or those living in temporary bodies of water the problem of desiccation 

 is paramount. 



The desert environment is not a particularly favorable one for mol- 

 luscan life. One who has searched for desert mollusks is not so much 

 impressed by their paucity as by the fact that they occur there at all. 



The problem of desiccation is met by mechanisms that involve both 

 diurnal and seasonal behavior patterns. A knowledge of these patterns 

 will materially aid the collector. For example, pulmonate land snails are 

 normally active on the surface of the ground only during the early morning 

 and evening hours. During the warmer, drier portions of the day they 

 retreat to the lower layers of the litter or under logs and rocks where the 

 microclimate is more favorable. However, during stormy periods when the 

 humidity is high these snails may be found actively crawling over the 

 surface regardless of the time of day. 



During the extremely dry, hot periods of mid-summer these land snails 

 live in a temporary state of suspended animation termed aestivation. They 

 retract into their shell, frequently attaching themselves to a leaf or twig to 

 seal off the aperture. One or more membranous partitions (epiphragms) 

 may be formed between the animal and the aperture. In some species the 

 epiphragm may be so heavily impregnated with calcium carbonate that it 

 resembles somewhat the operculum of the prosobranchiate snails. 



Aquatic gastropods inhabiting temporary pools or intermittent streams 

 may survive periods in which the bed is dry by burrowing below the surface 

 into soil which retains some moisture. Aestivating Planorbis and Lymnaea 

 have been kept alive under laboratory conditions for more than 2 years, and 

 it is well established that some land snails are able to survive for long 

 periods, up to 6 years at least, in a state of suspended animation. 



The low temperatures of winter are avoided by a behavior pattern 

 (hibernation) similar to aestivation. The chief difference between the two 

 is that hibernation is more pronounced, the snails crawling to the lower- 

 most levels of the litter and sometimes even burrowing for some distance 

 into the ground before constructing the epiphragms. 



Salinity and alkalinity are other problems with which mollusks of 

 desert areas must cope, especially the aquatic forms. The collector will find 

 that not all of the relatively few aquatic habitats are occupied by mollusks, 

 presumably because of these problems. It is known that a variety of gastro- 

 pods inhabited the fresh waters of Pleistocene Lake Bonneville, but as it 

 receded and became saline the snails finally disappeared. No mollusks live 

 in its remnant, the Great Salt Lake, at the present time. Our knowledge of 

 the present aquatic gastropod fauna of the Bonneville Basin tends to indi- 

 cate that the prosobranchs have a slight advantage in the more arid portions 

 of this region. 



Finally, it should be remembered that mollusks, like all other organ- 

 isms, vary from kind to kind in their ability to resist unfavorable environ- 

 mental conditions. The molluscan fauna of desert regions consists, therefore, 



(74) 



