A few Jurassic Coast pebbles

Pebbles from the Jurassic Coast World Heritage Site in Dorset, U.K.

A small basket of Jurassic Coast pebbles. It includes a small heart urchin fossil from Lyme Regis in the middle, and a blue-grey limestone pebble with a white heart-shaped mark from Charmouth. The holes in some of the stones are caused by burrowing marine animals: the large holes, like the ones in the pebble top left, are probably made by a bivalved mollusc called the Flask Shell, Gastrochaena dubia Pennant. The smaller borings in other pebbles are most likely the result of attached mud-tube-living marine worms like Polydora ciliata  (Johnston) in which the acidic waste products dissolve the stone.

The other pebbles include three mottled yellow, white, grey irregular-shaped flint nodules; a pebble with a fossil gastropod; and two strangely patterned stones with reddish designs reminiscent of some of the Chesil Cove pebbles that I featured in an earlier post. I think they are Triassic quartzite pebbles from Budleigh Salterton in Devon and the coloured markings are caused by oxidising iron within the stone.

Revision of a post first published 17 June 2009

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Ancient & modern Polydora ciliata type burrows in Flat Oyster shells

P1090297Blog1 Severe infestation damage by Polydora ciliata type marine worms in a Flat Oyster shell (Ostrea edulis Linnaeus) from Poole Harbour, Dorset, UK, 1989 (1) 

If you think the two pictures in this post look the same, then you are nearly right! They both show the same degree of infestation damage from marine worms of the P. ciliata type – but the image above showing a Flat Oyster shell riddled with u-shaped burrows was dredged from Poole Harbour in Dorset in 1989. The image below shows similar infestation characteristics. However, 1,200 years separate the two specimens since the shell below was recovered from the archaeological excavations of Saxon Southampton, Hampshire, at Melbourne Street.

Although the level of infestation in these individual shells looks alike, the proportion of oyster shells affected in the respective samples was different: with a smaller number of oyster shells like this in the Saxon sample. Examination of many oyster samples from all historical periods during the last two thousand years in Southern England seems to demonstrate that the level and intensity of infestation by marine polychaetes has increased through time and is at its highest level in the twentieth century. It is possible to speculate that this may be, at least in part, the result of present day nutrient enrichment of coastal waters by, for example, run off water from fertilised arable land. 

P1090457Blog2 Severe infestation damage by marine polychaete worms of the Polydora ciliata type in an archaeological Flat Oyster shell (Ostrea edulis Linnaeus) from Saxon Southampton, Hampshire, UK (2) 

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Flat Oyster shells with Polydora ciliata burrows

P1160683aBlog1 Detail of burrows made by the marine polychaete worm Polydora ciliata in a Flat Oyster shell from Whiteford Sands, Gower, South Wales (1) 

Much of the damage sustained by Flat Oyster shells (Ostrea edulis Linnaeus) in life or after, in recent or archaeological specimens, is the result of infestation by worms. Mudtube-dwelling marine polychaete worms such as Polydora ciliata (Johnston) are adept at utilising the nooks and crannies in shells, as well as in rocks and encrusting calcareous algae, for shelter.

The Polydora worms belong to the Family Spionidae which is part of the Class Polychaeta – the Bristleworms. All the species in this group make a u-shaped tube from small particles – usually of mud. The worm insinuates itself and its tube into small crevices in such a way that the two open ends of the u-shape tube open outwards to the seawater. In life, the worm sticks out two long palps which it agitates enthusiastically.

Acidic metabolic by-products from the worm gradually dissolve the shell or stone in which the worm is sheltering. In time this process etches a u-shaped burrow in the hard substrate. These small burrows can be found anywhere over the external surface of the living oyster shell. The presence of the worms and their tunnels does not usually affect the health of the oyster but they can disfigure the shell. In trade terms this means that the oyster cannot be served in its shell in restaurants.

There is a lot more to say on the subject of worm tubes and oysters but I’ll leave that to another day.

P1160682aBlog2 Close-up left valve external surface of a Flat Oyster shell showing damage by the marine worm Polydora ciliata, from the beach at Whiteford Sands, Gower, South Wales (2) 

P1160753aBlog3 View of the entire left valve external surface of a burial-stained Flat Oyster shell with Polydora ciliata borings - from the shore at Whiteford Sands, Gower, South Wales (3)

P1140002aBlog4 Large, thick, old, and stained Flat Oyster shell with moderate marine worm damage - as first seen on Whiteford Sands, Gower, South Wales (4)

P1130999Blog5 Flat Oyster shell (Ostrea edulis L.) in context with a scatter of cockle and mussel shells on the sand at Whiteford, Gower, South Wales (5)

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