The Common Snail Melampus bidentatus Occurs Throughout the Salt Marsh in Its Northern Range

The common marsh snail Melampus bidentatus is an omnivore-detritivore that is typically restricted to the upper marsh zone in salt marshes of southern New England and further south. However, in Maritime Canadian salt marshes (specifically in the Northumberland Strait), M. bidentatus occurs throughout the high and low marsh zones (mean summer densities ~ 50 individuals m⁻²). This study determined the within-marsh distribution of M. bidentatus near its northern range limit and investigated the mechanisms responsible for this distribution. Intensive spatial and temporal sampling in 2015–2016 confirmed that all stages (adults, juveniles, and egg masses) occurred throughout the salt marsh. Investigations of snail survival (using tethering assays) and movement (using mark-recapture trials) indicated that mortality was very low and independent of marsh zone, and movement was moderate, random, and circuitous, generally maintaining snails in local areas. Thus, lack of differential survival and movement between marsh zones support an unrestricted distribution. This wide spatial distribution of M. bidentatus within salt marshes in north temperate latitudes is likely due to the species’ high physiological tolerances, absence of competition from other gastropod omnivores-detritivores, and low predation pressure in the low marsh zone. Given these findings, further research on the snails’ role in, for example, trophic dynamics would provide further insights as to latitudinal differences in the ecology of resident salt marsh fauna.