Why do clams squirt water?

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📓 The short answer

Clams squirt water as a byproduct of filter-feeding, as an involuntary reaction when they snap shut to hide from predators, and — in certain mussel species — to shoot their larvae into the gills of passing fish.

📚 The long answer

I have to be honest. Before I received this question, I never really thought much about clams. I don't live near a beach or eat seafood, so I would have clammed up if asked about clam facts.

Until now.

Before we get into why clams squirt water, here are a few rapid-fire clam facts for ya:

1. Clams have no brain. They have a cluster of nerve cells called ganglia and sensory organs to detect the world around them.
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2. Many clams start out as males producing sperm and, later in life, transition into females producing eggs. This makes them sequential hermaphrodites.
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3. There are over 15,000 species of clams, ranging in size from 2mm (the pygmy pea clam) to 1.2m (the giant clam).

And, of course, clams can squirt out water. If you haven't seen this before, here's a symphony of clams putting on a show:

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A note before we begin: Given how diverse clam species are, their reasons for squirting water aren't universal, but many are shared across species.

Reason #1: Clams are filter-feeders and "spit out" what's left after filtering.

Clams have two siphons that they use to eat, breathe, and expel waste (and water.) One siphon brings in water and the other pushes it out.

The water they bring in is filtered through hair-like cilia covering their gills to capture food and oxygen. The leftover water and waste is then pumped out through the other siphon.

They are quite impressive filter-feeders: A single ~50mm clam can filter ~17 liters of seawater per day. Watch a 100-minute timelapse of two dirty aquariums and see the clams work their filter magic:

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A common reason clams squirt water is because they are simply breathing and eating. Once they've filtered out the oxygen and food they need from the water, they expel it out the other siphon. With enough gusto, this can cause the clam to appear like they're "spitting" into the air.

Reason #2: Clams quickly close their shells when they sense danger, resulting in a startled squirt.

While you'd be hard pressed to find them a shoe that fits, clams have feet. Many clam species use their muscular foot muscle to burrow themselves in beach sand. This allows them to feed on the incoming tide while remaining out-of-sight from predators like crabs, birds, and fish.

Here's another clam timelapse for you (5x-speed). Watch a pacific razor clam bury itself in sand and expel the sand it inadvertently collected in doing so.

Burrowing offers them protection, but they can't remain fully buried all of the time. So when a clam hears vibrations in the sand (predator warning!), they will quickly use that foot to bury themselves.

But there's one flaw with this defense strategy: clams accidentally reveal themselves to the predator. When a clam senses a threat, it snaps its shell shut to burrow, which inadvertently forces water out.

In 1885, a scientist put it eloquently:

Reason #3: Some mussels are parasites that use fish to disperse their larvae a greater distance.

When my parents first sent me off into the world on my own, I was lucky enough to be left with groceries, hugs, and reminders to call home.

For the thick shelled river mussel (Unio crassus), it's not such a gentle goodbye.

Found in the streams and rivers of Central Europe, U. crassus has a peculiar habit. It uses its foot to position itself on the edge of a stream, facing the water at a right angle. Then it sucks in water and powerfully squirts it back into the stream over a meter away. It continues this spitting routine up to several hours.

In 2005, scientist Heinrich Vicentini decided to solve the mystery of the spitting mussels (mussels are close cousins of clams). Vicentini collected water from the U. crassus's squirt streams and discovered it was full of mussel larvae.

In other words, these mussel parents are quite literally launching their little mussel children into the world.

U. crassus squirts its larvae in a stream of water so that they can get trapped in the gills of unsuspecting fish. This allows the mussels to travel a greater distance, so they don't have to compete as much for resources. Technically this makes U. crassus a parasite.

When the baby mussels are ready, they detach from the fish gills and go with their lives, eventually cannonball-ing their own children into the world.

Isn't life wonderfully strange?

--- Thanks for reading this week's newsletter! If you have any thoughts, questions, or favorite GIFs, my inbox is always open. Just hit reply to send me a note! :) ​ All my best, Caitlin ​Click here to read five facts about Caitlin​

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🌐 Wikipedia article of the week

​1593 transported soldier legend​

"A folk legend holds that in October 1593, a soldier of the Spanish Empire (named Gil Pérez in a 1908 version) was mysteriously transported from Manila in the Philippines to the Plaza Mayor (now the Zócalo) in Mexico City. The soldier's claim to have come from the Philippines was disbelieved by the Mexicans until his account of the assassination of Gómez Pérez Dasmariñas was corroborated months later by the passengers of a ship which had crossed the Pacific Ocean with the news.

Twentieth-century paranormal investigators giving credence to the story have offered teleportation and alien abduction as explanations.

👀 Catch up on other curious questions

• ​Why do we look away in conversation when we're trying to think?​​
• ​Why does drinking something cold make you cough?​​
• ​How is vegetable oil made?​​

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