Snippet 15: The continuing mystery of the piddocks

The tide is ebbing and, along the inner edge of a shallow channel on the shore, it has deposited a line of offerings, neatly sorted: predominantly mussel shells, some black, some striped, all shining wetly in the October sun; a sprinkling of broken, pale-grey whorls of the Common Whelk; a few pink tellins, their two valves agape; shards of razor shells; the legs of shore crabs, elbows bent, pincers grasping at air.

And the shells of piddocks. There are only a dozen or so, single valves, some broken but some intact. ‘Oh!’, I keep muttering to myself, astonished. ‘Oh, look! Another.’

They should not be there. Piddocks, which are bivalved molluscs, develop and grow inside burrows which they bore within soft rock and shale; they enlarge the burrows but not the entrance, so that once embedded they cannot escape unless the rocks are cracked open. But here on the shore of the Solway I have also found the signs of ancient piddocks – both their shells and their burrows – in newly-uncovered banks of peat, associated with the submerged forest. These peat banks come and go, exposed then hidden again, or broken up by the storms.

Were these piddock shells released from ancient but wave-damaged peat? Are they themselves ancient – or have new peat-banks been uncovered on the bed of the Firth, and recently colonised?

Zirfaea and three Pholas

Nearly all of the shells are of the Common Piddock, Pholas dactylus. But there is an odd one out, a species I have not seen before: it is chunkier, and exactly fits the description in my old Collins Pocket Guide to the Sea-shore: ‘shell oval, solid and coarse with furrow dividing valves into anterior and posterior regions.’ The ‘teeth’ at the posterior of this shell are a little worn, but it’s definitely an Oval Piddock, Zirfaea crispata.

The website of the National Biodiversity Network notes that ‘Single valves [of Zirfaea] regularly wash onto all Dutch beaches, while live animals can be found in washed-up blocks of peat.’

As always, the objets trouvés on the shore pose many questions; our answers can only be informed or optimistic guesses. But to find piddocks always provides a thrill.

See also Snippet 14, Long-lost piddocks and the peat, for a longer piece about piddocks and peat. For much more about the Solway’s submerged forest, see ‘Cold cases: land-scape puzzles on the Solway shore’ .

There is also a chapter on the Solway’s origins – which includes the changing sea-levels, forest, peat and clay, and more – in my book The Fresh and the Salt. The Story of the Solway (Birlinn 2020): for more details and photos, see the website.

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