They are the oysters’ reaction to an intruder in their shell. The pearl is the result of a kind of defense of the mollusc organism against an invader – an external organism that can be anything from a grain of sand to a parasite. Not all oysters form pearls, only the pearls that are part of the Pteriidae (saltwater) and Unionidae (freshwater) families. And it’s not all pearls that have commercial value either – just the ones that come out really round.
Most of them, however, grow attached to the oyster shell, as if it were a wart, and are in the shape of a half sphere, which makes it less likely to be sold.
PROTECTION MECHANISM
The jewel defends the oyster from invaders. The entry of “invaders”, such as worms and plankton, grains of sand, pieces of rock or coral, generates a kind of irritation – a process that the oyster uses to defend itself against this new organism. This reaction aims to defend the mantle, a very thin tissue, with several muscle lines, present in oysters and shellfish. It covers the entire body of the animal and protects its organs, such as the heart and intestines, called “soft parts”.
The mantle covers the invader with several layers of a substance called mother-of-pearl, or nacre, composed of calcium carbonate (about 93%), water, protein (conchiolin) and particles that give, for example, the color to the pearl. The process takes an average of three years. There is no maximum size reached by a pearl: it is usually taken with 12 mm in diameter, but it would be able to reach up to about 3 cm. More than that, it could deform the shell and kill the oyster.
You can grow pearls: a small plastic ball or a piece of mollusk is placed in the oyster to trigger the process
EVERY ONE, EVERY ONE
Only 2% of pearls are round. And they have different colors. The pink, red, or blue color is due to debris, protein, or the internal color of the shell. The rarest is the black one, from Tahiti and the Cook Islands.
The most common pearls are the semi-round ones. There are still those that grow in the form of a teardrop, drop, cone or baroque, very irregular.
SOURCES Vanessa Simão, specialist in molluscs; Luiz Ricardo L. Simone, Museum of Zoology of São Paulo; and Carlos Henckes, president of the Institute of Conquiologists of Brazil
Read too:
– How does the oyster produce the pearl?
– What is the most expensive thing in the world?
– How does a diamond mine work?