Piddocks – anything but boring

Just recently we were pottering happily along a pebbly beach on the island of Lismore, and some of the stones that we were picking up looked like this:


Just one pebble with a hole in it you could probably accept as a happy stroke of luck – but there were many more!   Some had just one hole, beautifully drilled and smooth on the inside, just wide enough to put your little finger in;   others were perforated with a network of tiny tunnels, so that if you held one up you could see a thousand pinpoints of light. But how do these holes occur?   They are certainly not caused by the sea.   The culprit is a mollusc, and it is called a piddock.


photo via Wikimedia

photo via Wikimedia

Piddocks are bivalves, and their specially-adapted oval shells are edged with fine teeth which they use to excavate burrows in rock.  Their fleshy foot grips the stone surface and helps to rotate the shell, creating a circular scouring action.   Piddocks will also drill into submerged wood, if it happens to be available.

Once the piddock has carved a safe tunnel for itself, it settles down inside and extends a siphon through the entrance, which it uses to filter food such as phytoplankton from the sea water.   Another siphon empties waste products back into the sea.   As the animal grows in size, it chisels at the walls of its rocky home to expand them.

Piddock siphons, credit pfly via Wikimedia

Piddock siphons photographed in Washington State, US. (Wikimedia)

Lismore - piddocks by Castle Coeffin

A snack bar for piddocks

Despite their tough-guy attitude, piddocks are elusive little things, and their shells are (amazingly) so brittle that, once exposed to the ocean, they are quickly broken up – which means that you are unlikely to find a whole piddock shell unless it’s still inside the stone.

However, if you’re walking along a beach and you pick up a pebble that is riddled with small holes – or one that looks like a half-inch drill bit has gone right through it – you’re probably holding a fine example of their handiwork.

Piddocks collage

Comparing piddock holes. Several other species of mollusc, along with some worms and sponges, can also create holes in solid rock.  These examples may have been carved by a number of different species.

Piddocks 4

The American piddock, Petricolaria pholadiformis, courtesy G U Tolkiehn via Wikimedia

The American piddock, Petricolaria pholadiformis, courtesy G U Tolkiehn via Wikimedia

Piddocks are also known as Angelwings, which I think is a lovely name:   you can immediately see why, when their shell is opened out.  There are many species of piddocks, but the one that is native to British shores is the Common Piddock (Pholas dactylus).   Surprisingly, it can grow to 15 cm in length.


Now, here is a fascinating story:  piddocks in the genus Pholas are known to phosphoresce, or glow with a greenish light, in their burrows.  That is delightful enough!   But the 1905 edition of ‘The Victorian Naturalist’, the Journal and Magazine of the Field Naturalists’ Club of Victoria (Vol XXI), contains this startling announcement: “Pliny spoke of luminescence in the mouths of people who ate Pholas, the rock-boring shell-fish, and of such importance is this phenomenon that it is even said to have gained the first king of all Scotland his throne.”

If this is true, what a huge role for a humble mollusc!   Sadly, I can find no more information to back up the claim.   But that doesn’t stop my imagination from conjuring an interesting scene, perhaps at a banquet or even on a windswept rocky shore where the knowledge of ‘magical’ creatures might have given a vital edge to an early contender for Scotland’s crown. I shall look at piddocks in a whole new light from now on!

Piddocks 3

Peeping piddock

Peeping piddock

The wonderful thing about piddock art is that every piece is unique.   I have several treasured examples – one with a shell still inside, which rattles when you shake it.   If you tilt it, you can just see the tip of the piddock shell, but the burrow entrance is too narrow for it to drop out.   

Piddock art gallery (the shells are top shells, not piddocks)

Piddock art gallery (the shells are top shells, not piddocks)

Piddocks aren’t the only molluscs capable of making holes in rock.   Several others, along with worms and sponges, are described by Jessica Winder in this wonderful post on her Nature Blog, illustrated with some superb photos taken on beaches in the south of England.   She also writes about piddocks, and has published some great photos of the creatures in action.


Photos © copyright Colin & Jo Woolf unless otherwise stated.

For more fascinating rocks, take a look at:



  1. This is absolutely fascinating for me who collects shells & stones from around the world! (I ask anyone travelling to places I don’t, to bring me back those “presents” which to me, are far more valuable than any other souvenir)
    As you wrote… I will certainly look with a different eye those stones 🙂
    Thanks for this truly refreshing post… I’ll treasure that information every time I walk the beach.

    • Thank you very much, I’m so glad to hear you enjoyed it! I am also lucky in that I have friends who have brought me stones and shells back from places I’ve never been – I have a few bits of lava from Iceland, some wonderful desert rose crystals and geodes from Algeria, and some jade from New Zealand. They are cherished too! Thanks for your comment and good luck with your rock hunting!

  2. There must be a fresh-water North American equivalent of this mollusk because I have picked up stones like this, riddled with smooth holes, in many places near water in the Midwest. Thank you for this interesting post!

    • That’s an interesting thought! I don’t see any reason why there shouldn’t be freshwater piddocks too… wow, they’re everywhere!! 🙂 Thank you for your comment!

