Podcast Summary
Podcast: But Why: A Podcast for Curious Kids
Episode: How do oysters make their shells?
Host: Jane Lindholm, Vermont Public
Guest Expert: Mike McCann, Director of Science and Research at the Billion Oyster Project
Date: December 12, 2025
Main Theme
This episode explores how oysters make their shells, why oysters are important to their ecosystems, and the work of the Billion Oyster Project to restore oyster reefs in New York Harbor. Through curious questions from kids, the episode covers oyster biology, environmental benefits, predators, and citizen science.
Key Discussion Points and Insights
1. Introduction to Oysters and the Billion Oyster Project
- Oysters were once abundant in New York Harbor, making the city the "oyster capital of the world" [03:00].
- Overharvesting and pollution nearly eliminated the population.
- The Billion Oyster Project aims to restore oyster reefs for ecosystem health and biodiversity.
2. What is an Oyster?
- Oysters are mollusks—a group including snails, slugs, octopuses, and clams [04:00].
- They are bivalves: animals with two shells hinged together [04:16].
3. How Do Oysters Make Their Shells?
- Mike McCann: "Oysters have this very special tissue called their mantle... they can take minerals that are in the water, primarily calcium... and they mix it with protein, and then they… build more and more layers of shell." [05:07]
- Shell growth is continuous during their life; they never molt or shed their shells [05:59].
4. Oyster Lifespan and Age Markers
- Restored oysters in NYC are typically 7-8 years old; some can live up to 20 years [06:31].
- While shells show growth rings, they’re not as precise as tree rings for aging [07:00].
5. The Oyster Shell Pile and Reef Creation
- The Billion Oyster Project collects 3 million pounds of oyster shells from NY restaurants [07:48].
- Shells are cured outdoors, cleaned, crushed, and mixed with eco-concrete to make "reef balls"—structures to anchor oyster reefs [09:00, 09:52].
- Reef balls look like huge, holey plant pots or "giant colanders" and weigh about 500 pounds [10:13].
6. Growing New Oysters
- Living baby oysters ("spat") are introduced in special facilities with reef balls, then deployed into the harbor [10:42].
- The goal: 1 billion oysters in NY Harbor by 2030 [15:51].
7. What Do Oysters Eat and How?
- Oysters eat microscopic algae called phytoplankton, not large seaweeds [12:32].
- They use a tissue called the "ctenidium" (like a comb) with cilia to filter water, select food, and reject debris or pollutants as "pseudo-feces" [13:02].
- Mike McCann: "The oyster can actually choose the particles that it wants to eat and choose the particles that [it] says, nah, that's not food..." [13:02]
8. Oyster Ecosystem Services
- Oysters filter pollutants from the water, improving water quality and clarity [14:32].
- They help remove excess nitrogen by trapping it in sediment or turning it into harmless nitrogen gas [14:32].
- A billion oysters could filter a significant part of NY Harbor in just a few days [15:46].
9. Why Oysters Live in Saltwater
- Oysters and most mollusks evolved in the ocean. Their bodies are specialized to handle saltwater (osmoregulation) and can’t easily adapt to freshwater environments [16:33].
10. Oysters Through the Seasons
- Adult oysters stay cemented in place year-round. In cold winters, they slow their metabolism and live off stored energy [17:50].
11. Oyster Predators
- Various animals eat oysters: blue crabs, mud crabs, oyster toadfish (named for their croaking sound), predatory snails (like the oyster drill), boring sponges, and humans [18:42].
- Fun moment: "Can we just pause on the oyster toadfish? Because first of all, it has a fantastic name, but also it is a weird looking fish." – Co-host [19:01]
12. Food Safety: Eating Oysters
- Oysters must come from clean waters to be safely eaten raw [21:02].
- Mike McCann: "It’s only safe to eat oysters if they’ve come from clean water. And any oyster you see in a store... [or] restaurant has come from waters that have been designated safe for harvest." [21:29]
13. How Pearls Form
- Most oysters have a shiny inner shell layer called "nacre" or "mother of pearl" [22:32].
- Pearls form when an irritant gets inside; only some oyster species make perfectly round pearls, most pearls found in NY oysters are irregular [22:32–24:10].
- Co-host: "Basically. Anytime you see a pearl, you can think of it as something that really annoyed that animal..." [24:10]
14. Student Involvement and Citizen Science
- Young people, especially students at the NYC Harbor School, play key roles in the restoration—operating boats, scuba diving, and constructing reef structures [24:41].
- Mike McCann: "Restoration is not just the job of adults. We think that young people and kids and students are the next—they’re the leaders of what the harbor is going to look like in the future." [24:41]
Notable Quotes & Memorable Moments
- Mike McCann [05:07]:
"Oysters have this very special tissue called their mantle... and they can secrete, layer by layer, that mantle tissue can sort of build more and more layers of shell." - Mike McCann [14:32]:
"They can actually get rid of [nitrogen] entirely... the microbial community... convert it into nitrogen gas, which then goes into the atmosphere, where it's perfectly fine." - Jane Lindholm [15:51]:
"So what would a billion oysters do?"
Mike McCann: "A billion oysters could filter, you know, a good chunk of New York harbor in a few days." - Co-host [19:01]:
"Can we just pause on the oyster toadfish? Because first of all, it has a fantastic name, but also it is a weird looking fish." - Co-host [24:10]:
"Basically. Anytime you see a pearl, you can think of it as something that really annoyed that animal, that what they made was something to put around something that was annoying." - Mike McCann [24:41]:
"Restoration is not just the job of adults. And we think that young people and kids and students are the next... leaders of what the harbor is going to look like in the future."
Timestamps for Key Segments
- Oyster Basics and Shells: 03:26 – 05:59
- How Long Oysters Live: 06:31 – 07:00
- Oyster Shell Pile & Reef Balls: 08:09 – 10:31
- How Oysters Eat & Filter: 12:32 – 14:32
- Oyster Ecosystem Benefits: 14:32 – 15:46
- Oysters in Salt vs. Fresh Water: 16:33 – 17:39
- Predators: 18:42 – 20:05
- Food Safety/Eating Oysters: 21:02 – 21:56
- Pearls & Nacre: 22:32 – 24:19
- Kids in Restoration: 24:41 – 25:24
Engagement and Call to Action
The episode emphasizes hands-on science and environmental stewardship for kids, encouraging listeners to get involved with local citizen science projects or restoration initiatives.
For More Information:
- Visit butwhykids.org/books for related reading
- Learn about the Billion Oyster Project
This summary brings together all the fun facts, discoveries, and moments of awe that make the episode an enriching listen for curious kids, teachers, and parents alike!