The beautiful luster of a pearl makes it highly valued as a gemstone. However, the pearl is unique among gemstones as the only one created by a living organism. The humble oyster creates pearls in the same manner that it creates its shell but why has recently been in dispute. Although pearls in the wild are rare, humans have developed pearl farming techniques to stimulate oysters into making these lovely gems.
The Old Theory of Why Oysters Create Pearls
It was once believed that oysters created a pearl in response to an irritant such as sand or food entering its mantle. The mantle is the organ responsible for ensuring shell growth and for creating pearls. The material that creates the shell and the pearl is a mixture of minerals and proteins called nacre. The Field Museum describes on its website page “Making Pearls” how oysters are filter feeders that extract food from water they pull over their gills to breathe. Sometimes an errant piece of food becomes embedded in the mantle and the oyster’s cells secrete nacre around the intruder to protect the oyster’s tissues. This piece of food enveloped in nacre becomes a pearl.
The New Theory of Why Oysters Create Pearls
Paul Southgate and John Lucas write in the 2008 book The Pearl Oyster that no evidence supports the “encapsulated irritant” theory. Pearl oysters have the highest recorded rate of mantle cavity ventilation rates among bivalves and so it is quite likely an oyster could expel any irritant from its mantle. Rather, oysters make pearls in response to a mantle injury. Sometimes a wound will heal in such a way that a cyst or pocket forms which fills with calcium carbonate from the mantle’s secretory cells. These excretions in response to the injury are what become the pearl.
How People Help Oysters Make Pearls
Natural pearls, or pearls made in the wild are incredibly rare. In the quest for more pearl necklaces people have developed techniques to stimulate pearl production in oysters. Western Australia’s Department of Fisheries describes this process on their “What is pearling?” page. Wild pearl oysters are caught and brought to pearl farms – generally panels floating in the sea. In process called seeding an irritant called a nucleus is inserted into the oyster’s gonads. The nucleus is usually a small piece of muscle shell. This prompts the mantle to secrete nacre around the nucleus to ease the irritation caused. A pearl is then formed.
Do Oysters Die When the Pearl is Taken?
Oysters can create several pearls in a lifetime. The “What is pearling?” article reports that it takes two years for a farmed oyster to create its first pearl of marketable value and size. After the first pearl an oyster can create subsequent pearls within one year. Pearl farmers expect to seed an oyster four times within its lifetime. Not until its last year is the oyster killed for the use of the nacre on its shell.
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