Rocks with holes made by Piddocks – Part 1

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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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14 Replies to “Rocks with holes made by Piddocks – Part 1”

  1. 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.


  2. 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.

    Liked by 1 person

  3. 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.


  4. 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.

    Liked by 1 person

  5. 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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