Giant Clams

In this episode, we explore the truth about giant clams—and their skills as farmers and sunlight-diverters! Our guest Alison Sweeney tells us about where giant clams live, how they get so big, and why they glow!

Guest bio: Alison Sweeney

Dr. Alison Sweeney is Associate Professor of Physics and Ecology & Evolutionary Biology at Yale University. Before going to Yale, Alison Sweeney was associate professor in the Department of Physics and Astronomy at the University of Pennsylvania. Alison’s group focuses on the mechanisms by which novel materials arise in natural evolution, and the mechanisms by which evolution finds novel routes to self-assembly. She was a postdoctoral scholar and research scientist in the group of Dan Morse at the University of California, Santa Barbara, focusing on marine biophotonic materials. Sonke Johnsen at Duke University advised her Ph.D. work on the evolution of squid optics. (Bio from Yale University)


MAGGIE: Mommy, why were the clams on the reef so far apart?

ABBY: Uh…I don’t know.

MAGGIE: They were seashell distancing!

ABBY: [Laughing] Oh my goodness.

ABBY: Very topical, since we started this show during the great social distancing period of your life. So well done, I’m impressed.

MAGGIE: Clams are also the topic for today’s episode of

ABBY and MAGGIE: Big If True,

MAGGIE: Where I, Maggie,

ABBY: And I, Abby,

MAGGIE: investigate the truth about big things. Before we get to the show, though, could we ask you for a quick favor? We’re trying to get 500 downloads of this episode, but we can’t do it unless you tell your friends. So if you have friends in your class at school, or your parents have friends with kids your age, why not tell them that you learn a lot from this show and they should listen to it?

ABBY: We love to listen to podcasts on Maggie’s lunch break, or while we’re getting the table ready for dinner. That’s a great time to listen to Big If True!

MAGGIE: Thanks for helping us out. OK, back to our topic: a big clam—a GIANT clam, to be exact! So here’s our quiz question to test your knowledge.

ABBY: Giant clams are, well, giant, so they don’t have a lot of natural predators. What type of animal is most likely to eat a giant clam?

MAGGIE: A. Great white shark; B. Sea turtle, C. Human, or D. Giant grouper.

ABBY: We’ll tell you the answer later in the show. Our guest today got started in her giant clam studies in an unusual way.

SWEENEY: I am Alison Sweeney. I’m a professor of both physics and ecology and evolutionary biology at Yale University. And I’m really interested in studying the really cool physics of animals and how those physical properties of animals evolved over time.

MAGGIE: Hi Alison! Thanks for talking to us. How did you get interested in giant clams?

SWEENEY: I was always interested in cool animals, and in the ocean and in questions about how the physical properties of the world like light and water flow and smells shaped animals in the way that they evolve. And so that’s sort of the big picture question of what I worked on. And then, specifically giant clams, I actually started out interested in squids and squid camouflage, and all the really cool things that squids can do to hide themselves. And one of the things that squids do to hide themselves is they make these really special sort of mirror-like cells called iridocytes, and they actually use mirrors in their skin to hide themselves. And giant clams have really similar mirror-like cells, but they don’t swim, they don’t attract mates, they’re not trying to hide, they just sit there. And so I was really curious why on earth a giant clam would have sort of a mirror cell, like a squid would. And the answer ended up being really exciting, and not at all what I expected when I first started looking at it.

MAGGIE: Ooh, a mystery! I can’t wait to hear what’s so special about these mirror cells. But first, there are clams in our local stream that I can pick up and dig out. How are the clams in our stream different from giant clams?

SWEENEY: They’re a lot more similar than you think. So they’re actually pretty similar to each other. And it’s sort of like comparing, you know, a mouse to a rat to a gerbil, you know, all three of those animals have the same parts. And if you know the parts of a mouse, you pretty much know the parts of a gerbil, and you know, they behave in pretty similar ways. And so both the clam and your stream and the giant clam in the Pacific Ocean that I study, they’re both bivalves, which means they have two shells instead of one shell. They have gills, they have little openings called siphons that they pump water through. And they live in part by filtering particles, so basically just dirt and bacteria, out of the water for their food to eat. So if you opened up a clam in your stream and a giant clam, it would actually have exactly the same parts, and the differences between them are actually relatively subtle.

MAGGIE: But there are SOME differences, like where giant clams live.

SWEENEY: They can get up to a meter in length, which is a big animal. There aren’t too many occasions when we, in our everyday life, see animals that are a meter long and almost a meter wide. So they’re very, they’re very impressive and very big in person. But those are the biggest ones. The far more common ones only get as big as like a softball. But there’s one species of giant clam that can be a meter long. They live in really clear water in the Pacific Ocean, so they live on coral reefs. So they live sort of in tropical latitudes, on coral reefs, but only in the Pacific Ocean. They don’t live on coral reefs in the Atlantic Ocean.

ABBY: Someday I would love to go snorkeling in the Pacific Ocean and see giant clams. Maybe I’ll do it while I’m working on my next book project.

MAGGIE: Can you take me with you?

ABBY: Sure, why not?

MAGGIE: Why do giant clams get so much bigger than the little clams in the river?

SWEENEY: They both filter particles out of the water, like I said, so they, they have this big pump inside of them, and they pull water through there to openings and they are able to filter out dirt and bacteria in the water. And that is very nutritious for them and they eat that.

