Ask Dr. Universe: When and Why Would a Clam Open Its Shell?

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

One thing that most species of clams have in common is that they can open and close their shells using two super strong adductor muscles. Some clams use these muscles to open their shells when searching for food. These filter feeders eat and breathe through a tubular part of their body called the siphon, which protrudes from the top of their shell.

A clam will use its siphon to bring a lot of water into its body for two main reasons. The clam receives oxygen from the water in order to be able to breathe. It also derives important nutrients, or its food, from water in order to survive.

If there are leftovers in the water that the clam does not need, it is filtered and taken out through a second tube-shaped siphon. If you’re ever lucky enough to see this happen, you’ll feel like the clam is spitting in the air.

When people harvest clams for food, they often use a knife to open the shells, and in doing so, they also cut the adductor muscles. This is why we see partially open clams 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 creatures such as starfish, sea otters, seagulls, and fish. Another reason a clam can naturally open its shell is to stick its foot out and dig into the ground.

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

A clam can use its foot to turn around and propel itself forward. He can use his foot to create this jumping motion on land and in 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 types of clams, including the genus known as geoducks.

Turns out, not all clams have a shell that can actually open and close. The geoduck has a foot so large that it cannot even fit into the shell. But the large foot helps the geoduck to dig very deep in sand or mud to escape predators.

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


Dr. Universe

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