Oyster is a colloquial term used to refer to different types of sea-water bivalve molluscs. At some point in our childhood, we have all visited a beach and returned home with a pocket full of seashells. Do you remember a white or off white shell, slightly convex, softer on one side with rings on the other side? That’s the bivalve I am referring to. While the first bivalves conquered the seas about 500 Million years ago, it took another 250 Million years for the first appearance of an oyster at approximately 252 Million years ago, around the same time the first dinosaurs appeared in the fossil record.
What is the first thing that comes to your mind at the mention of the word oyster? I bet it is either the creamy, buttery and salty seafood delicacy; or the smooth, lustrous, and iridescent queen of gemstones - pearl. In this article, I shall highlight the latter aspect associated with oysters. It is essential to understand that the oysters we consume (edible true oysters) are quite different from those that produce the coveted pearls (pearl oysters).
In the southern part of the Indian state of Tamil Nadu, nearly 600 kilometres from the capital city of Chennai, lies a city which has been traditionally referred to as ‘Pearl City’ - Thoothukudi (anglicised: Tuticorin). This place has been famous for its pearl production since the birth of the Pandya Dynasty about 2000 years ago when Korkai served as the capital city. The natural pearls sourced from the Gulf of Mannar were considered to be among the best Orient Pearls; more valuable than those sourced from elsewhere. It is quite ironic that ancient Tamilians exported just the ones that everyone discarded, which had a huge demand in the foreign markets. So, just try and imagine how magnificent the best of the pearls would have looked like.
The process by which the natural pearls form is quite intriguing. When foreign materials such as grains of sand intrude into an oyster shell and settle in the interior, it activates a defence mechanism. To relieve itself of the irritation, the oyster secretes a mixture of organic and crystalline substances known as Nacre (a.k.a mother-of-pearls) to cover the irritant. As multiple layers are deposited over it, the hard and glistening pearl is formed. Ironically, the creation of a pearl begins with an accident. The pearl has the same chemical composition as that the oyster that produces it - Calcium Carbonate (CaCO3). The Calcium Carbonate adopts the structure of Aragonite, the hard, dense and the most stable form at high pressures.
Pearls, natural or cultured, are famous for being the only gemstones produced by living organisms. Thus, their availability is limited by the availability of the oyster population in the fishing area. The genus of Pinctada comprises seawater oysters belong to the family of feather oysters and are popularly referred to as Pearl Oysters. There are very few oysters that are of interest today - P. radiata (Gulf pearl oyster), P. margaritifera (black-lip oyster), P. maxima (gold-lip & white-lip oyster), P. mazatlanica (Panama pearl oyster), P. fucata/imbricata (Akoya pearl oyster), and P. albina (Shark bay oyster). Over the years, natural pearls' availability has become a rarity owing to the diminishing oyster populations. Particularly in the Indian Ocean, changes in the environment and modern fishing equipment usage have resulted in the clear decline of the pearl fisheries. In the Gulf of Mannar, the last fishing event dates back to 1962.
A solution to this problem is Pearl Culturing. To obtain natural pearls we harvest adult oysters that have already produced a pearl. On the other hand, pearl culturing involves capturing the young oysters and introducing a foreign particle such as a piece of an oyster shell and letting them mature under observation. The young ones can either be collected from the wild during natural spawning seasons or be artificially spawned in hatcheries. Akoya pearls, freshwater pearls, South Sea pearls, and Tahitian pearls are some of the traditionally cultured pearls that come in a variety of shapes, sizes and colours.
The development of this technique is credited to 3 Japanese men - Tatsuhei Mise, Tokishi Nishikawa and Kokichi Mikimoto, who independently experimented with pearl oysters by introducing an oyster epithelial membrane. No matter how close the cultured pearls get to their natural counterparts, an X-Ray analysis clearly distinguishes the two. The development of this technique resulted in good quality pearls becoming easily accessible and affordable for all. Ecological changes and environmental deteriorations do not affect cultured pearl production as much as it used to affect natural pearl production. Today, large-scale fishing of natural pearls is rarely employed with the cultured pearl industry overshadowing the global pearl production.