Marsh Periwinkle

Introduction

Imagine you are walking out along a dock overlooking the Cape Fear River. There is a steady breeze that makes the marsh grasses sway as you pass. Looking out you, begin to notice these whiteish gray bulges appearing throughout the grasses. These abundant little mollusks are likely none other than the Marsh Periwinkle. These typically grayish white snails with red/brown streaks are abundant in salt marsh habitats and grown between 19 and 32 mm in height. They are often seen above the high waterline swaying and feeding on the stems of marsh cordgrass. Their shells are shaped like an elongated cone that is thick and broad with between 8 and 10 whorls (Munoz, 2005). The oval opening of the shell is a sharp outer lip with regular grooves. Sometimes these mollusks have a slightly green tinge which comes from algal growth on the outsides of the shell. Not much is known about the specific species reproduction, but other members of this genus reproduce by internal fertilization where the female lays the eggs and no other parental involvement occurs after (Munoz, 2005).

Evolutionary History

The Marsh Periwinkle has two distinct evolutionary behavioral adaptations. The first as discussed previously, is its ability to cultivate fungal growth on the Spiratina. This is done through a simple mechanical mechanism of opening up scars or cuts on the grasses to facilitate fungal growth (Silliman, 2017). Though the periwinkles do not show preference to feeding on the stems they have individually scarred, they have been observed playing a role in stimulating that fungal growth.

Another adaptation that marsh periwinkles display is in their predation avoidance techniques. These mollusks move up and down their cordgrass both as the tide moves in and out, but also to avoid being eaten (Warren, 1985). At low tide, there is an abundance of algae on the marsh floor, however there is also the danger of blue crabs and crown conchs feeding on the marsh periwinkle. Thus, the marsh periwinkle actually move up the cordgrass stalks as the tide goes out to avoid these hungry foe. The video to the left gives more insight into this evolutionary adaptation.

Conservation

Because the marsh periwinkle is sensitive to toxic agents and is so prevalent throughout coastal brackish water marsh ecosystems, it acts as an important estuarian health indicator. Toxicology studies actually use density of marsh periwinkle to determine estuarian health (Walters & Coen, 2006). Some of the human impacts affecting marsh periwinkle populations are loss of habitat and overfishing of their predator populations (Walters & Coen). The loss of habitat reduces the amount of marsh cordgrass available a food source and habitat and thus population sizes. Overfishing in the blue crab industry results in the loss of a major predator to marsh periwinkle that typically would keep their population in check. Without this balance, there are often major growth spikes in marsh periwinkle populations which also lead to decreases in seagrass.