNITIES IN OTHER INLAND WATERS 



Tropical fresh-water communities. — The discussion of animal 

 distribution in fresh waters has dealt thus far mostly with those of 

 the temperate zone, because these waters have been most studied. 

 The faunae of tropical and polar fresh waters differ from those of the 

 temperate regions in many ways. 



The relatively constant high temperature throughout the year is 

 the outstanding difference between the fresh waters of the tropics and 

 those of other zones. The annual variations in temperature are negligi- 

 ble, excepting in shallow basins of standing water. The difference 

 between the temperature at the surface and at the bottom in tropical 

 lakes of greater depth is also much less than in temperate lakes; 

 in Lake Nyassa the temperature in December measured 27.6°-29.7° 

 in the surface layer, and 22.75° at a depth of 193 m. The distribution 

 of oxygen in deeper lakes has not been investigated, but in swamps 

 of the South American Chaco, the general lack of oxygen is an im- 

 portant biological factor. 1 



The fauna of the tropical and subtropical fresh waters is more 

 diversified than that of the temperate or arctic zones, and the number 

 of animal groups represented is larger. This is explained by the fact 

 that stenothermal animals adapted to the warm regions find their real 

 home there, and though the eurythermal animals are not excluded, 

 many genera of snails and mussels are limited to tropical waters. A 

 few have remained in the waters of the temperate zone as relicts of 

 former warmer eras; thus 2 species of the snail genus Melanopsis are 

 found in the warm springs of the Maremme in Tuscany and in the 

 thermal waters (32°) in Hungary, 2 and 1 species of Melanin occurs 

 in the brooks and rivers of the Balkan Peninsula. 



Cyclops prasijius, which is found occasionally in Germany, oc- 

 curs everywhere in Africa, while C. strenuus, which is adapted to 

 colder temperatures, is missing there. Dolphins, Sirenia, hippopotami, 

 and crocodiles may be mentioned as amphibious vertebrates charac- 

 teristic of tropical fresh waters. The diversity of fishes in the tropical 

 waters is exceedingly great: 748 species have been reported from the 

 Amazon, nearly 400 from the Congo, 192 from the Nile, 170 from 



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