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Ask Dr. Universe: When and why would a clam open its shell?

Aug. 29, 2021 Updated Sun., Aug. 29, 2021 at 2:53 p.m.

A grocer displays live geoduck clams in Seattle. A geoduck has a foot that is so large, it can’t even fit inside its shell.  (Associated Press)
A grocer displays live geoduck clams in Seattle. A geoduck has a foot that is so large, it can’t even fit inside its shell. (Associated Press)
Washington State University

Washington State University

Dr. Universe: When would a clam open its shell? Why do the shells open? As far as I know, it opens when boiled for food. – Teng, 5, China

Dear Teng,

There are a lot of different reasons why a clam might open its shell. My friend Jonathan Robinson, a marine ecologist at Washington State University, told me all about it. If we spent some time where the ocean meets the shore, or the intertidal zone, we might observe how clams open their shells when they need to eat, breathe or move around.

One thing most clam species have in common is they can open and close their shells using two super-strong adductor muscles. Some clams will use those muscles to open their shells when they are in search of food. These filter feeders eat and breathe through a tube-like part of their body called a siphon, which sticks out from the top of their shells.

A clam will use its siphon to bring a bunch of water into its body for two main reasons. The clam gets some oxygen from the water so it can breathe. It also gets important nutrients, or its food, from the water so it can survive.

If there’s any leftover stuff in the water that the clam doesn’t need, it gets filtered up and out a second tube-like siphon. If you ever have a chance to watch this happen, it will look like the clam is spitting into the air.

When people harvest clams for food, they often use a knife to open the shells, and in the process, they also cut the adductor muscles. That’s why we see clams that are partially open on the dinner table – they can no longer open and close their shells on their own.

Humans aren’t the only ones who eat clams. Clams are an important food source for critters like sea stars, sea otters, seagulls and fish, too. Another reason a clam might naturally open its shell is to stick out its foot and dig into the ground.

Yes, you read that right: A clam has a foot. Of course, it isn’t quite like a human foot. “It’s one big muscle, and it kind of looks like a human tongue,” Robinson said. Some clams will use this foot to dig into the ground and hide away from predators.

A cockle clam can use its foot to sort of flip itself over and propel itself forward. It can use its foot to create this hoppinglike motion on land and in the water. Along with the WSU Beach Watchers, a group of volunteers who help protect the Salish Sea and Puget Sound, Robinson often explores the shores where there are several kinds of clams, including the kind known as geoducks.

It turns out not all clams have a shell that can actually open and close. The geoduck has a foot that is so large, it can’t even fit inside the shell. But the big foot helps the geoduck dig really deep down into the sand or mud to escape predators.

It’s great to hear you are making observations and asking big questions, Teng. Maybe one day you will help us learn more about the intertidal zones that so many living things call home.

Sincerely,

Dr. Universe

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