Pebbles with holes made by tube worms

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Pebbles with holes: An arrangement of sedimentary rock pebbles riddled with small holes and burrows made by marine polychaete worms such as Polydora ciliata (Johnston) (1) 

Continuing the theme of pebbles with holes made by sea creatures, the photographs here show pebbles of soft sedimentary rock which bear, not only the larger sort of hole made by boring bivalved molluscs, but also many much smaller holes.

These small holes may be seen in some instances to occur in pairs that represent both openings of the U-shaped tunnel that has been dissolved into the stone by the presence of a mudtube-dwelling marine bristle worm or (polychaete) such as the ubiquitous Polydora ciliata (Johnston).

The tunnels in the rock can often be seen like narrow open channels on the surface. This happens in the pebbles that are old and in which the outer layer has been eroded away by mechanical damage.

Earlier posts on the burrowing damage caused by small marine mudtube-dwelling polychaetes can be found in Flat oyster shells with Polydora ciliata burrows and Ancient & modern Polydora ciliata type burrows in Flat Oyster shells.

Pebble with holes and tunnels: A pebble of sedimentary rock in which the majority of borings are small and made by marine mud tube worms (2) 

Close-up of pebble with holes: Detail of infestation damage caused by small marine bristle worms like Polydora ciliata (Johnston) in a pebble from the Jurassic Coast UK (3) 

Pebble with holes: Small dark grey pebble with burrows made by small marine polychaete worms like Polydora ciliata (Johnston) (4) 

Pebble with holes: Small cream coloured pebble with infestation damage caused by marine bristle worms like Polydora ciliata (Johnston) (5) 

Revision of a post first published 13 November 2009


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37 Replies to “Pebbles with holes made by tube worms”

  1. It probably takes a long time……….but the polychaete worms are not chewing through the stone, they are “accidentally” dissolving it with their acidic excretory products!


  2. Hi Jessica… can you tell me if there is a generic name for these types of rocks with small holes created by sea creatures..?

    …thanks…. rgds…bob..


  3. Hello, Bob. Sorry but I don’t know of any special name for the rocks with holes in them made by marine invertebrates. I suppose you could say the rocks have been colonised, occupied, excavated or infested by seashore creatures. I keep finding them in unexpected places – like today, I discovered some on Rhossili beach which appeared to be as ancient as the rocks themselves. So this kind of burrowing activity and damage seems to have been going on for millions of years and sometimes you can’t tell from the holey pebbles you find on the beach when the holes have been created.


  4. I’m so happy I found this site. I have been collecting similar broken and rounded clam shell fragments that have been bored (I always thought by worms, but now I’m thinking sponges). Some I find are beautiful and remind me on carved jade (though not green). I have been making jewelry out of them. I will have to take some photos of them to show you. I love the intersection of art and nature and would like to learn more of the natural history behind these beach finds….North Carolina coast.


  5. Hello, Nancy. Nice to hear from you. I am pleased that you like the blog and have been able to find out something about how holes get in shells. I guess there are still more types of bore holes than I have already described. I must get down to publishing some posts of things like gastropod drill holes in shells. I would love to see some photographs of the jewelry that you have been making – and maybe I could identify what caused the perforations.


  6. Hi Jessica,

    Please could you have a look at this (related) discussion on the NHM’s NaturePlus forum

    We have a specimen where the holes are in triplets, and we are looking for an explanation.
    You obviously have an eye for this sort of thing. We would value your opinion, TIA.

    Mike Hardman (geologist, BSBI referee for Viola; amongst other things)


  7. These impressions are intriguing. I’ll need to give this some thought. It is a shame that the image is so low resolution. Am I right in thinking that some of the impressions may be in fours as well as in threes?

    Something in the regular, almost triangular, outlines to the depressions is making me think of hard mouth-parts – I believe that some marine worms have tripartite jaws – but I don’t think that can be the right interpretation. They are not like any borings and boreholes I have ever encountered.

    Was the stone collected because of the mystery impressions or for some other reason? As with your own thoughts, one possible explanation is that the marks were made by something after the (?soft) stone was picked up, while it was being carried home. It is so difficult to say without seeing and handling the specimen.

    I have looked up some work on marine fossils from sedimentary rocks in West Falkland but without finding any clues for the identification. My best suggestion at the moment is to contact NERC’s British Geological Survey which compiled the reports on the fossils from the Falklands (downloadable as pdf files on-line). BGS also holds house collections of Falklands fossils and rocks.

    If I have any further ideas I’ll get back to you.


  8. Jessica,
    Thank you very much. I have posted on NaturePlus; I hope we can procure a higher-resolution photo for you. When/if we get a good ID, I’ll post it here.


  9. Hello, Mike

    Thank you. I had already discovered them, saved, and enlarged them – but I am none the wiser.

    A few random, brain-storming ideas. I am, like yourself, keen to know what is the three-dimensional shape of the depressions. The outlines of the depressions are remarkably angular and not always consistent – some looking more triangular and others more rectangular.

    I am still exploring the possibilities related to tripartite or tri-cuspid structures – maybe associated with boring marine organisms. Shipworms, for example, have tri-cuspid shells so back in time there might have been something similar boring into muds and rocks.

    Also, could the depressions be the remaining base parts of longer cavities like boring polychaete burrows? That would tie-in with your idea that the depressions were not on a bedding plane surface. The greater parts of the cavities might have extended through the part of the rock which has been weathered and worn away.

    Not ruling out altogether the possibility of footprints, even vertebrate, though very small. What could have been around at the time? Tetrapods? I’m out of my depth here.

