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Distribution of the Marsh Periwinkle Littorina irrorata (Say) in a Virginia Salt Marsh

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Littorina irrorata varies over its geographic range in maximum size, preferred elevations relative to tidal datum planes, and in the type of vegetation it inhabits. On Wallops Island, Virginia, postlarvae of Littorina irrorata with shell lengths < 5 mm long live almost exclusively in dead, curled-up leaves of Spartina alterniflora at elevations near mean tide level, below elevations occupied by larger conspecifics. Snails longer than 5 mm in length increase in average size with decreasing elevation. This distribution is opposite to that found by Hamilton (1978) in a marsh in Florida. No difference was found in our study area in growth rate of marked snails at two different elevations, so the size-elevation gradient probably is not caused by differences in growth rate. Snails 15 to 19 mm long are more active when exposed to reduced salinities than snails > 21 mm long. The lowest salinities recorded in the marsh occurred at the highest elevations. This salinity effect, together with mortality from known size-selective predators, may account, at least in part, for the seaward increase in mean shell size.

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