364 ANIMALS IN INLAND WATERS 



alpine foothills. A water temperature above 10° is rare and is usually 

 found only in very shallow water basins. The valley basins which 

 are not exposed to the noonday sun, and are fed only by the waters 

 from near-by ice and snow, present especially unfavorable conditions. 

 They differ from conditions in polar regions in that the period of 

 illumination and of sunshine, of such importance to life, is shorter. 

 It is perhaps for this reason that such large animals as the Euphyl- 

 lopoda are missing in the mountains of central Europe in contrast with 

 those in Scandinavia. 



The abundance of life in high mountains varies greatly. Basins 

 which are open, contain plant growth, and have a sandy and gravelly 

 substratum, are most densely inhabited. In secluded cold glacial basins 

 with bottoms of coarse rubble the fauna is sparse. A few species of 

 rhizopods and tardigrades are all that are found in such waters, or they 

 predominate even when certain rotifers, copepods, and water mites 

 appear in addition. The cladocerans require relatively favorable con- 

 ditions; some insect larvae, water beetles, and bivalves {Pisidium) 

 also appear in the sparsely populated basins. 



All these animals must be cold-tolerant. They are either ubiquitous 

 eurythermal forms (many rhizopods, tardigrades, Limnaea trunca- 

 tula) or stenothermal cold-water forms (Cyclops strenuus, Diaptomus 

 laciniatus) . Some live actively under the winter covering of ice while 

 others hibernate. Many species are dwarfed, e.g., certain copepods, 

 Pisidium, Limnaea truncatula. Reproduction occurs in midsummer or 

 autumn, even among species which have their reproductive seasons 

 during the winter and spring in the waters of the plains.* The number 

 of generations is decreased; the Cladocera usually have only one 

 generation. In favorable summers, on the other hand, their fertility 

 is increased in comparison with that of their relatives in the plains. 



The difference between the inhabitants of the limnetic and the 

 pedonic areas which is quite noticeable in the deeper lakes on the 

 plains, as well as the difference in the conditions of temperature of 

 these two biotopes, 5 is blotted out. Animal species which are pro- 

 nounced deep-water forms in the lakes of the alpine foothills and the 

 plains can live in the limnetic region here.f The composition of the 

 plankton is rather monotonous throughout the whole region of the 

 high Alps. Zschokke mentioned only 55 species of plankton animals: 



* Hydra jusca, Planaria alpina, and Cyclops strenuus. 



t The typical deep-water rhizopods of the alpine border lakes such as Difflugia 

 mammillaris and others, the unusual flatworms, Plagiostomum lemani and Oto- 

 mcsostoma auditivum (see p. 333), or the water mites, Lebertia rufipes and 

 Hygrobates albinus. 



