24 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487 



among the four species that may be characterized as terrestrial {Suoainea retusa, 

 Caryehium exile, Helicodiscus parallelus > and Vertigo modesta) , only S. retusa 

 is represented in more than one faunal assemblage, and by more than one shell. 

 In short, the molluscan species in the assemblages under study here are over- 

 whelmingly aquatic in habitat requirements. 



Unfortunately, little is known specifically about the habitat requirements 

 of freshwater mollusks . As long ago as 1932 Morrison studied the molluscan 

 faunas in the Northeastern Wisconsin Lake District and noted the hydrogen ion 

 concentration (pH) and dissolved carbonate in the waters of the various lakes; 

 the latter datum he reported in parts per million. Subsequently Morrison has 

 been widely quoted on the assumption that his observations establish the limits, 

 in terms of pH and dissolved carbonates, of the molluscan species reported. 

 In point of fact, his observations can be considered no more than what they 

 were — observations. It is probable that the lakes do extend beyond the limits 

 of tolerance as far as acidity is concerned, but no limits were even hinted at on 

 the alkaline end of the scale, and no laboratory experiments were carried out to 

 determine tolerance limits. Mollusks which have shells require carbonate for 

 their development; these animals obtain carbonate directly from ambient waters 

 or from plants which they consume. The range of pH in which freshwater mollusks 

 may live and reproduce is extremely great; Hunter (1964, p. 87) reports that four 

 species of freshwater pulmonates that transmit schistosomes were shown by 

 laboratory experiments to be able to breed in waters varying from pH 4.8 to pH 

 9.8. Hunter found (1964, p. 86) that while pH may be a limiting factor in the 

 distribution of freshwater mollusks, "... the most important chemical factor 

 is dissolved calcium, varying more than a hundred-fold in fresh waters with 

 mollusks. " 



Morrison points out that, normally, thin-shelled unionid mussels in 

 waters with low pH and low "fixed carbon dioxide" have shells so flexible that 

 they may be distorted by twisting without breaking (1932). Waters with less 

 than 3 mg/liter of calcium support very few mollusks; in Britain such waters 

 were observed to support a species of Lymnaea, one of Ancylis (Ferrissia) , 

 and several species of Pisidiwn. Waters having 8 to 10 mg/liter of calcium 

 support up to 17 molluscan species, while an Irish lake having about 50 mg/liter 

 of calcium supports 32 species (Hunter, 1964, p. 86). Boycott (1936) reports 

 that of 62 species of freshwater mollusks in Britain, 26 can live in waters con- 

 taining less than 10 mg/liter of calcium and 6 occupy lakes and streams having 

 more than 10 mg but less than 20 mg/liter, while the remainder required waters 

 having at least 20 mg/liter of calcium. 



Temperature is not likely to have been a problem at the two bog lakes 

 under investigation, as it is well known that mollusks can survive wide ranges 

 of temperature. Odhner (1923) in a report on Novaya Zemlya, an island off 

 northeastern Siberia well above 70° N latitude, found Pisidiwn oonventus active 

 in the brief summers, where winter temperatures are extremely severe. Lymnaea 

 peregralives in thermal waters at 45° C in the Pyrenees, while the same species 

 has been observed active under ice in lakes in Scotland (Hunter, 1964, p. 86). 

 Even from these brief observations and from the modern distribution in North 

 America of the species listed in figure 3, it can be concluded that temperature 

 could not have been an important factor in the survival of the species recovered 

 from the two bogs. 



From available evidence, it is probable that the trophic state of the two 

 bog lakes was favorable to the growth of mollusks; the accumulation of organic 



