One of the most striking features of our beaches is seashells. Their whorls, curves, and shiny iridescent insides are the remains of animals. Most shells come from soft-bodied mollusks. Snails, clams, oysters, and others need the hard protection of their shells. This tough outer covering protects the tasty body hiding inside. Other animals, such as crabs and lobsters, also make a tough outer covering, but here we focus on mollusk shells.
Where do shells come from? The animals make them. Mollusks have an outermost layer of tissue on their bodies. Called the mantle, this layer connects the animal to its shell. The mantle also creates that shell.
Specialized cells in the mantle build the shell using proteins and minerals. These are secreted—released into the space outside the cells. There, the proteins create a framework that provides support for the growing shell. The proteins in the framework also determine which minerals are used in specific parts of the shell.
Calcium carbonate, the main mineral found in shells (including eggshells), binds to the protein. If you have ever seen construction workers build with concrete, this is similar. The protein is like the steel rebar that gives shape and support. Calcium carbonate is like the cement that fills in all the gaps.
Calcium carbonate can form two different types of crystals. One is called calcite. This incredibly common crystal can be found all over the world. Calcite makes up chalk, marble, coral, limestone—and seashells. The other form is aragonite. This crystal has a different arrangement of calcium carbonate. Both calcite and aragonite are found in seashells.
A mollusk’s shell has three layers. Each is made up of similar materials. But how those materials are arranged gives them each a different look and feel. The outermost layer is mostly protein. It’s often rough and may have bumps or spikes. Proteins in the middle layer cause calcium carbonate to form calcite crystals. These fill in the spaces, making the shell tough to break.
The innermost layer is the one in contact with the mantle. It’s a smooth, iridescent layer called nacre or mother-of-pearl. Nacre is made up of protein and calcium carbonate. But it looks and feels completely different from other parts of the shell. That’s because the mantle secretes different proteins for different layers. Different proteins cause calcium carbonate to crystallize in different ways. Those used in the middle layer create calcite. Those used in the innermost layer create aragonite.
As the animal grows, its shell must grow along with it. This happens along the outer edges. A snail adds to its shell around the opening, where it pokes its head out. For a clam or mussel, it’s the outer edges where the two shells separate. The result is growth rings, like those in a tree, that allow us to measure a mollusk’s age.
When the animal inside dies, its shell is gradually pounded against the rocks and sand. Over time, shells break down. They become part of the sand. White beaches have sand made almost entirely of tiny bits of shells.
LEARN MORE ABOUT SHELLFISH AND OCEAN ACIDIFICATION
Gröcke, D.R. & D.P. Gillikin. Advances in mollusc sclerochronomology and sclerochemistry: tools for understanding climate and environment. Geo-Marine Letters, vol. 28. 2008. doi: 10.1007/s00367-008-0108-4.
Horne, F. How are seashells created? Or any other shell, such as a snail’s or a turtle’s? Scientific American. October 23, 2006. https://www.scientificamerican.com/article/how-are-seashells-created/
King, H.M. Calcite. Geology.com. https://geology.com/minerals/calcite.shtml. Accessed on January25, 2021.
Kristl, M. et al. Evaluation of calcium carbonate in eggshells using thermal analysis. Journal of Thermal Analysis and Calorimetry, vol. 138. August, 26, 2019. doi: 10.1007/s10973-019-08678-8.
McDougall, C. & B. M. Degnan. 2018. The evolution of mollusc shells. WIREs Developmental Biology, vol. 7. doi: 10.1002/wdev.313
Zhang, C. & R. Zhang. 2006. Matrix proteins in the outer shells of molluscs. Marine Biotechnology, vol. 8. doi: 10.1007/s10126-005-6029.