It is common to find pebbles and rocks which have holes in them when you walk on the seashore. These holes are frequently the result of various marine invertebrates that have burrowed into the rock. The larger, nearly round, holes about a centimetre or so across are often made by bivalved molluscs called Piddocks – these have a special shells and processes for mechanically boring into the rock.
Most commonly, you find stones, or shells, or bedrock, with holes – but the inhabitants have long disappeared. Other times, you glimpse the shell within the burrow but removing it is very difficult to do without totally destroying the shell. If you are really lucky, then you might spot a colony of living piddocks in their burrows at the lowest point of the tide where they will be sporadically squirting water out of their protruding siphons.
In any event, it is quite difficult to see all the details of the shells and thereby identify the creatures to species. Shells that occasionally get washed up on the beach may be abraded and worn, and vital parts for identification are always missing. So, I was delighted last Sunday to see slabs of broken shale, complete with piddocks in burrows (albeit dead specimens), washed up on the shingle of Monmouth Beach in Lyme Regis which is in Dorset, England.
This provided a wonderful opportunity to see what these boring bivalve molluscs and their shells really look like and observe the particular shell features that adapt the organism for this strange lifestyle – but which are normally missing from beach worn specimens on the strand-line.
As far as I can make out, all the specimens that I photographed on the beach were the Common Piddock (Pholas dactylus). I took a few home to clean them up and photograph them in better conditions. The light at the time was very poor, winds were very strong making it a problem to keep the camera still, and the air was laden with salty moisture that persistently misted the camera lens. I’ll show the photographs I took at home in the next post. Meanwhile this post will focus on the piddock shells in situ.
The shell of the Common Piddock is elongate, roughly elliptical and can grow up to 150 mm long – although the specimens I saw were a lot smaller. The anterior (front end) of the shell has a strange beaked appearance and surrounds a permanent pedal gape through which the muscular foot can be extended. The surface sculpturing at the front end of the shells consists of many short, sharp spines that develop at the junctions where the concentric ridges of the valves intersect with the radiating ribs. The spines enable the mollusc to use the shell like a rasp to file away the rock (or shell, or wood, or peat) as it bores down and creates its personalised living accommodation within the protective confines of the substrate.
This boring mechanism is aided by several extra features of the shell – accessory plates on the outside and a long curved process called the apophysis on the inside. The external accessory plates are the paired protoplax, the paired mesoplax, and the single metaplax. Rhythmic contractions of the muscles attached to these plates, and to the apophysis, enable the mollusc to perform a twisting action that aids the drilling process. The whole procedure is further enhanced by a forcing outwards of the two valves against the rock walls using hydrostatic pressure (sucking water in through the siphon and holding it temporarily to create pressure).
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Jessica's Nature Blog 
What an amazing creature! Thanks for posting.
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Thank you!
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Jessica. I did a drift dive off the Purbeck coast and bought home a piece of shale with a hole in out of curiosity, I noticed the sea bed covered in pieces of rock covered in holes. My internet search took me to your blog which has answered all my questions and more and has enlightened me so much. Thank you.
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Thank you, Stuart. I am glad you found the information you were looking for.
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Husband and I found a beach in the Upper Peninsula of Michigan (USA) where the beach was covered with these rocks. Some of them so large that you were unable to move them.
One of our children placed one of these rocks into the face of her fireplace.
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Thank you. They are apparently found in many places in the world. I have a selection of various rocks with animal burrows on the hearth of my own fireplace.
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TY you solved a family rock mystery in Maine USA!
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Hi, Martha. I have sent an e-mail to you.
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Well hello there!
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Question – do you think shale with piddock holes or similar qualify as holey/hag/adder stones, usually defined as a pebble with a natural hole through it? I have several in my collection, but the clay/shale/slate pebbles are so much more fragile than their flint counterparts from down south.
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Hello, Mal. Holes made by piddocks cannot be called Hag Stones. Hag Stones are a completely different physical or chemical phenomenon that is found in hard rock types such as flint – they do not originate in the same way as those which have been made by invertebrate animals in softer rocks.
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Hi Jessica,
I’ve been reflecting on your explanation of the holes in stones ever since I stumbled upon your wonderful bog a decade or so ago.
I draw an analogy with spiders which make their burrows in banks and then produce a web which acts as a funnel into the hole.
Am I correct in imagining that during the millennia of hole boring these mollusc homes would have been excavated in submarine vertical rock faces? Or perhaps horizontal ones?
That generations of common piddocks or similar species would have been spawned and returned to the same ‘nest’ to continue the boring process to create a bigger, deeper, safer dwellings?
