If you’ve spent any time exploring the rocky Maine shoreline — or running your fingers along the rocks in our 200-gallon touch tank — you’ve almost certainly met a periwinkle.
They’re the small, spiral-shelled snails clinging to the rocks, scooting along so slowly you’d swear they weren’t moving at all. They’re easy to overlook. But these tiny grazers have one of the most surprising backstories of any animal on our coast, and they’ve quietly shaped the very shorelines we love.
Meet Maine’s Three Periwinkles
When most people say “periwinkle,” they’re picturing just one snail. In fact, three different periwinkle species live along the Gulf of Maine, and each has carved out its own neighborhood in the intertidal zone — the stretch of shore that’s underwater at high tide and exposed at low tide.
The common periwinkle (Littorina littorea) is the one you’ll spot most often. It’s the largest of the three, with a dark, sturdy shell that can grow up to an inch or two long, and it ranges across the middle and lower parts of the intertidal zone.
The smooth periwinkle (Littorina obtusata) has a rounder, blunter shell and comes in a surprising range of colors — yellow, olive, brown, even orange. It lives lower on the shore, tucked among the rockweed it depends on for food and shelter.
The rough periwinkle (Littorina saxatilis) is the smallest and least common of the trio. As its name suggests, its shell has a rougher, more sharply pointed spire, and it lives highest up the shore, in the splash zone where only sea spray and the highest tides reach.
Even their family lives are different. Common periwinkles release their eggs straight into the sea, where the larvae drift as plankton. Smooth periwinkles lay their eggs in jelly-like masses on rockweed. And rough periwinkles skip the egg stage entirely, giving birth to live, fully formed baby snails.
“We have all three species as residents here at the Shaw Institute,” said Mikey Vittum, Lead Aquarist at the Institute. “Between our 200-gallon touch tank and our other aquariums, they play an important role in not only enhancing these mini ecosystems we’ve created, but also in allowing us to teach visitors about the value of the amazing sea life we have here in the Gulf of Maine and beyond.”
Built for Life Between the Tides
The intertidal zone is one of the harshest neighborhoods in the ocean. Twice a day it’s pounded by waves and submerged in cold seawater; twice a day it’s left high and dry, baking in the sun or freezing in winter air. Periwinkles are beautifully equipped to handle it.
The secret is a little trapdoor called the operculum. When a periwinkle pulls back into its shell, it seals the opening with this tough, plate-like cover. That single adaptation does two crucial jobs: it locks in precious moisture so the snail doesn’t dry out, and it shuts out hungry predators. Sealed up tight, a common periwinkle can survive out of the water for days.
To eat, a periwinkle puts out a remarkable tool called a radula — a ribbon-like tongue covered in rows of microscopic teeth. It works almost like a tiny chainsaw, rasping algae and biofilm off the surface of rocks and seaweed. You probably won’t catch the radula in action, but you can see the evidence: those faint scrape marks zig-zagging across wet rocks are periwinkle dinner tracks.
Watch one move and you’ll see a single muscular foot doing all the work, leaving a faint trail of mucus behind. Look closely and you might spot two little stalked tentacles poking out, which the snail uses to sense light and dark as it forages.
A Snail That Sailed to Maine
Here’s the twist that surprises almost everyone: the common periwinkle — the most abundant snail on our shores, the one that seems like it’s always been here — isn’t actually native to Maine.
Littorina littorea comes from the rocky coasts of northern Europe. It was first documented in North America at Pictou, Nova Scotia, around 1840, and from there it spread rapidly down the coast, reaching Maine by the late 1860s and early 1870s and pushing south past Cape Cod within a couple of decades. Today it’s found from Labrador to Maryland.
How did it get here? The leading theory is that periwinkles hitched a ride in the rocky ballast that sailing ships carried to stay balanced on long ocean crossings. When ships dumped that ballast on Maine’s shores, the stowaway snails came with it. Genetic detective work by researchers, including biologist Dr. Susan Brawley of the University of Maine, has even traced North American periwinkles back to ancestors in Britain and Ireland — a clue that ties the snail’s arrival to the busy transatlantic timber trade of the early 1800s.
The common periwinkle has been here so long, and become so woven into the shoreline, that some scientists half-jokingly call it an “invasive non-invasive.” It’s a great reminder that the natural world we think of as timeless is often more dynamic — and more entangled with human history — than it first appears.
“The debate about whether or not periwinkles are doing more harm than good or vice versa I feel will always rage on here amongst the marine ecologists of the North Atlantic. In my opinion, the snails have only partially out competed the native smooth periwinkles, which are still thriving. If anything I feel the larger non-native periwinkles provide more food for intertidal animals like whelks, jonah crabs, sea stars, and more. As well as providing larger homes for the native hermit crabs you can find at low-tide. Regardless of my opinion this is a classic case of how animals can be displaced by mistake and come out settling comfortably somewhere else.”
How One Tiny Grazer Shapes the Shore
A single periwinkle scraping algae off a rock seems harmless enough. But multiply that by the thousands of snails packed into a stretch of shoreline, and their appetite reshapes entire ecosystems.
Because common periwinkles graze so heavily on algae, they help determine which seaweeds get to grow where. Scientists suspect that before this snail arrived, Maine’s rocky shores may have been far more colorful — fuzzy with green, red, and brown algae that the periwinkles now keep mowed down. Their grazing has nudged out other species, influenced which seaweeds dominate the rocks, and rippled through the whole intertidal community.
Periwinkles aren’t at the top of the food chain, either. They’re a favorite meal for shore birds, fish, and crabs — including the green crab, another European newcomer to our coast, which crushes their shells to get at the soft body inside. People eat them too: in much of Europe, “winkles” are a traditional seafood, boiled and pulled from the shell with a pin, and they’ve supported a small commercial harvest right here in Maine.
The Humming Trick
Here’s a fun one to try the next time you visit. There’s an old bit of shoreline lore that says if you hold a periwinkle gently in your palm and hum to it, the snail will come out of its shell. Many people swear it works — likely because the snail responds to the gentle vibration. It’s the perfect low-stakes experiment for curious kids and patient grown-ups alike.
“Periwinkles are one of the first animals a lot of our visitors really connect with, because they’re small and gentle and they’re right there at fingertip level,” says Mikey Vittum, Lead Aquarist at the Shaw Institute. “I love telling people that the ‘plain little snail’ in their hand actually sailed across the Atlantic and reshaped the whole shoreline. You watch their face change — suddenly it’s not just a snail anymore. That’s exactly the kind of moment a touch tank is for.”
Come Say Hello
You can meet our periwinkles up close in the 200-gallon touch tank at the Shaw Institute’s Environmental Education Center in downtown Blue Hill. While you’re here, take a slow moment to find one on the rocks, feel the smooth curve of its shell, look for the little trapdoor operculum, and — if you’re patient — try humming it out for a closer look.
The Education Center is home to much more than the touch tank, too. You’ll find marine mammal skeletons and bones, aquariums, an interactive virtual ocean explorer, original artwork, a gift shop, lab tours, and more. We’re open 10 a.m. to 4 p.m., Wednesday through Saturday, and we’d love to see you.
Next time you’re on the shore, you’ll never look at those little spiral snails the same way again.
Main image at the top is also from An Bollenessor, a talented photographer and marine biologist based in Cornwall, England. Please support their work.