By Sarah Rosenbaum
Most people have never thought about where a seashell comes from and even fewer people realize that as the mollusk grows, so too does the shell.
Empty seashells have provided a home for hundreds of sea life since the beginning. Octopuses, crabs, fish, corals, sponges, and many other creatures can make use of an old, uninhabited shell. But where did that shell come from? How did it come to be that size?
It’s no secret that Florida seashores are covered with all manner of seashells, ranging from common clams to vibrant scallops and ornate gastropods painted with eye-catching patterns. Among the most sought after shells is the Florida horse conch (Triplofusus giganteus), named the official state shell of Florida in 1969.
While there are a variety of mollusks that have unique growth phases, I will use the Florida horse conch to explain in simple terms how a seashell comes to be. This is to help you have a better understanding of the animals that create the gorgeous shells we all love.
The egg casings of Florida horse conchs resemble the corn chips called “Bugles” in that they are ridged triangular pods wider at the top. If you look closely or hold fresh egg casings up to the sun, you can see each pod is filled with fluid and tiny shells. Each egg pod will have about a dozen baby shells inside and once the eggs are ready to hatch, the tiny mollusks poke a hole in the top of the pod and escape into the water column to start their life.
A female can lay tens of thousands of eggs in one year and reproduces at about six years of age. In a recent study, researchers determined the females will mature into a much larger animal (and therefore shell) than the males, and overall the mollusk’s lifespan is about ten years.
When it hatches, the mollusk already has a protective shell on its body. While still the size of a pin head, this shell provides the animal inside a place to call home and a foundation for what will become a masterpiece.
How does the shell get bigger?
Shells are mostly made up of calcium carbonate, meaning it’s a hard, stiff and rigid structure for the animal to live in. Throughout its life, the mollusk continues to add calcium carbonate to the edge of the shell where the aperture, or opening, is, similar to how our fingernails continue to grow out.
Here’s an easy way to think about it: when you’re born, you already have muscles, organs, and a skeletal structure. These mollusks are the same. They’re hatched with all their body parts ready to go and a nice solid structure around them. As they grow, so does the shell; just like your bones as you grow. The mollusks do not jump from shell to shell. That would be like me jumping into your bones. They are attached inside to their “skeleton” and the genes of the animal literally program the shape, color, and texture of the shell.
The Florida horse conch is hatched with a white shell, but as it grows, the shell growth is a bright orange color. In juvenile shells, it’s easy to see the distinct white section at the top of the spire showing you just how tiny this creature was when it was born. The animal’s flesh itself is a bright orange color with a dark brown nail-like operculum attached to the foot.
Through the years, as the shell grows out, the colors may change to a yellow-beige or darken into a burnt orange color which is considered more collectible.
A solid white albino is even more rare. Juveniles and adults may often be found with the periostracum on the shell which is a skin-like black membrane.
Florida horse conchs are predatory sea snails. In fact, as adults they’re the largest species of shell to be found in the Atlantic and Gulf of Mexico. They are the second largest predatory sea snail in the entire world. The largest horse conch shell on record is 24 inches in length and can be seen in the Bailey-Matthews National Shell Museum in Sanibel Island, Florida.
During their life they hunt other mollusks and other small vulnerable animals. Small clams and scallops make easy meals due to their mobility challenges, but the conchs are particularly fond of other gastropods like Banded Tulip Snails.
As an adult the mollusk’s foot becomes easier to see as it moves about. The mollusk has a rather large and pronounced orange foot, two eye stalks and an operculum of about two inches (on average). These opercula are oval shaped with a slight pinch on one end.
As mentioned before, the reproductive age is about six years and the eggs are often laid in large bundles attached to an anchoring object such as another heavy shell, a rock or log. If you come across an egg casing that is still full of fluid in the pods, return it to the water so they can hatch. Horse conchs reproduce sexually and multiple males may fertilize an egg casing. This means that any individual pod of embryos may have different genes and parentage.
Florida horse conchs may be preyed upon by other, larger horse conchs, as they can be cannibalistic. They are also sometimes attacked by giant red hermit crabs seeking their shells for shelter. These crabs can grow almost as large as lobsters. Other creatures such as octopuses, sea stars, whelks, beach crabs, and birds may also take a liking to the horse conch meat.
Their main predator now, however, are people. Humans have been harvesting conchs for their meat and shells, and unfortunately overharvesting has become an ecosystem problem. Researchers have declared the Florida horse conch may be closer to the endangered list than previously believed.
I encourage shellers everywhere to respect the ecosystem’s balance and leave live shells in their habitat. While human causes are not the sole risk to the Florida horse conch, it would be a shame to see them eliminated due to our actions. I suggest taking a great photo and putting them back.
Even after the mollusk itself has died, the calcium carbonate shell remains in the watery wonderland, waiting to be found by whomever may come across it first; whether it be animal or human. Shellers and beachcombers in the Southeast United States dream of this shell rolling in across their path.
Shells may become structure for corals, algae, sponges, barnacles, other mollusks’ egg casings,
or hiding places for small aquatic critters.
If you’re seeking one of these giant trophy shells, keep an eye out after big storms when the waters have been churned up heavily. These large and dense shells are not likely to be along the dry beaches but rather stuck on sandbars, often submerged in water due to their weight.
Most of the ones I have come across were found at a very low tide and were located far out in the water, away from the dry beach. These are often mistaken for rocks that are buried, only a knuckle or two of the shell is exposed above the sand.
If you’re out wading in the waters in search of one of these, I suggest shuffling your feet and keeping your eyes peeled for a rock-like, barnacle-covered item that’s mostly buried. Always investigate it if you’re unsure if the item is a shell or not. You might get lucky. Or, like my many experiences, it could just be a stump or rock. But you never know!
Resource article: University of South Florida Newsroom, “Florida’s state shell at higher risk of extinction than previously thought,” April 2, 2022.
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.
This article appeared in the Beachcombing Volume 38 September/October 2023.
All photos courtesy of Sarah Rosenbaum.