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Pearl Oysters

A pearl is a hard rounded object formed in the mantle (flesh) of a mollusc with a shell. Just like the shell of a mollusc, a pearl is made up of tiny crystals of calcium carbonate which have been deposited in a series of rings.

Pearls come in a variey of shapes with round, smooth ones the most highly valued. Almost all species of shelled molluscs can produce pearls but only those from pearl oysters are highly valued. Pearls from the sea are valued more highly than freshwater pearls.

A pearl oyster containing a pearl

WIld pearls only form in about one in a thousand oysters so cultured or farmed pearls from pearl oysters make up the most of those that are currently sold. The formation of a natural pearl is the oyster's defence mechanism to an irritant that enters the shell. If the irritant (e.g. a piece of sand, larva or parasite) picks up a a piece of the mantle tissue on the way a pearl can form (see next page)

The pearling industry in Australia is based on the silver lipped pearl oyster (Pinctada maxima), which produces high quality cultured pearls known as South Sea pearls. These pearls are white in colour, with a brilliant shine. Pearls are sold for use in jewellery especially necklaces.

This species is found across the central Indian and Pacific Ocean region from India to New Guinea and the Philippines, and, in Australia, from Carnarvon in Western Australia to south of Cairns in Queensland.

Pearl oysters begin their lives as males, when they reach tthree to four years of age and a length of 11 to 12 mm they begin to change sex and become females. By the time they reach19 cm in length they all are females. Breeding takes place from September or October to April and May. Females can release between 2 and 12 million eggs which are fertilized by spem released into the water by males.

They are part of the plankton for fout to five weeks during which more than 99% of the larvae die, most eaten by predators. After this time the remaining larvae start to settle on the sea floor and change into young oysters.. They can grow to 12 cm in less than three years.