The flat oyster, Ostrea edulis Linnaeus, is not as common as it used to be – but out in Poole Bay, and possibly in parts of Studland Bay where there are firmer substrates, we are lucky to have one of the few natural populations of oysters remaining in Britain. By natural, I mean that the population is a native British species; is a breeding and self-perpetuating one; not one that has been transplanted from elsewhere; or seeded with young oysters cultivated in a laboratory. There is historical and archaeological evidence that it has existed for at least two thousand years – and probably from time immemorial judging by the number of fossil oysters that can be found in shoreline rocks along the Jurassic Coast of Dorset.
Oyster shells frequently wash up on the sandy beach at Studland Bay. Usually the thicker, older shells are the ones that survive the persistent pounding by the surf, the periodic burials, and the exposure to the elements. This means that the shells are often worn smooth. The shell in the photograph above has not been flotsam for too long – relatively speaking – because it still shows a lot of its original features.
Oysters are bivalved molluscs in which the two valves (shells) look very different. The left valve has a shallow cupped shape. The outer surface is broadly ribbed from the ligament or hinge end to the outer edge. There is also a series of concentric ‘frills’ which are consecutive growth shoots. You can see these ‘frills’ in the close-up shot at the top of the post. The sharp edges show that this oyster shell has not been around long enough to get broken or eroded.
The picture above shows the smooth inside surface of the same left shell. It has a single central adductor muscle scar (unfortunately, obscured by the sand). The adductor muscle is what the living oyster uses to keep the two shells tightly closed. The single muscle, and its single scar on the shell, is one of the features that distinguishes the Flat Oyster shell from a Saddle Oyster shell. Have a look at the previous posting on Saddle Oysters from Studland Bay to see the differences.
Pictured above is an example of a right valve from a Flat Oyster. This one was actually dredged from Poole Bay a few years back by local oyster fishermen. Unlike the left valve of this species, it does not have radiating ribs or frilly concentric growth shoots. It does, however, have distinct concentric growth lines and bands. Also, in this dredged shell, in contrast to the washed up shell from Studland, the outer surface has a fine dark outer horny layer which is typical of the right valves of fresh specimens.
A close up look at the right valve reveals that it is encrusted by Bryozoan sea mat. The small holes and tunnels are caused by marine polychaete worms that live in mud tubes in the crevices of the shell surface.
The final picture shows a very thick old left valve of a Flat Oyster, also dredged from Poole Bay. On the smooth inner surface, acorn barnacles have begun to colonise the empty shell after the death of the oyster. Similarly, the little black dots and threads are the byssus threads by which mussels attach to hard surfaces – the fragile young shells have not survived. Around the edges, between the layers of shell, are numerous small U-shaped tunnels which have been created by the acidic waste products of marine worms like Polydora ciliata (Johnston) dissolving the shell while sheltering in the crevices during the life of the oyster.
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