Clumped oyster shells (1)

Sometimes the beachcomber on  British seashores, or archaeologists excavating food middens on UK sites, find shells of the Flat Oyster (Ostrea edulis Linnaeus) joined together in clumps. Such finds have a special significance for the interpretation of the oysters’ natural history and for the understanding of the site. The presence of clumping in oyster shells provides a strand of evidence for the nature of the oyster bed or population from which the oysters originated.

Archaeologists are always curious to know whether oysters eaten in the past by the occupants of a particular archaeological site have been merely fished or farmed and cultivated. In many parts of the world, from very early times, oysters have been farmed using various methods of differing levels of complexity. If it can be demonstrated that oysters were cultivated, then it is possible to infer something about the level of exploitation of this marine bivalved mollusc by the people who ate the oysters.

The photographs immediately above and below show the same clump of oyster shells in which four individual valves are joined together: internal view above and external view below. These beach shells are coloured by deep burial in the seashore sediments to a black, metallic, pewter-like finish. The large left valve has three smaller oyster shells attached to its inner surface. These attached shells are barely discernible from the outer surface view. 

A simple traditional way of farming oysters has been to fish for small, young oysters from wild, natural beds in the sea. These young oysters are then relocated to shallower waters, often in places where freshwater flows into them, and where nutrient levels are much higher than in the deeper waters where they are found naturally. The outcome of this ‘transplanting’ is that the meat of the oysters grows much bigger and more palatable. Additionally, the oysters are more accessible for harvesting.

However, when the natural beds are dredged for oysters to stock the relaying beds inshore, the young oysters are frequently, maybe mostly, stuck together in clumps of both living individuals and empty ‘dead’ shells. The reason for this is that newly-developing young oyster spat, after their initial exploratory swim around, prefer to settle where other oysters live – and usually upon other older individuals or empty oyster shells.

However, by settling in groups that are closely stuck together, the oysters may not able to develop their full potential for growth because they are all competing for space and the resulting shells are often mis-shapen. So oyster-fishermen would take a special large knife called a cultack to carefully prise apart the individual oysters before relaying them separately in new, shallower-water grounds. This process enabled each individual oyster to grow without hindrance to its normal shape. [Of course, these days young oysters can be grown in the laboratory floating in a nutrient medium before ‘planting’ out as separate oysters under protection; and these also can potentially achieve full potential growth in shape and size].

All the above contributes to one of the explanations why oyster shells, found on either the beach or in archaeological excavations, which are still attached in clumps or with many small spat oysters covering their surfaces, are most likely to have originated in a wild or natural oyster bed. This oyster bed would be one where the population was successfully breeding or self-propagating – as opposed to a farmed or relaid population where conditions are good for fattening the meat but not usually optimal for breeding.

Above and below is another group of attached oysters. This time they are stained orange because they have only been shallowly buried in beach sediments. Again there are four oyster shells but this time three equal sized valves are attached to the outer surface of a larger left valve. This means that the three newcomers may well have settled simultaneously on a living oyster specimen. This contrasts with the specimen in the top two post pictures where the three smaller shells of different ages and sizes settled on the inner surface of an empty oyster shell. 

The object upon which the minute spat oysters settle can be referred to as cultch. This is frequently another oyster shell but can be other types of shell and object – I’ll talk more about that in a later post.  

The final specimen in this post comprises just two oyster shells cemented together. One has settled on the smooth inner surface of an empty, thick and old oyster left valve shell. Both shells have the blackened gun-metal appearance of deep burial in the beach sediments. The clump is shown from the inner surface, the outer surface, and side view in the following pictures.

The older cultch shell bears evidence of several types of encrusting or infesting epibiont organisms which have also found it a suitable settlement substrate. I will talk in more detail later about the way these kinds of clues can help to build up a picture of the environment in which the oyster was growing, and sometimes even provide evidence for the region or location from which the oysters were collected. This is important when considering ancient shell remains. It potentially allows archaeologists to think about shellfish trade and economy in the past, in periods and places for which there is no written record.

The oyster shells illustrated in this post were found at Studland Bay and Gower beaches.

Other posts about oysters in this blog include the following and more:

Black oysters at Rhossili Bay

Colour variation in oyster shells (1)

Oyster shells from Studland Bay

Oyster Thieves at Studland and Ringstead Bays

Fossil oysters from Ringstead

More about fossil oysters from Ringstead Bay

More clumped Flat Oyster shells

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