The Story Behind the Seashells By the Seashore

Celebrate the first day of summer by learning how seashells form and what they can tell us

The inside of a lumpy white and brown seashell is smooth and pink.
Not only is this regal murex seasnail shell stunning in appearance, but the stark contrast between the inner and outer shell highlights the complexity of seashell formation. Smithsonian Institution

With summer finally here, many of us are ready to hit the beach to beat the heat – which means it’s officially time to look for seashells.

Seashells, often found lining the seashore, stand out among the sand like precious gems. From smooth to rough, spiraled to dished, and pure white to vibrant colors of the rainbow, shells come in all shapes, sizes, and textures. Due to their varied charm, humans have prized seashells for as long as they have collected pretty things.

But by the time they wash ashore, seashells have already had an extensive history. Constructed by everything from sea snails to scallops, seashells are beautiful biological treasures that contain hints of the marine environments they came from. As a result, seashells are useful to both aquatic critters and scientists on dry land – including those at the National Museum of Natural History.

Known for their vibrant patterns and glossy appearance, tiger cowry shells are found from the eastern coast of Africa all the way to Hawaii. Smithsonian Insitution

More than a sum of their parts

Most seashells come from a group of animals called mollusks, which includes everything from oysters to octopuses. The word mollusk originates in part from the Latin word “mollis,” meaning soft. As the name suggests, all mollusks have soft, squishy bodies, so many species create firm shells for structure and protection.

Which begs the question — what are these sturdy shells made of?

While they may look like bumpy bones or sculpted stones, they are neither. Shells are actually part of a category of objects called “composite materials,” meaning they are composed of a combination of different components. In the case of seashells, they are made of both mineral and biological features. The biological aspects include protein and chitin – the same material found in the shell of a crustacean like a lobster – and they form the flexible scaffold that minerals fill the gaps of. The mineral component of shells is calcium carbonate – the same material eggshells are made of. When present in seashells, the carbonates take on the crystalline form of either calcite or aragonite, depending on the shell type.

“This makes seashells very special because composite materials have unique material properties,” said Gabriela Farfan, the museum’s curator of gems and minerals, who specializes in studying carbonates.

Farfan explained how composite materials like seashells behave differently than their pure components. For instance, mother of pearl – the innermost layer of some seashells – is well known for being far sturdier than its carbonate component alone.

Many sea snails fashion swirling shells, like this emperor's slit shell, that are decorated with intricate patterning. Smithsonian Institution

The hardworking mantle

How do these squishy creatures construct their strong shells? It all starts in the mantle, the thin layer of flesh connecting a mollusk to its shell. Cells in the mantle secrete all the proteins, chitin, and minerals necessary to create the shell. Mollusks never shed their shells, so the creatures must increase their shell’s size as they grow. They do this by building their shells little by little from their margins, like a tree creating new rings. “Generally one can think of growing shells as growing tubes,” said Chris Meyer, research zoologist and curator of mollusks at the museum.

Each mollusk’s shell contains three distinct layers – all made of the same material but arranged in different structures. There is a smooth and shiny inner layer, a chalky middle layer and a rough outer layer.

The innermost layer is the layer that directly touches the mantle. Its proteins cause the calcium carbonate to form fibrous aragonite crystals. The shell’s middle layer includes different proteins that lead to the formation of calcite instead, which is why it looks different than the inner layer. The shell’s outer layer is not calcified at all, but is a thick layer of protein.

When the shelled animal dies, it leaves behind its shell like a skeleton. They either fall to the seafloor or are washed ashore to be found by beachcombers or house-hunting hermit crabs later.

Some shells, like this one that was created by a clam-eating sea snail, are covered in large, defensive spines. Smithsonian Institution

Pearly time capsules

Seashells are more than just valuable for the creatures that grow them – they are also useful to scientists studying Earth’s past. Since mollusks form shells in incremental layers, each layer captures a snapshot of the surrounding environment during its creation.

Farfan, who studies round collections of smooth inner shell called pearls, explained that scientists have been able to associate variations in shell layers with different aspects of its local water conditions at the time, such as its salinity, temperature and the amount of dissolved oxygen.

“We hope that by understanding how these processes work in modern systems, we can then look at paleo systems and understand what the Earth was like in the past and make predictions about how the Earth may be in the future,” Farfan said. “All based on these tiny shells and pearls.”

In the shell of this green abalone, a large sea snail, the difference between the inner and outer shell is stark. The shimmering inside of the abalone is known as the mother of pearl. Smithsonian Institution

In addition to information about the mollusk’s water’s chemistry, shells can also reveal to scientists where bodies of water once lied. “Finding a seashell anywhere in the world tells you that once upon a time there may have been water,” Farfan said

If you come across a shell away from the beach this summer, you might be beachcombing on an ancient coastline without even knowing it!

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