SWEENEY: The difference is that the giant clam has actually learned how to sort of eat light. So the giant clam is a farmer in that it grows, plant cells, in essence inside its body, and that’s not something that a clam in your stream would be able to do. And they actually farm those plant cells, they grow, they actually grow the little cells in rows, almost like corn or soybeans. And they eat the sugar produced by solar energy inside of these these plant cells, which is one of the things that makes them so cool.

And so that’s one of the reasons that the giant clam gets giant is that it has this extra sort of vegetable food source inside of it.

Look at the glow! Photo by Flickr user Piktour UK.

MAGGIE: One of the things we learned from reading about your work is that giant clams kind of glow! What makes them glow?

SWEENEY: So that glow, it sort of depends on what you mean by the word glow. Some animals do produce their own light. And that is typically what we call sort of glowing animals like a like a firefly, so fireflies glow. And giant clams definitely have cool optical properties that you could call sort of glowy. But it’s a little different than a firefly making its own light. So giant clams don’t make their own light, they reflect the light from the sun. But the reason that we see them as so sort of beautiful and glowing is because, so I mentioned they have almost like, you know, soybeans or corn sort of growing in their skin. But obviously clam skin is, you know, much less thick than a soybean field, for example. So they actually have to come up with a pretty clever way to make sure that sunlight gets all the way through that layer of plant cells that they’re getting food out of. And so the way they do that is they use these mirror-like cells called iridocytes to take the sunlight and basically push it down into the clam skin and make sure that all the plant cells in there, get sort of a happy healthy dose of sunlight. So they really are actually very good farmers.

ABBY: That is so cool!

SWEENEY: So that’s sort of what those glowy mirror cells are actually for. But because they have such a special and particular interaction with light to make sure that you know, all of the farm is doing well, they end up having sort of a structured fancy reflection as well. And so that like little bit of accidental reflection that we see as a beautiful colored glow is actually sort of a byproduct of the fact that they’re trying to be farmers.

MAGGIE: What an amazing animal! What lessons can we learn about light or farming from these giant clams?

SWEENEY: They ended up teaching us a pretty elegant but straightforward way to do really efficient photosynthesis. Photosynthesis is the science word for plant cells making sugar from sunlight.

SWEENEY: It has come up with this really clever way to use every last bit of the sunlight in its environment, which is not something that we knew how to do before we studied giant clams. And so we both learned that it’s possible for natural systems to use all the sunlight really efficiently to make sugar or do photosynthesis. And we also discovered how you would do it. And the secret is in these special little glowy mirror cells that we just talked about. So it was pretty cool to learn both that being really efficient at using sunlight was possible and how you would do it if you wanted to copy it. I think it’s really just satisfying and cool to solve one of these natural puzzles. Before we started working on it, nobody knew why giant clams glowed. And it was a lot of hard work. But it was just really satisfying to work really hard on a puzzle and then figure it out. And now there’s new information in the world that wasn’t there before we worked on it. And I find that kind of work really fun along the way and really satisfying when we finally figure out the puzzle.

MAGGIE: Wow! So, are giant clams under threat from anything? Are they at risk?

SWEENEY: So the biggest risk to clams right now is just being eaten. So yeah, people really like to eat clams. There are many places in the world where there used to be a lot of clams, and now there are none or only a few. The good news is, you know, there’s still enough that if people stopped overfishing them, then they would be able to recover. So they’re not, they’re not extinct or that close to extinction yet, but it would be good for people to stop eating them. And then another sort of bad thing that happens with clams is there’s illegal aquarium trades. So people pay a lot of money to be able to put a giant clam in their aquarium, and so they can be illegally shipped around that way.

MAGGIE: Hey, there’s the answer to our quiz question! The animal most likely to eat a giant clam is…a human!

ABBY: But giant clams are also under threat, like everything in the ocean, from another human problem: climate change.

SWEENEY: if coral reefs go away, which is possible with climate change, giant clams are going to go away also. And so anything that kids can do to let their government know that they know about climate change, and they think it’s really important and that we think they think we should have policies to help mitigate climate change will help you know, all animals and all coral reefs, you know, giant clams included.

ABBY: We’ll have some links in our show notes to organizations like Oceana and Sea Legacy, which are specifically dedicated to the preservation of the oceans, and other climate change organizations, where you can advocate for cleaner and healthier oceans.

MAGGIE: Alison, thank you so much for talking to us about clams—they are more complicated creatures than I thought!

ABBY: So now, listeners, it’s your turn to create something cool about clams. This week, why not do a little more research on coral reefs and the oceans where giant clams live, and draw some artwork or write a poem that advocates for the health of the oceans? If you draw or write something and send it to us, we’ll send it along to the elected officials and organizations who need to know that kids care about a healthy ocean. You can send it through our show notes on our website,, or you can ask your adult to post it on Instagram or Facebook and tag us. All the info about our social media accounts is on our website.

MAGGIE: And that’s our show for this week! Join us next time when we continue to investigate things that are

MAGGIE and ABBY: Big If True!

ABBY: Special thanks to Alison Sweeney for being our guest this week. If you want to read more about her work, check out our shownotes. Our theme and episode music are, as always, by Andrew Cote. Thanks for listening, and tell your friends!

More fun stuff

Here are the links to the organizations we mentioned in the show. They advocate for cleaner and healthier oceans, and they could use your help!

We want to see what you come up with! You can tell us what you made here!

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