Pearl oysters are two shelled aquatic creatures. Due to their two shelled body, they are called Bivalves. They fall in the phylum Mollusca. The pearls from fresh water are generally obtained from a mussel called Margaritifera margaritifera (Critically Endangered by IUCN red list) while from the marine waters, they are obtained from oysters of the genus Pinctada which belong to the family Pteriidae. Apart from these, there also exist other pearl forming bivalves that have the ability to produce “Nacre” which is also referred to as the “Mother of Pearls”. For example, most of the cultured pearls in America come from Amblema species where as in Asia these are a product of the Hyriopsis species of the Unionidae family.
When an irritant, such as a grain of sand or a parasite etc. makes its way into the space between the shell and mantle of these molluscs, they create a pearl sac to seal off that irritant. So pearls are a defence mechanism against potentially threatening irritants. In another words, pearls in molluscs are the result of an immune response similar to the one which takes place in the human body – The capture of antigens by antibody. This defence saves any attack from the outside from injuring the mantle tissue. The mollusc mantle wraps the irritant in multiple layers of calcium carbonate in the aragonite form which is a mixture of aragonite and calcite cemented together by conchiolin. This combination of conchiolin with aragonite is called ‘Nacre.’ Nacre can be generally referred to as an organic-inorganic composite material which traces its existence from the ancient lineages of bivalves, gastropods, and cephalopods. This concentric layer of nacre solidifies and forms a pearl.
In the wild, a naturally occurring pearl is made completely up of aragonite and conchiolin (both aragonite and conchiolin are polymers with different crystal structures but of the same chemical formula). Most commonly pearls are oval or ellipsoid in shape and the perfect round ones are considerably rare. Hence a round pearl is expensive while the oval or ellipsoid varieties are relatively less expensive. A natural pearl has a brown centre formed by calcite and wheatish or yellowish-white outer zones. A cultured pearl differs from the natural ones in that the irritant is artificially induced by man. The rest of the process is same in both naturally forming and artificially cultured pearls. The formation of a pearl is time consuming. To get a good quality pearl, it requires about 3 years in general. This so that a thick nacre deposits around the irritant, where as a relatively poor quality pearl may be obtained in about a year from the time of the irritant’s being introduced – the latter is also relatively small as too thin or too few layers will be deposited around the irritant.
There also exist fake pearls in the market, usually called as ‘imitation pearls’. These are designed to mimic a natural pearl. These are commonly made by dipping a glass bead into a solution made out of fish scales. Since this dipping forms a thin coat, it usually wears off. You can identify this by rubbing it across the teeth or a similar surface. The fake ones will be soft and glide off against the surface, while natural ones due to the layers of nacre, feel gritty.
The growing demand for pearls now days due to their relative affordability over other gems and precious stones, is eliciting the pearl culture units to produce beyond their normal standards. As a result, more numbers of pearl oysters and mussels are being employed by the pearl culture industries by displacing them from the wild to the culture units. This is leading to a rapid decline of their wild population. These molluscs are filter feeders. They help in cleaning the water body that they live in by filtering out organic wastes. A threat to them is a threat to the whole marine ecosystem such as the corals in the vicinity, and subsequently to man.
Due to improper handling of oysters and mussels during pearl harvesting, they die and are replaced by new ones mostly obtained from the wild, thereby threatening the molluscs’ population in the wild.
Deepak Tarun is a post graduate in Zoology. Not one to embrace anything at its face value, Deepak believes in really digging to the roots, before arriving at a decision. A budding wildlife biologist and passionate knowledge seeker, Deepak likes to read, and watch stand-up comedy in his spare time.