Marsh Friends: The Periwinkle Snail

The Marsh Periwinkle is one of the most common and characteristic inhabitants of salt marshes along the Atlantic coast from Massachusetts southward to central Florida and in the northern Gulf of Mexico to Texas. These whitish globular snails (approx.1 in.) are most commonly observed perched along the stems of Spartina in the mid to upper parts of the intertidal zone. The absence of Littorina in southern Florida re¬flects the replacement of Spartina by mangroves in that part of the state.

One of the interesting behavioral features of this little snail is the pattern of vertical migration up and down the Spartina plants with the rise and fall of the tides. At high tide Littorina is typically found high up on the plants above the waterline. As the tide falls, the snails move down the plants and onto the muddy substrate below. This behavior apparently evolved as a means of avoiding predation by blue crabs, one of the major predators of the snails.

Other predators include wading birds, mud crabs, and Diamond Back terrapins. The ability of these snails to spend most of their lives out of water indicates that the gill chamber has become modified as a sort of functional lung, allowing gas exchange both in air as well as in water.

Littorina feeds by grazing on the thin lm of detritus and microalgae coating the Spartina plants or at the surface of the mud at the base of the plants. The snails also rasp away the epidermis of the plants, creating wounds which become infected with fungi which are then fed upon by the snails. The fungi are fertilized by feces of the snails. So Littorina literally farms the fungi on which it feeds and is the only mollusk known to practice fungiculture.

However, the fungal infected feeding scars are detrimental to the plants and there are some indications that where populations of Littorina have increased in many areas there has been a corresponding die-back or decline of Spartina.

As with many other estuarine animals, the reproductive and life history events of Littorina are closely attuned to the rhythm of the tides. Mating occurs during the warmer months and involves copulation between male and female snails. This is followed on the next high tide by the release of tens of thousands of fertilized eggs into the outflowing water. The transport of these planktonic eggs out of the marsh and into nearshore coastal waters removes the eggs and developing larvae from the proximity of many of the more important predators found in the marsh. After several weeks of larval development, the young snails are transported by in¬flowing tides back into the higher levels of the marsh where they complete their metamorphosis into adult snails.