  3. Altogether astonishing! When you mentioned shaking a stone with a shell still inside, it triggered a memory of me doing that with a stone I found on a beach once, but it’s just a vague memory. From now on I’m going to be paying attention to any stones with wee holes in them. Thank you for the lesson on piddocks!

    • Aagh, now you’re going to be wondering where it was! I hope you find another! They are amazing little creatures, aren’t they?! I’m really glad you enjoyed finding out about them as much as I did!

  4. This is totally fascinating! They have so much character in them. Anyone one of them would be a wonderful sidekick to Bashful, *my* pet rock 🙂 XOXO – Bacon

    • Thank you, glad you found it interesting! I couldn’t believe someone had a pet rock called Bashful, but I checked your site and there he was! Say hi to Bashful for me! (Just going to count my own pet rocks now!) 😀 Good to hear from you, Bacon!

      • I will definitely say hi to the little fellow for you. Right now, he is currently in France visiting friends. I don’t know if you checked out some of my posts from last week but he was on there with some of his adventures in France.
        Earlier in the year, I wanted my own pet. Pigs get lonely too you know 🙂 I wrote to a company and they sent him to me for free. I was so pigstatic! A little friend all to my own. XOXO – Bacon

      • Thanks, Bacon! I will check out your rocky friend’s adventures! 🙂

  5. Absolutely enchanting!. It is lovely to see that even the humblest of creatures can create such a wonderful universe. The nursery is an endearing sight. Thank you for this informative article with great pictures. Now I”ll take a look at my rock collection!.

    • Aren’t they lovely?! They do create their own universe, you’re right! I’m so glad you found it interesting. It’s really nice to meet other people who are just as passionate about rocks!

  6. So that’s what they are from! We always come home with rocks and shells when traveling. I’ve seen rocks with holes for years, but never thought to find out what created the holes. I thought it was just soft spots in the stone that were worn away. Fascinating!!!

    • Well, I’m pretty sure you have got some piddock holes in your collection by the sound of it! I guess not all holes in pebbles are made by piddocks but they are quite easy to identify once you have seen one or two. Now you’ll be looking for rocks with the piddocks still in them! 🙂

  7. Wonderful, Jo! Love the “peeping piddock” especially and the bit about the first king of Scotland’s glowing mouth–Kenneth MacAlpin, wasn’t it? Marvelous. Scotland’s cuisine can sometimes have a sort of stunt-food factor to it, so this doesn’t sound too far fetched. Heh heh!

    • Thank you, Hank! The whole ‘king of Scotland’ thing is mind-boggling. I’ll have to consult Neil Oliver about who was the first King of Scotland. I think he slightly disputes Kenneth MacAlpin’s claim! But then maybe he’s never heard of piddocks?!

      • Oh! We love Neil Oliver! Love, love, love him! Especially that wee flip of his locks when he’s dashing off on a particularly interesting topic. Heh heh. What a great storyteller!

      • He’s so good, isn’t he?! He gets really passionate about the subjects he’s talking about! I love his books too – he has a great way of writing. A real storyteller, you’re right!

  8. well I never – I have never been to Lismore but it is on the cards to do so – and when I do this blog I am sure will creep back in to my mindseye

  9. Amazing post, its so interesting, those rocks I found look very nice in our rock collection! Can’t wait to keep collecting more! P.S Love your new background 🙂

    • Thank you! 😀 We found some good rocks that day, didn’t we! I’ve still got to find homes for some. Nice that you’ve started a collection too!

  10. Very ineteresting!
    I would never thought that a small mollusc could drill something like that! Incredible!

    • No, I know, it’s amazing, isn’t it?

    • Debbie Richards says:

      Great, I actually sat with my little boy the last few evenings down our beach in Southsea, and there were lots of them, I kept going on about them as I don’t remember seeing maybe one, let alone lots of them, just couldn’t understand how they got there right through rock, found it really baffling & odd! So thankyou for the info

  11. Your post and photos sent my mind back to visiting Hawkes Bay beaches where there were so many rocks with holes or hollows in them. I had simply assumed (!) they were the result of water action on the greywacke rock and sandstone that is so prevalent in that area. But looky here: We have similar boring wee critters drilling happily on our beaches too. I am off to consult my Native Animals of NZ book to learn more.

    Kids would love to have glowing mouths just as potential kings obviously did!!!

    Fascinating post Jo. Thank you…..and I love the word Piddock and the Angel Wings are simply beautiful.

    • Wow, so you have piddocks in NZ too! Thank you for the page link – I love the reference to shipworms and their damage to the ‘bridge over troubled waters’! Since writing this post I’m hearing of piddocks all over the world, (well maybe a slight exaggeration!) which is lovely! Thanks for this, Lyn.

  12. What great photos! And your collection looks so familiar, just like an Oregon beach. I loved the “peeping piddock.” Goodness, we’ve learned a lot this week!

  13. Fascinating. Much more so than anything on the course I’m doing at the moment (Understanding our Oceans, Southampton University). Thanks for the links, too.

  14. I have a few pieces of ‘piddock-art’ myself, but had never thought to investigate what a piddock looks like – beautiful, I love the ‘angelwing’ name and amazing that something so fragile can bore through rock.

