Seatown Rock with Piddock Holes

A contributory factor in the erosion of the beach rock at Seatown in Dorset is the burrowing activity of the marine bivalve mollusc called the piddock. Low on the shore millions of holes in the soft calcareous mudstones are evidence for the burrows made by Pholas dactylus. The holes are almost circular in shape reaching up to two centimetres in diameter,  and can occur as a scatter or as dense populations wherever the rock remains wet between the tides. They seem to prefer the darker layers rather than the alternating light layers – although they are found in both. The rock on the east Seatown shoreline is composed of alternating almost horizontal layers of pale (carbonate-rich and carbon-poor) mudstone, and darker (carbon–rich and carbonate-poor) mudstone from the Belemnite Marl Member of the Charmouth Mudstone Formation.

Where successive generations of this boring mollusc have colonised the strata, the mudstone has been reduced to an irregular honeycombed mass. Most of the holes seem unoccupied and small pieces of orange-coloured gravel have filled them. In some burrows the empty white shells of the piddock can still be seen. Some of the burrows are undoubtedly still occupied but I did not have an opportunity to locate any for photographs since the area was only exposed for half an hour. The shells of the living animals may not have been visible because they tend to lie deep within the burrow but the living specimens can often be detected by the fact that their siphons extend from the shell to the surface and these periodically squirt out water during low-tide.

Pebbles and beach stones which have neat circular holes in them are frequently wave-washed and beach-tumbled pieces of rock that have broken away from intertidal rock layers that have been riddled with burrows made by rock-boring molluscs such as piddocks, in the way shown in these photographs from Seatown in Dorset, England, along the World Heritage Jurassic Coast.

7 thoughts on “Seatown Rock with Piddock Holes

  1. Interesting. What do you suppose accounts for the larger holes shown in the foreground of Seatown Rock with Piddock Holes 9? Is it just where the smaller holes have run together?

  2. Yes, you are right. The largest irregular shaped holes are an amalgam of several burrows created one after the other at varying angles to the surface. This means that the shaft of a new burrow intersects old empty ones. All is revealed when the surface of the rock wears away.

  3. One hole has been made by a single piddock in its lifetime – I am not certain how many years that would be. The piddock occupies the burrow that it has been steadily enlarged throughout its lifetime and it is a “prisoner” within it because the entrance hole is smaller than the width of the adult shell. Its only connection to the outer world is via two tubular siphons. When the piddock dies, its shells remain in the hole. Subsequent generations of piddocks may create burrows at angles that cut across the shaft of the old burrow, making large irregular cavities which are only revealed when the outer layers of rock are worn away.

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