That at some point violent tectonic shifts and collisions would have destroyed the holey cliff face and broken it up into fragments of varying sizes. These stones, some broken across a piddick tunnel, had circular holes all the way through. Others would have been the end of a burrow, hence holed only partially? And some would be at intersections thus multi-holed?
Centuries of tidal attrition across beaches would have rounded off these stones to varying degrees.
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Hello, Hamish. Thank you for commenting. I am sorry that it has taken such a long time to reply to you but sometimes life gets in the way.
You ask first if the piddocks would have excavated their holes in vertical submarine rock faces or horizontal ones. The answer is probably both – but more often into horizontal surfaces if the modern examples are anything to go by. I have written so many posts about piddocks and similar rock-boring molluscs on this blog over the years with accompanying photographs and the burrows are almost always downwards into a horizontal surface but occasionally I have seen into vertical ones. The vertical surfaces have been shallow ones and not vast cliff faces. The Pholas dactylus species grows in the shallow sublittoral down to around 35 metres. But when you see them exposed at low tide there are only limited vertical surfaces colonised. As this piddock only occurs along British southern coasts, it is difficult to envision vast cliff faces under water in the English or Bristol Channels.
You ask if piddocks or similar species return to the same “nest” to continue the boring process and make bigger deeper dwellings. I think this is unlikely to occur in the way you suggest. I have not read anything mentioning the re-use of old burrows by subsequent generations. I would imagine that a process like this this might be hindered by the fact that the shell of a dead piddock tends to remain in the burrow because the method by which the piddock drills the hole means that the entrance hole is smaller than the final size of the shell. A living piddock can never leave the burrow it has made for itself. It is probably in the nature of this creature, when spawned and settled by a byssal attachment on a rock or similar hard surface, to gradually create its own burrow customised to its increasing growth. It would be more accurate to think that newly settling spawn would find the same kind of surfaces or substrates ideal for settlement rather than any any attempt to recolonise burrows created by past generations. The burrows can however be colonised by other smaller creatures making use of the shelter they provide.
There is a question about how old these colonies of piddocks might be. I have not so far found evidence or research to indicate how frequent this species was in the distant past. When you find a bed of piddock holes exposed at low tide, you know immediately if it is a modern colony if the bivalves are alive and maybe spouting waste water. Holes with empty shells could be any age perhaps. Some of the exposed beds I have seen in clay or peat beds at low tide on Gower in South Wales probably date from about 10,000 years ago. I haven’t found any references to piddock shells from earlier years – but then the shells are very fragile and would not survive long in the geological sense.
The burrows would of course remain in the rock but you would need to know not the date when the rock was formed but when the rock was exposed for colonisation.
Unlike yourself, I cannot imagine vast cliff faces underwater, riddled with piddock holes, which are then broken up by violent tectonic shifts. It would seem more likely that the relatively soft strata in the intertidal and shallow sublittoral zones where piddocks like to burrow would have been gradually eroded by wave action and broken up into pieces or slabs. This is featured in one of my blog posts where I describe how slabs with living piddocks still in their burrows were broken up by stormy conditions some years ago at Lyme Regis.
Once a slab of rock or similar substrate has broken away from the layers at the edge of the sea, it will continue to be weathered by tides, rain, and frost, and break up further, and become more smooth in outline.
Where a burrow has extended downwards through a succession of rock layers, those layers may eventually separate – each section bearing a part of the original burrow. The burrows come in different sizes depending on the age of the piddock on death. The burrows are not the same shape or size from top to bottom, and not necessarily equal in width from entrance to base, not cylindrical, because of the shape of the creature. This means that the holes that remain is small pieces of the original rock layer, are of different circumferences and depths. Stones (pebbles or rocks) that are perforated right the way through by the hole would have originated from the outermost section or the piddock burrow or entrance. Holes that terminate with a curved base, represent the base or innermost part of the burrow. Stones with more than a single hole represent a section of the original colony of piddocks. And since burrows can be created from the horizontal surface downwards through a rock layer or series of layers, as well as horizontally from the vertical faces at the exposed edge, holes can intersect, with new ones cutting across old ones.
Complicating the interpretation is the fact that many species of mollusc or other marine invertebrate can burrow into the same rock surface – resulting in a variety if shapes and sizes that are nothing to do with piddocks (Pholas dactylus).
In this post link, there are list of some of the many articles about the subject:
And these links will take you to several other posts about piddocks added to the blog at a later time.
Hope this helps answer some of your questions.
Best wishes
Jessica
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Hi Jessica,
I really appreciate you taking the time and trouble to reply in such detail and so clearly.
The revelation about a burrow being the lifetime work of a single piddock is so interesting as is the fact that they live and die in it.
So this adds another layer of meaning to my âTidal Drillâ (named before I discovered the true process through youâ¦but too late to change).
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With many thanks and best wishes,
Hamish
07977-269778
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