Effects of current and distance from headwaters on shell size of the snail Physella gyrina.

Abstract

The effect of current and distance from the headwaters of a stream on the obesity of snail shells was studied by comparing the size index of snails in the Maple River system of Emmet County, Michigan. Physella gyrina was collected from four different locations. Two sites were on the main branch of the Maple River at Maple River Road and Brutus Road, one site was on the west branch at Robinson Road, and one site was on the east branch at Douglas Lake Road. The shell obesity index was calculated by dividing the width of the shell at its widest diameter by the length (Goodrich 1937). The expected results were that shell obesity would increase in a downstream direction and would increase with velocity. The only significant results from this study was that the shell obesity decreased between two sites and this correlated with increasing current velocity, which did not support the expected results. The results of this study were inconclusive because only small numbers of snails were available from areas with measurable currents and because the stretch of river studied was relatively short. Goodrich's 1937 study involved hundreds of snails collected from the entire Alabama River system, and it would seem that only by testing snails from measurable currents in a large river system that one could draw any conclusions about the relationship of current, shell size, and postion in the length of the river.