Ask Dr. Universe – Seashells

How are seashells formed? And why are they different colors? Can seashells live or die? – Caroline, 9, Crestwood, Ky.

Dear Caroline,

Seashells come in an astounding variety. Some are curved and round, others long and tube-like. Some are smooth, others bumpy. Some are large, others small. Plus, they come in a rainbow of colors: red, green, brown, purple, pink, and more.

All that variety comes from the same source: little animals called mollusks, with a mighty muscle called a mantle.

I found out all about them from my friend Richelle Tanner, a scientist at Washington State University. She is very curious about the ocean and knows a lot about mollusks, a type of animal with a soft, moist body.

There are many kinds of mollusks: both on land and in the sea, with and without shells. If you’ve ever seen a snail or a slug, you’ve met a mollusk in real life.

Unlike humans, cats, and other animals with backbones, mollusks don’t have skeletons inside. Many move through life with just their soft bodies. But some grow shells for protection, as a kind of traveling armor.

That’s where seashells come from, Tanner explained. “A seashell is a protective outer coating secreted by the animal’s mantle, which is one of their muscles,” she said. The mantle forms the soft outer wall of their body.

The mollusk’s mantle builds the shell from the bottom up. It absorbs salt and chemicals from the water around it. When it has enough of the right ingredients, it uses them to form a hard substance called calcium carbonate.

Strong, healthy seashells are made mostly of calcium carbonate. (So are eggshells!) A mollusk produces calcium carbonate from its mantle, laying down layers of it over its lifetime. Together, those layers form the seashell.

You can think of a seashell kind of like your own hair. Your hair grows and is part of you, but it isn’t alive on its own. A living mollusk produces a shell with its body, but the shell itself isn’t alive.

When a mollusk dies, it leaves its shell behind. But even after the life of the mollusk inside has ended, its shell is important. Seashells provide shelter for fish and hermit crabs, nest material for birds, and even nutrients for other animals to build their own shells.

You’re right to notice that seashells can come in many different colors. The way the shell forms helps explain where the color comes from.

“The material for the color comes from the mollusk’s environment—so it’s either taken out of the water or from what they eat,” Tanner said.

For example, seashells from warm waters tend to be more colorful than those from cold areas. This might have to do with their diet. Warm Caribbean waters have more colorful foods than the cold ocean near Maine.

We know seashells’ colors come from their environment. But scientists don’t know yet how the colors get spread around, creating brilliant patterns.

If you keep asking questions and hunting for answers, maybe you could help figure this out.

Dr. Universe

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