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Weekly “What is it?”: Marsh Periwinkle Snails

Salt marshes are composed of endless thickets of salt and water-adapted grasses, rushes, and sedges. Photo credit: Carrie Stevenson, UF IFAS Extension

The first thing you’ll notice about a salt marsh is the endless sea of grass. These emergent grasses along the water’s edge have their roots underwater, and despite the odds thrive in wet, salty, often turbulent conditions. Along our shorelines you’ll see a thick border of bright green saltmarsh cordgrass (Spartina alterniflora), with tall dark black needlerush (Juncus roemerianus), just inland of it. These waters are teeming with wildlife, as the grasses provide food and plentiful hiding places.

The marsh periwinkle snail lives and feeds on blades of saltmarsh cordgrass. Photo credit: Chesapeake Bay program

But if you look closely at the blades of Spartina, you might notice a small (1/2” to 1”) grayish-purple speck clinging to it. Keep looking, and you’ll notice more…dozens, even hundreds attached to the grasses. These are marsh periwinkle snails (Littorina irrorata), and they make up half of one of the more interesting predator-prey relationships in the salt marsh.

Upon close examination, an observer will notice dozens, even hundreds of snails on cordgrass blades in a salt marsh. Photo credit: Carrie Stevenson, UF IFAS Extension

Periwinkle snails are a favorite food of the blue crab, which scuttles along the bottom and swims throughout the marsh waters. When the tide is low, the populations of snails will crawl down the stalks near the water’s edge to feed on algae, staying just out of reach of the blue crab’s claws. When the tide comes in, however, the higher water level allows the swimming crabs to reach higher up the grass blades. Seeking to outrun their would-be captors, the snails will all climb higher up the stalks, staying above the water line and just out of reach. You can watch the timing of daily tides simply by observing the snails’ daily migration up and down the grass blades.   This defense is not foolproof, as crabs have actually been observed shaking the grasses, trying to knock snails off their slimy perches. Diamondback terrapins and mud crabs also eat periwinkle snails, so they have plenty of predators to try and avoid.

Next time you’re out paddling or fishing in our local salt marshes, look closely! You’ll see these marsh periwinkle snails everywhere.