But Wait, What is Sally Selling By The Seashore?

How do seashells form and what dictates their appearance

From spiral to ridged, and striped to spotted, seashells are some of the most visually diverse sea creatures. While seashells themselves are not living, they are the exoskeletons of mollusks, a phylum of invertebrate animals, and home to more than 50,000 species including snails, squid, clams, oysters, and octopuses (1). Yet how is it that even within the same species, seashells vary so widely in their appearance? It all has to do with their intended structure and chemical formation.

Seashells are comprised of 95% calcium carbonate (CaCO3) and 5% proteins and sugars (1). The latter provide shells with strength and the durability to not dissolve in water in order to maintain their structural integrity. While the chemical formula of calcium carbonate does not change, its 3D chemical shape, or crystal structure, varies; this difference has led to two different types of CaCO3: calcite and aragonite (2).

On the right is calcite and on the left is aragonite (3). 

How this difference in CaCO3 structure occurs, scientists aren’t really sure. Currently, a group of researchers at MIT who are studying this question, believe that magnesium levels in the water might play a role. Most species develop calcite layers on the outside of their shells for added length, while they develop aragonite on the inside for added thickness (3). Due to the difference in their crystal orientations, species are less prone to shell fractures because calcite and aragonite do not break in the same way. 

In order for seashells to grow, precise conditions need to be facilitated through a process called biomineralization. Mollusks use an external layer called the extra-pallial liquid to extract calcium carbonate from their surroundings, and another coating called periostracum to protect developing structures against water erosion (4). These two factors work together to facilitate shell growth; the structure of the shell itself consists of repeated layers of protein and aragonite crystals which are held together by a stable polysaccharide called chitin. In each chitin layer, there is room for proteins to lay down, which through chemical charges, attract crystals to start growing into the next layer of chitin. In many cases, seashells act as “homes” for mollusks, which means that as the animal inside grows, so does the shell (4).

The beauty of seashells really comes through in their complex shapes. Most seashells fall under two categories: bivalve, meaning they contain two sides, and spiral. (4) Each added external characteristic is aimed to serve a purpose; for example, spikes are helpful for protection, ridges act as added strength, and alternative textures prevent the seashells from sinking in the sand. 

The color and size of seashells are dependent on the shell’s location. While colder areas closer to the pole are great for small, slow-growing seashells, warmer areas host a great diversity of sizes from 0.7 millimeters to 1.2 meter length (4). On the other hand, their color depends on the diet of mollusks where once again, warmer waters lead to a larger diversity in pigment. While the colors of seashells can be advantageous for camouflage, animals seem to utilize them more for their dieting and geographical guidance. 

Seashells present some of the most beautiful, unique shapes across marine life. From their chemical development to their patterning, a magnificent number of mollusks have evolved to utilize the biochemical and behavioral traits to flourish on ocean floors. So next time you walk by the shore and pick up a seashell, take a moment to appreciate its fascinating structure and complex chemical composition.


  1. The Mollusca. The mollusca. (n.d.). Retrieved October 7, 2022, from https://ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php#:~:text=Mollusca%20is%20one%20of%20the,scallops%2C%20 oysters%2C%20and%20chitons.
  2. Public Broadcasting Service. (2018, September 10). Reactions. PBS. Retrieved October 7, 2022, from https://www.pbs.org/video/how-seashells-are-made-vikxvz/ 
  3. David L. Chandler | MIT News Office. (n.d.). Mystery solved: Why Seashells’ mineral forms differently in seawater. MIT News | Massachusetts Institute of Technology. Retrieved October 7, 2022, from https://news.mit.edu/2015/why-seashell-mineral-forms-differently-in-seawater-0302#:~:text=Calcium%20carbonate%20can%20take%20the,can%20ultimately%20transform%20into%20calcite. 
  4. Amy.dobos. (2017, November 7). She sells sea shells… Curious. Retrieved October 7, 2022, from https://www.science.org.au/curious/earth-environment/sea-shells 

Image 1: https://images.unsplash.com/photo-1589805719243-774da93c3cb8?ixlib=rb-4.0.3&ixid=MnwxMjA3fDB8MHxwaG90by1wYWdlfHx8fGVufDB8fHx8&auto=format&fit=crop&w=774&q=80

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