328 ANIMALS IN INLAND WATERS 



of the water likewise occurs more rapidly in the shallow shore waters, 

 for the cooled surface waters cannot set up a circulation like that of 

 open water. For this reason ice first forms along the shores in winter. 



The breaking of the waves against the shore is of importance not 

 only in killing organisms but also in grinding up their excrements and 

 remains, providing thus for the development of detritus and the 

 occasional stirring up of these nutritive materials. This is especially 

 advantageous to the numerous detritus feeders and furthers their 

 development. Fishes congregate where the bottom food supply (e.g., 

 Tubificidae, mussels, and gnat larvae) is plentiful, and experience in 

 fishing shows that exposed shores furnish a richer catch of fishes than 

 shores protected from wind. 14 A region of strong breakers, however, 

 is fatal to many forms of life; on plantless shores where strong waves 

 heat when the wind is high, rocks and pebbles are tossed about, and 

 in winter, icefloes are dashed against the banks, crushing any in- 

 habitants. 



Most harmful for the fauna of a littoral region, however, are the 

 regular variations in the water line in many lakes. This is especially 

 true in mountain lakes where the periodic increase and decrease in 

 size of tributaries from glaciers or melting snow cause a marked rise 

 and fall of water level, in Lake Constance a yearly average variation 

 of 2.2 in., with a maximum of more than 3 m. A similar condition 

 occurs in the lakes of the plains. Even in the Great Lakes there are 

 extensive variations in lake level both from temporary seiches and 

 over periods of years. 



Only that part of the shore fauna which can resist or evade the 

 unfavorable conditions can live in such parts of the littoral zone as 

 are dry a part of the year: animals with powers of locomotion, which 

 can follow the fluctuations, such as Crustacea, insects, water mites; 

 animals which can protect themselves from desiccation by encysting, 

 like many Protozoa; or which can stand drying (threadworms, tardi- 

 grades, many rotifers and copepods) ; or, finally, animals of amphibian 

 habits, such as frogs and many snails. Sessile or scarcely mobile ani- 

 mals, such as sponges, Bryozoa, and mussels, are absent. 



In the region of profuse plant growth, in and among the leaves and 

 on the bottom, are found a diverse and important group of plant and 

 detritus feeders, and the animals which prey upon them. Protozoans 

 and flatworms, leeches, annelids and rotifers, live here in abundance, 

 being especially plentiful where the growth of Myriophyllum is thick- 

 est. 15 With them live a host of Entomostraca, partly in the mud (Fig. 

 95), partly among the leaves of plants, along with isopods and amphi- 

 pods. A large number of snails and mussels also live there. Above 