    What type of rock is it and what period? Knowing that fact would help narrow down the range of organisms possibly responsible for the depressions. I think a lot of the sedimentary rocks in West Falkland are Devonian and Silurian.

    Sorry to ramble on. Hope there is something useful in this.


  10. ‘base parts … longer cavities’: I see where you’re coming from, but the idea doesn’t fill me with a Eureka sensation! I guess I would expect to see more of the cavities that happened not to be at the bottom.

    Tetrapods: As good as any of my thoughts, but size is a problem; see the scale on this photo – I don’t know if tetrapods could have been small enough.

    I have asked further questions on NaturePlus; hope we might get an idea if the rock is limestone or something else; and whether the marks extend around the edges and on the other side. There’s not much chance of finding out the age of the rock since its place of collection is unknown (and being from a beach would make it uncertain even if we did know the locality).

    Thanks for your continuing ponderances.


  11. Hello, Mike. Some further thoughts. Sea Lilies can attach to rocks and firmer sediments by means of a structured holdfast at the base of the stem or stalk. The holdfast could leave an impression. One such holdfast is illustrated in Figure 20 on page 16 in the book Fossil Crinoids by Ausich, Brett, Hess and Simms which shows the anchor-like holdfast at the end of the stem of Ancyrocrinus bulbosus.

    There are similarities between the shape shown here and the mystery impressions in the rock – although it is not a match because the indents caused by the Ancyrocrinus holdfast would be approximately triangular with the points radiating outwards, whereas the actual indents in the rock have the points of each cluster directed inwards. However, the Ancyrocrinus holdfast structure has elements that would produce indents of similar overall shape, angularity of outline and edge. The four-part construction of the holdfast could easily result in two, three or four indents depending on the angle at which the holdfast dug into the substrate. The small size of the rock indents could be, in this scenario, attributable to first settlement of small crinoids specimens. Therefore I am wondering if the impressions in the rock were made by some type of Sea Lily attachments.


  12. Hi Jessica,
    That ‘s a good find; thanks; and thanks for the reference; I’m on the same wavelength.
    It would be nice to find some fragments of crinoid in the specimen (especially the holdfasts themselves), but I suspect we won’t; wish we had the outcrop to look at!. …There are some other unexplained marks, perhaps fossils intersecting the surface and subjected to erosion/solution. I can’t equate them with crinoid, though 😦
    I tried Googling “crinoid holdfast” “trace fossils” but didn’t get anything close.
    I did, however, stumble across this photo of ‘barnacle borings’ –
    search for ‘August 7th, 2011’ in
    Keeping fingers crossed, I went Googling for barnacle borings, but came up with almost nothing (adhesive pad marks, yes). I wonder if you have any experience with barnacles in this respect?…
    Another vague possibility is something like Catellocaula vallata (; and more on that in the preceding link (search for ‘Catellocaula ‘). That’s thought to be due to a soft-bodied parasite, such as an ascidian tunicate like the modern Botryllus (paraphrasing).


  13. Looking into sessile tunicates a bit more, I notice that the larvae may have anchoring spikes, in threes – which might leave anchorage marks a bit we need? I have not found any photos, but have a look at this diagram – search for ‘Processes for attachment’ in Look at the top-right&left-hand corners…
    Do the adults also have tripartite anchorages, I wonder?
    I am not having much luck researching that idea, but I wonder if you have some marine bio books that might help?…


  14. Thanks. I do think crinoid attachment is a strong candidate for the impressions. Thanks also for the link to the sponge borings – useful. I’ve not come across ‘barnacle borings’. I am only familiar with what I used to call barnacle scars – usually the cement impression or actual basal plate of sessile barnacles. I would never have thought of anything so soft-bodied at a tunicate/ascidian for creating the impressions!


  15. You are right about the three anchoring spikes on the larval tunicates – they would account for the groups of three impressions but not the fours. I can’t find anything clear in my books about adult tunicate attachment hooks etc – they all sort of gloss over that part – except a mention of stolons that are basal and look root-like and are shown sometimes tri-partite in the diagrams – but they are the structures from which budding occurs and their shape is too amorphous to fit the bill.


  16. I wish my PhD supervisor, Pete Crimes, was still with us; ichnology was his speciality.
    I have emailed another of his students, Liam Herringshaw (Durham), also into ichnology, to see if he can shed any light.
    We are also awaiting answers to the questions I recently put to the original poster, most importantly whether the marks are just on this side, or all round.


  17. Whether they are modern of fossil traces, Liam may have a pertinent opinion.
    If the original poster tells us the marks are all around (and with similar morphology), the implication is that the marks are modern (too much of a coincidence for the surface of the modern pebble to also have been an ancient surface). Otherwise, the marks may be fossil or modern; I would still favour modern because the surface doesn’t look like a bedding surface yet the marks are fairly consistent.
    (Statistically, there’s also a chance the modern pebble was also an ancient pebble, recently eroded from a conglomerate. But that just confuses the picture!)


  18. I have rocks like this with slanted holes of different depths and were found in Belize as artifax from 400bc to 600ad as possible jewelry Amerindian , although they were very primative with jewelry and not much made.Some of the artifacts were found in a ocean grave site with 10 fosselized bodies of natives along with a white fosselized finger bone.I came into possesion in a identified group of scientists and purchased over a three year collection tagged and after a lot of searching and knowing where these came from found the report studies and tests done.


  19. How very interesting to think of people all those years ago using stones burrowed by marine worms as jewelry! It is amazing that you have been able to acquire these cultural artifacts. A friend of mine found on the beach what appears to be an axe-head made of limestone through which a piddock type of mollusc had burrowed and left a hole right the way through.


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