An Oyster Shell with a Black Pearl

I found this shell on Rhossili Beach. Oyster shells often wash ashore there. The European Flat Oyster used to grow in abundance around the Gower Peninsula in South Wales and was commercially fished until about the 1940’s when stocks declined to such an extent that it no longer remained a viable proposition. They are presently trying to re-introduce the oyster fishery.

Fresh shells brought up by the tide would seem to indicate that Ostrea edulis still lives and breeds in the locality. The older shells have evidence that they have been around for a long time, possibly decades. Many are very thick showing that they lived for a long time. Commercially fished or cultivated oysters are usually cropped at three or four years before the shell has achieved its maximum growth and are therefore relatively small and thin. Left undisturbed, O. edulis can live for fifteen years or more. However, after a certain time, the diameter of the shell more or less ceases to increase and the animal’s energy is concentrated on thickening rather than widening the shell.

The longer the oyster lives, the greater the possibility of its shell assuming unusual shapes and abberations. Some of the mis-shapes result from the animal’s defensive reaction to infesting or encrusting organisms on or in the protective shell. Occasionally, irritation of the fleshy interior by foreign objects causes changes in the way the shell is laid down by the internal nacreous layer. This is the way pearls are formed. You may be surprised to learn that commercially fished pearls, and cultivated pearls, do not actually come from oysters. The Pearl “Oyster” – is a mis-nomer. It is in fact a Pearl Mussel. The Latin name for the Pearl Oyster species (of which there are several) is Pinctada. and the species belongs to the Family Pteriidae a close relative of the true oysters – the Ostreiidae. When Julius Ceasar came to Britain with the invasion and extolled the beauty of British pearls, which he then exploited and exported back to Rome, he was referring to pearls from freshwater mussels Margaritifera margaritifera (Linnaeus).

It is not common to find pearls in true oysters like Ostrea edulis but they do occur. They are not considered to be as valuable as those from mussels and in some cases are prone to disintegrate with time. I have seen good examples in the museum at Colchester, Essex, which is an area reknowned for its oyster fishing industry dating back to at least Roman times.

Pearls as we commonly know them usually form as distinct separate bodies within the fleshy mantle of the oyster. Occasionally, the pearls are attached to the inner nacreous layer of the shell. They can be attached by a short stalk. That is what we have here in this beach-combed oyster shell. The “pearl” is attached to the inner surface of the right valve of the shell next to the pale kidney-shaped adductor muscle scar. [The strong adductor muscle joins the two valves in life and is used to close the shell when necessary. The default position of the oyster is to have the valves open and apart and it is automatically kept in this position by the ligament at the umbonal or hinge end of the shell.] The black colour of the pearl and the shell itself is the result of spending a considerable time buried deep down in anaerobic sediments. Black oyster shells are common on Gower beaches.