THE ADAPTATION Of ANIMALS TO THE ENVIRONMENT 797 



shallow-sea forms, exposed to wave action, have hard, thicker shells. Even 

 the greatest depths are inhabited, for tube-dwelling worms have been 

 dredged from depths of 24,000 feet, and sea urchins, starfish, bryozoa and 

 brachiopods have been found at depths of 18,000 feet. 



360. Fresh-Water Life Zones 



Fresh-water habitats may be divided into standing water— lakes, 

 ponds and swamps— and running water— springs, creeks and rivers— 

 though of course each intergrades with the other. The biologic communi- 

 ties of fresh-water habitats are in general more familiar than the marine 

 ones and many of the animals used as specimens in zoology classes are 

 from fresh water— amebas and other protozoa, hydras, planarians, cray- 

 fish and frogs. 



A lake or other large body of standing water can be subdivided, 

 much as the zones of the ocean are distinguished, into the shallow water 

 near the shore— the littoral zone— the surface waters away from the shore 

 —the limnetic zone— and the deep waters under the limnetic zone. Some 

 aspects of the ecology of a fresh-water lake were discussed in section 

 326. The ecologic factors which may be limiting a fresh water habitat 

 are temperature, turbidity of the water, the amount of the current and 

 the concentration of oxygen, carbon dioxide and salts, especially phos- 

 phates and nitrates. The organisms of the fresh-water community may 

 also be subdivided into plankton, nekton and benthos. The most im- 

 portant animal members of the community are fish, insects and Crustacea 

 and the plant members are algae and aquatic seed plants. 



Fresh-water habitats change much more rapidly than other life 

 zones; ponds may become swamps and swamps become filled in and form 

 dry land in a few hundred years. Streams are constantly eroding their 

 banks and changing their course. Consequently the kinds of plants and 

 animals present may change markedly and show ecologic successions 

 analogous to those on land. The large lakes, such as the Great Lakes, 

 are relatively stable habitats and their populations of animals and plants 

 change much less rapidly. A large, deep lake will show vertical stratifica- 

 tion with marked differences in temperature, dissolved gases, light and 

 other factors. Particular species of fish and other animals are more or 

 less restricted to a certain range of depths. The deeper waters of many 

 lakes become almost depleted of oxygen during the summer In the 

 summer the top layer becomes much warmer than the water below and 

 the circulation of water is essentially restricted to the warm upper layer. 

 The increased activity of decomposer organisms in the lake depths ex- 

 hausts the supply of oxygen and the lack of circulation prevents its 

 renewal by the algae and other plants in the upper layers. 



The ecologic factors which are most important in limiting the dis- 

 tribution of animals in running water are the speed of the current the 

 degree to which basic nutrients can be obtained from the adjacent land 

 or from connected lakes, and the amount of oxygen present. Running 

 streams are in general well oxygenated and the --^.1^;-"^ J^ - 

 usually have a very low tolerance to reduced oxygen tension. The pollu- 



