How do oysters make their shells?

By Jane Lindholm,

Published December 12, 2025 at 9:05 AM EST

A large pile of while shells. — Oyster shells cure on Governors Island for about a year before they are made into reef balls to eventually become habitat for oysters.

On a rainy, windy day, But Why hopped on a ferry and headed to Governors Island in New York City with one thing on our minds - oysters! We met up with Mike McCann of the Billion Oyster Project to learn more about these very special bivalves. How do they get their shells? How long do they live? What do oysters eat and who eats them? Why do people eat oysters without cooking them?

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Oyster reef balls are grey spheres with large circular holes in them. — Oyster balls are created out of concrete and crushed oyster shells on Governors Island New York. They will eventually be homes for oysters.

New York Harbor used to be the oyster capital of the world. You could buy oysters from carts on the street. Oyster shells were even used in building construction! But the population declined because of over-harvesting and water pollution.
Oysters are bivalves, a type of animal that has two shells.
New York and most of the Eastern United States is home to one species of oyster known alternately as the Eastern Oyster or Atlantic oyster.
Oysters make their own shells shortly after they hatch by pulling calcium from the water and mixing it with protein. They have a special tissue called a mantle that can create their shell. Their shell grows with them as they get bigger.
Oysters can live to be 20 years old!
Oysters are food for other animals like crabs, fish, snails and of course, humans!
Oysters eat microscopic algae called plankton.
Oysters can also remove nitrogen from the water. An individual oyster can filter up to 50 gallons of water a day!
The Billion Oyster Project is trying to bring oysters back to the New York Harbor to help with pollution and also because oyster reefs foster biodiversity: a wide variety of animals and plants that make up a healthy ecosystem.
The oysters growing in New York Harbor these days are inedible due to pollution, but as the water gets cleaner, they could be harvested some day.
The Billion Oyster Project collects oyster shells from New York’s many restaurants. They crush the shells and mix them with an eco-friendly concrete to make structures called reef balls.
They drop the reef balls into the harbor along with millions of baby oysters. The oysters that survive latch onto the reef balls and attach themselves, helping to build up a healthy reef system.
The Billion Oyster Project hopes to restore a billion oysters to the harbor by 2030!