86 



Journal of the Kentucky Academy of Science 66(2) 



snails per square meter, with an estimated to- 

 tal of 2500-5000 individuals (Stine 1989). 



Wood snail shells are carried by some shell 

 dealers, but generally tropical landshells and 

 marine shells are more popular with collec- 

 tors. At least one U.S. population may have 

 been introduced to serve as a shell source 

 (Cowie and Robinson 2001). 



Wood snails also make easy to care for, if 

 unusual, pets. However, most land snails are 

 considered potential pest species by the 

 US DA, and there are restrictions even on 

 state-to-state transport of living snails (Dees 

 1970; Thompson 1996), dashing the hopes of 

 those in the exotic pet industiy hoping to 

 spark a nationwide snail craze. 



ECOLOGICAL AND GENETIC STUDIES 



The variety of shell colors seen among 

 banded wood snails has long fascinated natu- 

 ralists, and many papers catalogue diversity 

 within populations (Brooke 1897; Howe 1898; 

 Johnson 1928; Judd 1953). The genetics of 

 most color variations have been determined 

 via crossing studies (Cain et al. 1968). At least 

 five shell color loci are linked into a "super- 

 gene" (Jones et al. 1977). These control the 

 shell's base color and four banding features: 

 presence or absence, intensity of band and lip 

 color, whether bands are continuous or dotted, 

 and their spread (Jones et al. 1977). Four oth- 

 er unlinked loci also affect banding, with the 

 number of bands controlled by two, one con- 

 trolHng darkening along the length of the 

 bands, and one determining whether bands 

 are black or orange (Cain et al. 1968; Jones et 

 al. 1977). Epistasis between some loci also 

 plays a role (Jones et al. 1977). Considering 

 that there are no fewer than six alleles for base 

 color of the shell, and that banding is affected 

 by at least eight loci and 18 alleles (Jones et 

 al. 1977), it is not surprising that early workers 

 enumerated hundreds of shell varieties (Howe 

 1898). 



Researchers have wondered how such high 

 levels of variation are maintained. With long 

 distance gene flow often limited by the slow 

 spread of individuals, and many populations 

 founded by small numbers of snails, one 

 would expect to commonly see fixation of shell 

 morphs through loss of alleles. However, fixed 

 populations are rare. For example, a survey of 

 1000 French populations revealed only two 



that were monomorphic for shell coloration 

 (Murray 1964). In a similar survey of 3000 

 British populations, fewer than 20 were mono- 

 morphic (Jones et al. 1977). Two factors are 

 thought to play a crucial role in maintaining 

 this diversity. Because these snails are her- 

 maphrodites, mating is possible between any 

 two individuals, increasing the potential allele 

 combinations available to offspring (Murray 

 1964). Also, wood snails generally mate at least 

 twice prior to laying eggs, and can store sperm 

 from multiple matings, effectively increasing 

 the population size (Murray 1964). Thus, even 

 small populations of snails may harbor more 

 genetic diversity than would be seen in other 

 types of organisms. 



Founder effects do have an impact on di- 

 versity, though, especially in U.S. populations, 

 most of which arose from introductions of 

 small numbers of individuals (Brussard 1975). 

 A study scoring shell polymorphisms and nine 

 isozyme loci showed that the major differenc- 

 es between U.S. populations seemed to be 

 based on which part of Europe the snails had 

 been introduced from, rather than the envi- 

 ronment they were currently in (Brussard 

 1975). Later isozyme studies have also sup- 

 ported the founder effect as having a major 

 impact on the genetic variation within U.S. 

 populations (Selander and Foltz 1981). 



Climate also has great influence on the di- 

 versity of shell colors. Wood snails, commonly 

 found in cool temperate climates, are sensitive 

 to overheating (Arnold 1969; Jones et al. 

 1977). One study of populations on sand 

 dunes found a disproportionate number of 

 brown and pink shelled individuals dying from 

 heat shock (Jones et al. 1977). Cfimatic selec- 

 tion is thought to play a major role in large- 

 scale patterns of shell color, with pale shells 

 being selected for in hotter climates (Jones et 

 al. 1977). Indeed, there is a cline for shefl col- 

 or across Europe, and in the hottest parts of 

 their European range, yellow shelled wood 

 snafls are the most common type (Jones et al. 

 1977). Additionally, observation of shells dug 

 from archaeological sites in England shows 

 that, historically, brown shells were more com- 

 mon during periods with colder climates 

 (Jones et al. 1977). However, interpretation of 

 the interplay between climate and color is 

 complicated by the fact that small scale envi- 

 ronmental conditions may also have an effect 



