II. 

 GENERAL ECOLOGY 



ECOLOGY now has such a wide significance and includes so many 

 diverse subjects that it is necessary to define what is embraced 

 under this term in this chapter. It is here made to include every- 

 thing that affects the animal either from external or internal sources, in- 

 cluding such subjects as the habitat, climate, altitude, chemical conditions 

 of the water body, pollution, parasites, relations in food chains, reproduc- 

 tion, development, etc. 



GENERAL ENVIRONMENTAL CONDITIONS 



Representatives of the family Planorbidae may be found in almost any 

 body of fresh water, from the largest lakes to the smallest pools. Streams 

 of all sizes harbor one or more species of the family. Certain groups, as 

 Menetus, Planorhula, Tropicorbis, and some Gyraulus, are abundant in 

 small pools which may become wholly or partially dry during diy seasons. 

 ]\Iany of the large forms, such as Helisoma campamdatum, Helisoma 

 corpulentum, Helisoma pilsbriji, and some varieties of Helisoma anceps, 

 prefer larger bodies of water like the larger lakes. Helisoma trivolvis is 

 common in small lakes or bays of larger lakes, in shallow areas where 

 the shore is bordered by Typha and sedge, where food in the form of algae 

 and other vegetation is abundant. In such habitats the snails may be 

 seen with the foot attached to the under side of the surface film, the shell 

 hanging downward, the animal busily eating such small organisms as may 

 lie on the surface. In Florida, members of the subgenus Seminolina live in 

 limestone pools of limited size. 



Almost all of the members of the family Planorbidae are littoral 

 animals and are not usually found in water deeper than fifteen feet, the 

 usual limit of rooted vegetation in water bodies. The maximum numbers 

 occur in shallow areas not exceeding six feet in depth. A few inhabit deeper 

 water and in the deep lakes of Europe and Asia, as lakes Geneva, Con- 

 stance, and Leman in Switzerland, Lake Balaton in Hungary, and Lake 

 Baikal in Siberia, they may occur in abysmal depths of from forty to 350 

 meters. In our own lakes, deep dredgings have produced no members of 

 the family Planorbidae. The family is on the whole a distinctly shallow- 

 water group. 



The Planorbidae are able to withstand unfavorable features of their 

 environment better than most groups of mollusks, owing to their ability to 

 breathe free air. Thus water of some alkalinity and salinity, as well as 

 water containing sewage, may be used by these animals successfully as 

 habitats. The Lymnaeas also share this ability to live under unfavorable 

 conditions. It has been observed, however, that chemicals and oil, also 

 wood wastes from factories, are inimical to the Planorbidae and Lym- 

 naeidae and none have been seen which could resist this deadly type of 

 habitat. Clear, cold mountain streams, especially if rapid, contain no 

 planorbids, and usually no other mollusks, as far as personal observations 

 have indicated. ]\Iany such in New England have been examined and a few 

 in Idaho and ^Montana. 



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