Operculums: Doors to the Sea

By Jessica Anderson

You’ve spent the lazy, misty morning exploring the tide pools, munching on your favorite seaweed snacks. The air is still; the only sounds are the trickle and whoosh of cool water lapping against the rocks.

The sun shifts higher, burning off the marine layer. The water begins to recede with the tide. The heat is uncomfortable, but even more disconcerting is the rapid draining of the tidal pool. Saltwater is life—you can’t survive without it.

The water’s retreating faster now, far faster than you can chase it. Finally you accept the grim reality: you’ll just have to wait until the next tide comes in.

You, my friend, are a marine gastropod—Megastraea undosa, to be exact—and I’ll bet you’re really appreciating your operculum right about now.

Wait—what’s an operculum?

An operculum is a calcareous structure created by many sea snails that serves as a little “trapdoor” to safely close them inside their shell. When, say, the tide goes out, stranding a sea snail too far from the water, the gastropod can draw itself deep into its shell and pull the operculum closed behind it. Some operculums (or opercula), like that of the wavy turban snail, match the shape of its shell opening exactly, allowing the snail to seal itself inside, high and—well, not dry. It also comes in handy when protecting itself from predators such as sea stars, octopuses, and carnivorous snails. (That Kellet’s whelk in the next tide pool over did seem to be eyeing you rather hungrily.)

Typically, an operculum is attached to the snail’s muscular foot (the source of it’s scientific class name Gastropoda, meaning stomach-foot). When the snail extends itself outside of the shell, the operculum swings open and tucks out of the way; when it’s time to return to safety the snail’s soft-yet-strong body pulls that operculum tight against its aperture, or opening in the shell.

Not all sea snails create hard, shell-like operculums: some species secrete thin layers of a cuticle-like substance to protect themselves. But as those operculums are much weaker, it’s the calcareous “shells” that have caught the attention of humans for thousands of years.

Various cultures, from ancient to modern, have used operculums in incense, grinding the shells into a powder that’s used to stabilize and preserve other fragrances. Some historians speculate it may have been an ingredient in the sacred incense burned in the ancient Temple of Solomon 

A far more common usage of operculums—as with so many other seashells—is in jewelry and adornment. Most operculums are round or oval, with an attractive spiral pattern on one side. One popular operculum comes from the Turbinidae snail, a species found in Indonesia. Commonly called a “cat’s eye” or “Shiva eye” shell, its white, flat side displays its iconic spiral, while the rounded side boasts colors that do eerily resemble an eye. Both sides are used in jewelry and decoration, though it’s much easier to secure it in a setting with the flat side down.

Due to the massive international seashell trade, these tropical operculums are easy to find in coastal souvenir shops and incorporated into jewelry and decor. A more sustainable approach (both to the environment and the snails themselves, which are harvested and processed alive) is to beachcomb them yourself. I’m sure it goes without saying that only completely-vacated shells should ever be collected, and even then, many shells can be used by other creatures as a second-hand home. Operculums, though, can’t house hermit crabs, so there’s less risk in bringing them home.

Of course, in order to go operculum-shelling you’ve got to be near the ocean, on a beach that allows beachcombing, and preferably one where operculum-creating sea snails are commonly found. If you’re lucky enough to visit the beaches along our Southern California coast, you just might come across one of my favorite ocean finds: the oval, ridged operculum of the wavy turban snail—Megastraea undosa from this morning’s tide pool adventure.

The wavy turban snail, along with a few other turban snails, stand apart as makers of particularly unusual operculums. Instead of the circular, half-domed Shiva eyes, these sea snails make teardrop-shaped operculums with deep ridges that sweep along the length. I find the curving lines of these shells uniquely elegant, which is why I enjoy featuring them in my jewelry.

Various species of turban snails can be found across the world, but our Pacific Coast variety create, in my humble opinion, the most lovely. Something about those curving ridges echoes the sway of seaweed in the current and the rolling lines of waves. Perhaps it’s the introvert in me that appreciates the turban snail’s way of shielding itself when the outside world is less than hospitable. And who can resist the excitement of discovering a wonderfully wave-smoothed operculum shell and knowing, like an ocean-guarded secret, that you’ve found something truly special?

So when the tide pulls out and leaves behind that mysterious oval, curved and smoothed, hold it in your hand and think of the little marine gastropod it has protected. This “door to the sea” protected a living creature, allowing it to survive in the ever-changing intertidal zone. And who knows, maybe it will lead you into a salty new adventure!

This article appeared in the Beachcombing Magazine July/August 2021 issue.

Learn more about seashells

Learn more about identifying shells, the history of seashell collecting, great shelling beaches, and the lives of the animals who make the shells we find on the beach. Articles ›

No live shelling: Be sure shells are empty and sand dollars, sea stars, and sea urchins are no longer alive before you bring them home.