Anyone who has hiked and crossed a mountain stream or took a cold drink out of a spring has likely seen periwinkles, which look like small roundish black pebbles scattered about in the water. Closer inspection reveals that they are fresh water snails.
I’ve been told by my older kin that seeing these little guys in a stream indicated that the water was clean enough to drink. I wouldn’t go that far about the purity of the water, but they are partially right in that these snails are environmentally sensitive and good indicators of unpolluted streams.
First let me clear the name up a little. If you’re into landscaping, you know that periwinkle is also the name of a flowering plant that grows as a ground cover and produces a pretty five petal purple bloom. But periwinkle is also the Appalachian name for these little aquatic snails. Periwinkle is an Old English name for a group of edible snails found in Europe, and the original spelling was “pinewincle”. The “wincle” part meant snail and the pine or pin part meant just that, a pin. The thought is that folks long ago would eat certain snails by prying their edible bodies out of their shell using a long sharp object (a “pin”). So the term periwinkle (also pinnywinkle) is another one of those old mountain words that came with ancestors from Europe and hill people corporately held onto it and continued using it when everybody else forgot about it long ago. That’s one thing I like about us hillbillies, we hang onto old traditions. Oh, and I don’t really know how the flower came to be called periwinkle. The only clue I have is that the flower petals form in a pinwheel pattern, and the “pin” part also originated from the Old English name.
Anyway, back to the little snail dude. Periwinkles are in a class on animals called Gastropods, which includes all snails and slugs. They are in the group called “gilled” snails, meaning they have a breathing mechanism that allows them to breath underwater. The other group of aquatic snails is called “lunged”, and must come up for air. Gilled snails are found in fast moving, highly oxygenated water such as mountain streams, while lunged snails can tolerate less oxygenated still water such as ponds and river eddies. Another distinguishing feature of gilled snails is that they have a “trapdoor” called an operculum that they can close over the shell opening for protection from predators.
Periwinkle snails are pretty small, averaging a quarter to half inch in size, and if you look close their black shell has a bit of a spiral to it. Their day consists of slowly moving from rock to rock feeding on algae and dead plant material while hanging on for dear life to keep from being swept away by the water current. This moving and hanging on is carried out using a “foot”, which is a blobby muscle you can see sticking out of the shell. Procreation is done in the usual way, with boy snails and girl snails cooperating with each other. The female lays eggs in a gelatin mass stuck on submerged rocks or other protected surfaces.
While not very exciting representatives of the animal kingdom, aquatic snails are actually pretty nice to have around. They keep algal growth in check, and are a food source for some fish and bird species. Because gilled snails are sensitive to water degradation, they act as an indicator of unpolluted aquatic ecosystems. So if you see them, the stream you’re at is pretty healthy.
Steve Roark is with the Tennessee Department of Agriculture, Forestry Division.