Survivorship and size selection for smaller shells in the freshwater snail Campeloma decisum.

Abstract

Selection is the driving force of evolutionary change. We studied selection on the freshwater snail Campeloma decisum. The purpose of this experimental study is to determine which phenotypic characters of C. decisum are selected for, and to explore the factors that lead to this selection. In order to examine survivorship, we collected both living and dead C. decisum along the shore of South Fishtail Bay in Douglas Lake, Cheboygan County, in northern lower Michigan. We took eight measurements of the shells, (overall width, aperture length, aperture width, distance from the last bar to the aperture, age, shell thickness, and number of whorls) of the live and dead snails, and we weighed shells of the dead snails. We performed several statistical analyses of the data, including F-tests, t-tests, and regressions to determine how the measurements were related and whether selection appeared to be occurring. The results of our statistical tests revealed that the living snails had consistently smaller average values for the shell measurements than the dead snails. We further discussed our results in the light of previous research on size-selection of snails by predators, environmental factors, and lifetime fitness.