A Modern Oyster Shell Midden

Oyster shells in a heap at Whitstable for use as cultch for collecting oyster spat

I have spent a large part of my life studying oyster shells that have been excavated on archaeological excavations of sites in the British Isles dating from the last two thousand years. The shells have been found in a variety of contexts including middens which are heaps of kitchen waste including oyster and other marine mollusc shells. Strangely, I had never seen a modern equivalent until this week when I visited Whitstable on the north Kent coast. I had read all about the famous Whitstable Oyster Fishery but somehow had never got around to visiting the place.

I did not choose a very good day to see Whitstable for the first time. It was very cold, dull, and windy with the choppy sea high up the shingle beach and salt spray continuously misting my camera lens. Nonetheless, I had a great time and made some interesting discoveries – not least of which was my desire to go back ago and explore some more.

Almost the first thing I saw when I hit the shore after a coffee at the Horsebridge Gallery, was the building of the historic Royal Native Oyster Stores belonging to the Whitstable Oyster Company. It included a seafood restaurant closed at the time, and outside were two substantial heaps of empty oyster shells – middens – one against a wall and the other on the shore. A casual observer might wonder why mounds of empty shells had been left lying around and not properly disposed of. There is a good and logical reason.

The shells are being kept for cultch. The youngest form of an oyster is a free-swimming larval stage which needs to find somewhere suitable to settle down and grow. It is very particular about the type of object on which it will land and attach its embryonic shell. It has a limited time, maybe just a couple of weeks if the temperature is optimum, to find just the right place. It likes all sorts of hard substrates but it likes oyster shells best – sometimes empty ones and sometimes live ones.. Traditionally, this preference is catered for by the oyster fishermen who put down quantities of empty oyster shell as cultch on the seabed to encourage the settlement of young spat oysters. They also string old shells together to act as suspended spat catchers in the water. It is interesting to see these historic practices still in operation in an age when many oysters are bred in laboratories before being grown on in metal mesh bags on trestles covered by the tides. They use both old and new methods here.

The pictures in this post show more than one type of oyster shell. The Native British Oyster, also known as the European Flat Oyster (Ostrea edulis) is the type for which Whitstable is most famous. They also use the Pacific or Rock Oyster (Crassostrea gigas) which grows faster and is therefore a good commercial proposition, especially when in recent times the numbers of our native species have reduced.

Colour variation in oyster shells (1)

P1070625aBlog1 Colour banding in shell of Common Flat Oyster from Whiteford Point, Gower, South Wales. 

The most variable of all bivalved marine molluscs, the shells of the Common or European Flat Oyster, Ostrea edulis Linnaeus, reflect the differences in -and changes to – the local environment in which the animal has been living and growing; as well as the conditions that have affected them after death.

General details about Flat Oysters have already been discussed in the earlier post Oyster shells from Studland Bay. In this post I am going to talk about colouring in flat oyster shells. I have also briefly touched on why oyster and other shells are sometimes black. You can link to the post Black oysters at Rhossili Bay here.

P1070154bBlog2 Orange coloured Flat Oyster shell, Ostrea edulis Linnaeus, left valve outer surface on the beach at Whiteford Sands, Gower, South Wales, UK. 

Just to recap what was said in the earlier post, the orange and black colours of empty oyster shells that you find on the beach are stains on the shell that result from burial in the sand and mud of the seashore. The deeper and the longer the shell has been lying in the sediments, the blacker it will be stained. Shells buried nearer to the surface are likely to be stained orange. Because wave action can turn the sediments over, together with the objects held in them, you sometimes find shells that are both black and orange. Shells that have only been partially buried – with a part remaining above the surface -may be partly stained orange with some of the shell remaining the original colour. 

The precise depth in the sediments at which the staining will change from orange to black will depend on how far down in the loose sediments oxygen survives. As depth increases so does the likelihood of oxygen depletion. Surface layers are oxygenated but deeper layers have no oxygen and are termed anoxic. The depth of the oxygenated sediment will vary from one part of the beach to another, and from beach to beach, and from time to time. However, the boundary is usually between 5 and 15 cm down and is marked by a thin grey layer.

Bacteria live attached to the sediment particles. The kinds of bacteria vary according to whether the sediments are aerobic or anaerobic. Some bacteria can only live in the presence of oxygen; others thrive in the absence of oxygen. In the upper layers there is enough oxygen to reduce all the waste products of the micro-organisms; but lower down it is different. The activities of the bacteria react with iron compounds occurring naturally in the sediments. Anerobic bacteria use fermentation or other processes to break down organic compounds and create hydrogen sulphide, ammonia or methane as by-products. The hydrogen sulphide then reacts with the iron in the sand resulting in black iron sulphides that cause the black staining.

When the black iron sulphides are moved upwards through the sand by burrowing animals or agitation by wave action, the oxygen between the sand grains in the higher levels converts them to ferric oxide which is yellow and stains buried shells orange.

The large old orange-stained oyster shell above was photographed on the strandline at Whiteford Point in Gower, South Wales.

The thick old blackened oyster shell shown below was washed up at Rhossili Bay and is pictured on a piece of driftwood from the strandline.

P1060830bBlog3 Black left valve shell of Flat Oyster (Ostrea edulis L.) interior view, on driftwood at Rhossili Bay, Gower, South Wales, UK. 

Sometimes parts of oyster shells may be coloured purple. This colouring is usually confined to patches or concentric bands. Unlike the black and orange stains which occur after the death of the animal and are due to burial conditions, pink or purple patches are part of the oyster’s natural and original colouring and develop during the life of the animal – although not all oyster shells have these colours.

In the picture below you can see an oyster shell that is mostly stained black but has purple patches as well. This specimen was seen in the surf on the water’s edge at Rhossili Bay. The pink or purple colouring survives after death and burial. The reason that the pink and purple colours remain is that they result from pigments that are bound up in the shell structure.

Blackened left valve shell of Flat Oyster (Ostrea edulis L.) with patches of purple pigmentation, in the surf at Rhossili Bay, Gower, South Wales.  

The old heavy oyster shell illustrated below was seen on Whiteford Sands, Gower, and it is coloured both black and orange with purple concentric bands and patches.

P1110825aBlog5 Orange, purple, and black-coloured left valve shell of Flat Oyster (Ostrea edulis L.) on the strandline at Whiteford Point, Gower, South Wales, UK. 

The reddish, pink or purple pigments are thought to have been derived from the food of the living oyster. It is a filter feeder, eating small particles including various types of plankton such as the microscopic organisms that constitute red tides.

As the animal grows and lays down new layers of shell (a cross-section through the shell would look a bit like flakey pastry because of all the layers), the pigments are enmeshed or incorporated into the actual crystal structures. Pigments have even been preserved, wrapped up in crystals like this, in fossil brachiopods and fossil bivalves – and have been successfully extracted for the study of the organism’s diet by palaeontologists.

The final photograph of the post shows a fine example of the survival of this purple pigmentation in bands along the outer edges of some of the annual summer growth shoots. Growth of shell in Flat Oysters is greatest in warmer weather when food is also more abundant. Believe it or not, this fresh-looking, unworn, sharp-edged oyster dates from the 9th century. It is nearly 1,200 years old and was recovered during archaeological excavations of an ancient oyster midden underneath the Old Town Cellars at Poole in Dorset, UK.

P1090120aBlog6 Colour banding in a 9th Century left valve shell of Flat Oyster (Ostrea edulis L.) from an archaeological excavation of an oyster midden beneath the Old Town of Poole, Dorset, UK. 

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