﻿SNAIL HOST OF ORIENTAL LUNG FLUKE — ABBOTT 103 



Bionomics of Guam colonies. — A natural sequence to the bionomic 

 study of the Lithia Spring populations was an investigation of the 

 same species living on Guam Island to see if similar environmental 

 influences on size existed in areas natural to this species. In making a 

 statistical study of the 13 colonies collected on Guam Island it was 

 found that two serious drawbacks existed that, indeed, are common 

 to any material of this nature. The first was that in most of the 

 colonies an insufficient number of preserved animals were available 

 to ascertain at what point maturit}^ was reached. Secondly, the speci- 

 mens were collected by hand and not in a strictly random fashion, as 

 would be the case had a sieve or hand net been employed. The natural 

 tendency of the collector is to choose the lai'gest specimens first, then 

 the smaller ones, until patience or time halts collecting. 



Despite these handicaps, it was felt that a reliable index of size 

 could be obtained by using the 10 percent of that part of the colony 

 that represents the largest specimens. This method would be open 

 to criticism were we dealing with animals which stopped growth at 

 maturity, and which possessed some distinctive morphological adult 

 character. When neither of these markers are present, as is the case 

 with this genus of mollusks, we seek the maximum growth to which 

 the snails grow and eliminate an abnormally low mean, owing to 

 high percentage of young. The choice of the 10 percent figure is 

 arbitrary, although, in population samples of 50 or more, the chances 

 are that this will include only adults. It is unwise to choose simply 

 the largest one or two specimens, for it is common knowledge that 

 abnormal giants or perhaps polyploids are apt to be present in any 

 population and will not serve as representatives. This is the reason 

 why, in smaller samples, of 20 or less, it is best to choose the 20 or 30 

 percent representing the largest specimens, for comparative purposes. 



What we are essentially seeking in a study of shell length is the 

 largest size to which individuals will grow in any one population. 

 Since growth is continuous throughout the life of the individual, two 

 factors will delimit the length of shell — genetic and environmental. 

 We have seen in our study of the Lithia Spring populations that some 

 environmental factor is most likely responsible for length of shell, 

 and it is not unreasonable to assume that such is the case in any dif- 

 ferences found on Guam. On this island the streams are close to each 

 other, and accidental dispersal by birds and other creatures is likely 

 to keep the chances of a lengthy genetic isolation very low. To compare 

 the 10 percent representing the largest specimens of each population 

 is, in essence, to compare the ecologic conditions of each stream. 



In order to test the validity of using the 10 percent referred to of 

 each colony, histogi'ams were made of the 13 colonies, and the means 

 of shell length were calculated for the total population and for the top 

 10 percent. Figure 43 is a summation of this study from colonies A to 



