Common Piddocks – rock-boring molluscs

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Common Piddock dorsal view

Here are some close-up photographs of the Common Piddock – Pholas dactylus Linnaeus (Mollusca; Bivalvia; Pholadacea; Pholadidae) showing details that are important for its specific identification. The specimen in the first three images still has the dead animal within the shells. This is one that I collected from those I found at Monmouth Beach in Lyme Regis (see the previous post) where a slab of shale, complete with the rock-boring molluscs still inside the burrows, had been thrown up on the shore by stormy seas. The empty shell shown in images 4 – 8 is a beach-worn specimen picked up on Knoll Beach at Studland a few weeks ago.

Although the length of the shell of the Common Piddock can be up to 15 – 24 cm or 6 inches, the examples shown here are smaller – with an  actual size of shell for the Monmouth Beach specimen of 50mm, and 108 mm in the shell from Knoll Beach.

Full details of the shell characters used for identification can be found in the references given below. This bivalve mollusc bores into sand, peat, marl, wood, shale, slate, chalk, limestone, red sandstone, schists, firm clays, and even thick old oyster shells, from low on the seashore to depths of a few fathoms. It occurs in The British Isles from Kent along the south and south-west coasts, including South Wales, and as far south as the Atlantic coast of Morocco. Of particular interest is the phenomena of phosphorescence or luminescence exhibited by the living animal which has has bioluminescent properties and glows with a blue-green light in the dark.

Earlier posts on Jessica’s Nature Blog that refer to the holes in rocks and pebbles made by piddocks and other seashore creatures include:

Rocks with holes made by Piddocks – Part 1

Beach Stones with Holes at Worms Head Causeway

Peat ‘pebbles’ with piddock holes

Pebbles with holes made by boring sponges

Pebbles with holes made by tube worms

Pebbles with holes made by sea creatures

Driftwood with holes made by Gribbles & Shipworms

Benjamin & the pebble full of holes

Shells with holes made by boring bivalves

A rocky beach near Portland Bill


Tebble, Norman (1966) British Bivalve Seashells – A Handbook for Identification, published for the Royal Scottish Museum by HMSO – Edinburgh, 2nd Edition 1976, ISBn 0 11 491401 X, pp 175 – 180. [Out of print but now available on CD from Pisces Conservation Ltd.]

Hayward, P. J., & Ryland, J. S. (Eds.) (1995) Handbook of the Marine Fauna of North-West Europe, Oxford University Press, 1998 reprint, ISBN 0 19 854055 8 (Pbk), pp 619 – 622. Still in print and available from Amazon and other booksellers.

MarLIN about Pholas dactylus

Wikipedia about Pholas dactylus

World Register of Marine Species about Pholas dactylus

Marine Species Identification Portal – about Pholas dactylus

Common Piddock right side view

Common Piddock ventral view

Common Piddock exterior view left valve

Common Piddock exterior view left valve

Common Piddock anterior end left valve

Common Piddock interior view left valve

Common Piddock shell showing umbonal reflection


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9 Replies to “Common Piddocks – rock-boring molluscs”

  1. Hello Jessica! I have been searching rather fruitlessly for someone capable of identifying what animal/mollusk/worm/fungi/fish/plant/alien? is creating the rock formations I have found on a small area of my family farm in Fredericksburg, Virginia. After finding and reading your blog, I wonder if somehow the rock-boring species you mentioned was brought to the farm during the last century. Please, as soon as you are able, send me a message at so that I may send you some pictures and see what you think..

    I truly hope you somehow are able to get this comment and respond to my email.. the answer to my curiosity and questions has been weighing me down for some time.

    Liked by 1 person

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