Holey rocks! What happened?

Holey rock from piddock clams

Walking along the beach in Southern California, you might come across holey rocks and wonder, who or what in the world made all those holes? Perhaps the result of some alien testing to determine what planet Earth is all about? Or someone with way too much time on their hands? Not exactly…

The answer is: piddock clams!

Piddock clams are a type of clam that can bore into rocks. This happens to be an excellent strategy if you need to protect yourself from pounding waves. Instead of living with the constant threat of being washed away, it’s much safer and secure to create a permanent home in a rock.

Like many other marine animals, piddock clams start their lives as plankton, floating along at the mercy of ocean currents. While they are floating, piddock clams start looking for a place to settle by tasting and smelling the water to find fellow piddock clams that have already found a great rock home.

Rock with shells of piddock clams

Once a piddock clam finds a suitable rock community of like-minded clams, it grasps the rock’s surface with a broad foot and starts to grow a shell. The shell has ridges or tooth-like serrations the piddock clam uses to burrow into the rock by twisting back and forth.

Piddock clams continue boring into the rock until they are fully grown. Then they secrete shell material over their foot and officially take up residence. But how do you eat if you can’t leave your rock home?

Luckily, piddock clams have two siphons. To eat, piddock clams pump in water and filter food from the water with one siphon. Then they pump out waste with the other siphon. This all takes place as they are snugly situated in a rock.

And there you have it! The reason you may find holey rocks.


  1. Scott Ritchie says

    This writer is fantastic!! The article was most informative and answered a deep mystery long held by entire coastlines of the world and one guy in Ecuador named Phil. Well done!!

  2. virginia arenas says

    Impressive! I used this simple explanation in pre-school classes for many years. I got the info with great difficulty as the internet was still in the fascinating future.

  3. Patricia says

    I have always wondered what causes the holes, thanks for sharing. I have a very large holey rock and it looks like it had been used as an anchor at one time. Very interesting to see. I’m looking for someone who might be interested in buying it. Any suggestions?

    • Carolyn Kraft says

      Hi Patricia, Sounds like you have a cool rock! I’m not sure who would be interested in buying that type of thing, sorry! Maybe try searching rock collectors online?


  1. […] While the unknown mysteries will stay unknown to me for now, there was one mystery that I was able to solve. We all have seen those rocks at the beach that are covered with perfect shaped holes in them, and at the Little Corona Del Mar tide pools there are a lot of these such rocks. I was curious how these rocks were made, because why would rocks have a bunch of random holes in them while others don’t. I assumed it was due to the water grinding away in some way, but water does not play nearly as large as a role as I was expecting. The answer, piddock clams. Piddock clams begin as plankton and when they find a rock they find good, they latch on and begin to grow themselves and their shells. The shells contain ridges, and the clams use these ridges to burrow into the rock, twisting back and forth. It is very common for multiple clams to choose the same rock (http://oceanwildthings.com/2012/02/holey-rocks-what-happened/). […]

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