    • I know – piddocks themselves are quite hard to find, although they leave their marks everywhere! You’re right, it is amazing that they can bore through rock, and make such perfectly round and smooth holes. I just love them! 🙂

  15. Callie Jones says:

    I found a rock at grover beach, California. And i dont know what it is. Can someone help me. I know its a fossil. But what is it. It was arock and i dropped it and inside is an egg like structure. Almost perfectly intact. Email me for photos

  16. Gabrilelle says:

    Hi! I don’t know if you’ll ever read this, but thank you for this awesome article! I looked up sea rocks with holes because I went to a beach in Goleta, California and was intrigued and surprised to find rocks with intricate & unique holes in them. This is an awesome discovery. These rocks are so amazing & I found some that are small enough to wear as a necklace. I’m giving them to my friends as gifts 🙂

    • Hi Gabrielle, That’s amazing! Thank you for letting me know! What a great discovery, and I’m so glad you found this article interesting. I am always finding piddock-holed stones myself and it’s wonderful to hear of other people finding them too! 🙂

  17. Thanks for this excellent post on the humble piddock, which I stumbled across whilst thinking about holey stones, as opposed to holy stones! I’ve mentioned your blog, which I am now following, in my post ‘a light shines in the mouth’ on my blog at:

    • You’re most welcome, and I’m very glad you found some holey stones! 🙂 Once you’ve found one, you’ll be finding lots more (or that’s what happened for me). Thanks for following my blog! I hope you’ll find lots more of interest.

  18. Hard to figure a clam boring a hole in a rolling stone. I think they were occupying the holes opportunistically. These types of stones are found globally, and remains to be seen if the specie of clam is that wide-spread.

  19. Actually I can find a lot of whole shell of the creature. I’ve also found one alive. It was on a sand bar so I placed it back in a tide pool. I never knew what they where until now.

  20. vapor1vixen says:

    I’ve moved from Pennsylvania to Tennessee 15 years ago I have a stone ring it looks like that it would fit on a child’s finger and I found a perfectly shaped egg rock also . I posted it on my Facebook page I thought that they were Native American since there so many arrowheads on my property

  21. What a great article – I have learned so much and am thrilled to know what those fabulously perfect holes are in stones and how they were made. My granddaughter will be delighted to know too. Thank you so much.
    perry94022 at hotmail dot com

  22. Fascinating article

  23. Pam Sequeira says:

    Amazing! I live in southeastern Oregon, and have found many rocks with holes in them. Would I be correct in assuming that the high desert area I live in was once under water and a home for Piddocks?

    • Thanks for your question, Pam! I don’t really know enough about the geology of Oregon to say with any certainty. If you know that the region was definitely under water at some stage, I guess it’s a possibility. But if the region is volcanic, I know that some volcanic rocks such as basalt can also have little holes (vesicles) in them that were formed by gas bubbles. This page might help:

      • Pam Sequeira says:

        I’ll bet you’re right. There was a LOT of volcanic activity here.
        Thanks so much for the reply. 🙂

  24. Awesome, amazing and wonderous!!! Thanks for sharing.

  25. Wonderful! So many of my rocks make a lot more sense, now!

  26. Diane Bailey Beger says:

    I really enjoyed this informative article!

  27. I hope you don’t mind but I have linked to your blog in a post on FB and Twitter. I have three stones with holes made by Piddocks that I use on my stand when promoting The Wildlife Trust. They attract an enormous amount of interest and I found your blog when researching them. Thanks for putting such fascinating information on line.

    • Most welcome, and thank you for sharing it! 🙂 People seem to love finding out about them – and there’s something really pleasing about piddock ‘artwork’! Well done for spreading the word!

  28. Hecataine says:

    Reblogged this on The Witch Hecataine.

  29. Where I live in Chichester harbour, I grew so knowing them as lucky stones!

  30. Really interesting article. I’ve lived by the sea all my life and never heard of piddocks, so I’ve learned something new today. Will soon be off to the beach to look for some! I wonder if they bore through Lewisian gneiss, though? Will have to go and have look.

  31. I have found a piece of sandstone with several holes made by piddocks. Some of the holes still have shells in them. The odd thing is that I found it in my vineyard near Calistoga, California. Either the San Francisco bay reached this site or it is a fresh water species?


    • That’s very interesting! I have not heard of freshwater piddocks but I am not a scientist so maybe you would be better advised to consult the marine science department of your local University. I guess there is a chance that it was carried there, either by tidal action or human effort?! Thanks for your comment!

  32. i have found these in a creek bed in Lewis co. Ky.

  33. Excellent article.

  34. Julie Holm says:

    Wonderful to find you.. I knew that some Mollusk did the ‘drilling’, but love knowing more.. The pictures are fantastic–make me a bit envious..
    Thanks for sharing and giving me something to enjoy and learn..

  35. Edith Douglas says:

    Had no idea therenwere such things as piddocks, though I have seen those holes. Thanks for adding to my knowledge. The photos are excellent, I think.

    • You’re most welcome, Edith! Really glad you enjoyed reading about them. I keep finding them everywhere myself, now I know what to look for!

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