KIDS ASK! How Are Seashells Made?

Beautiful, colorful seashells can be found all over the world! But… who’s making them, and why?
© Vyacheslav Argenberg /, CC BY 4.0, via Wikimedia Commons

Here at HTHT, we teach a LOT of science, and the best part about it is feeding young scientists’ curiosity about this amazing world we live in!  Although our programs are jam packed with experiments, we make time to let our young scientists ask us whatever questions they’ve always wanted to ask a scientist. 

In the coming weeks, we will be sharing a special series of articles answering some of the most frequent questions that come up from our young partners in science.  Our question this week is:


They can be spiky, and sharp like a knife!  They can shine in every color of the rainbow!  Some are so big you could hide in them.  Some are so tiny you need a microscope to see them.  Lots of them are older than the dinosaurs.  You can even play them like a trumpet.  And, mermaids love them!  Get your flip flops and a towel, science friends, this week we’re going to the beach to check out SEASHELLS!

The sharp, super-spiky Venus Comb shell.
Didier Descouens, CC BY-SA 4.0, via Wikimedia Commons

Think you could hide in this Giant Clam shell?
Alicia Fagerving, CC BY-SA 3.0, via Wikimedia Commons

Even if you have never been up close to the ocean before, you’ve probably seen some seashells in movies or art.  Maybe you’ve put one up to your ear and heard the wild ocean waves calling to you (it’s actually the shape of the shell amplifying the sound of your own heartbeat, which is also pretty cool).  Because seashells are so beautiful and there are so many incredible kinds to see, it’s easy to admire the shell, but forget about the awesome animals that make them: an ancient, diverse, worldwide group of animals with the excellent name of MOLLUSKS.  Isn’t that a great word?  It’s fun to say and the animals in what scientists officially call the Phylum Mollusca are even more awesome than their name. 

This cutie pie is a Giant African Land Snail, and a member of Phylum Mollusca!
Sonel.SA, CC BY-SA 3.0, via Wikimedia Commons

Mollusks are a HUGE group that includes squid, octopus, snails, clams, and even slugs.  There may be as many as 200,000 different species of mollusks, while there are only 6,500 species of mammals.  They are one of the most successful types of animals in the ENTIRE HISTORY OF THE WORLD.  Look up!  Mollusks in the trees.  Look down!  Mollusks all the way to the deepest part of the sea.  Look back in time, before there were even dinosaurs.  Who was there?  That’s right, MOLLUSKS!  The only place you will never find one is flying in the air. 

How deep do mollusks go? Introducing the Magnapinna Squid, found at 20,000 feet.
NOAA, Public domain, via Wikimedia Commons

So, why do these ancient, world-wide powerhouses make so many beautiful shells?  It starts with the fact that their bodies are very soft, just like a slug’s.  Mollusks are invertebrates, which means they don’t have a spine or any bones in their body. Some mollusks, like squid and octopus, do have a radula, a powerful scraping beak that helps them eat.  The rock-scraping radulas of the mollusks known as limpets are thought be the strongest, toughest biological material ever found in any animal in the WHOLE WORLD.  So, even though mollusks are soft, they are still mighty! To be strong and soft at the same time, many mollusks create a hard outer covering for themselves, which is where seashells come from.  How can they make so many beautiful, different kinds of shells?  Well, mollusks have been mastering the art of making shells for at least 400 million years!

A scientist’s reconstruction of the ancient mollusks called Ammonites, which lived on earth 400 million years ago, long before the dinosaurs!
Bramfab at Italian Wikipedia, CC BY-SA 3.0, via Wikimedia Commons
This is an incredible Ammonite shell fossil that was covered with iridescent minerals over millions of years. We told you mollusks are shell-making masters!
JamesPFisherIII, CC BY 3.0, via Wikimedia Commons

Every seashell you see was made by one mollusk, to be its home for all its life.  Their shells start small and grow, little by little, as long as the mollusk lives and grows.  Many of our mollusk friends are actually born with a tiny, soft, colorless shell.  This tiny shell, called the protoconch, is a miniature blueprint of the shell it will grow during its life. 

See the very smallest whirl in the center? That’s the protoconch. The rest grew as the mollusk did!

How does something hard like a shell GROW?  There’s more to the mollusk’s magic.  A mollusk takes nutrients and minerals into its body from the things it eats and from the water around it.  These nutrients and minerals then feed into the shell gland, a part of the mollusk’s body that mixes up just the right formula to make a shell.  A mollusk’s shell is mostly made of a mineral called calcium carbonate and also a little protein called chitin.  The shell mixture is released through the mantle, the soft body of the mollusk.  The shell mixture crystallizes, hardens and basically becomes a form of rock!

How strong are shells, really? These are the famous White Cliffs of Dover in England. They are white because the entire cliff is made of hard, compressed SHELLS from ancient mollusks that lived millions of years ago.
Immanuel Giel, CC BY-SA 3.0, via Wikimedia Commons

A mollusk’s shell grows only from the edge of the shell.  This means that the tiny center point of a shell is the oldest part, the protoconch, that was there when the mollusk was born.  The rest has grown over time.  If you look closely at a seashell, you can see the “growth rings” where the mollusk has added another layer of growth. When you see a big shell with many rings, it means the mollusk was growing for a long time!

See the ridges here? Those are where the shell grew outward! Next time you look at a shell, look for the rings or ridges on its surface.
RealGatba, CC BY-SA 4.0, via Wikimedia Commons

Aren’t we lucky to live in a world that has beautiful things like seashells in it? Although shells are formed by completely natural processes, they are as beautiful as anything an artist could create. So now you know, science friend, that shells take a mollusk a long time to grow, and the shell is their only home. If you go to the beach this summer, have fun looking for shells! But if you pick up a shell with an animal at home in it, please put it back and let the animal enjoy its home — they built it themselves, after all!


The delightful Dr. Helen Scales introducing everything wonderful about mollusks and shells:

A quick introduction to shell formation:

A deep introduction to shell formation:

Ammonites and Ammonite fossils:

The deepest-dwelling mollusk:

Just beautiful shells and shell facts for you to look at:

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