Anatomy of finfish and shellfish (2+1)

Unit 13 - Mollusca

13.2.4. Pearl formation
A pearl is the result of any injury to the mollusks. It is secreted by the mantle as a means of protection against some foreign body. Whenever a foreign body, such as a grain of sand or a small parasite, such as a larval stage of a fluke, gets between the mantle and the shell it becomes enclosed in a sac of mantle epithelium which is thus irritated. The irritation stimulates the mantle epithelium to secrete thin concentric layers of mother of pearl around the foreign body. The amount of deposition is in direct proportion to the degree of irritation, At the end of several years, a pearl will be formed. Pearls are often found in clams and edible oysters but these are not nacreous and therefore or little value. The most precious pearls are found in the pearl oyster (Pinctacla vulgaris), which is closely allied to the freshwater mussel. The Japanese have developed a technique of producing pearls artificially by inserting foreign bodies such as glass beads, into the mantle of oysters which are retained in wire cages or crates until pearls are produced which can be later removed and sold in the market. It requires 3 to 4 years to produce a pearl of considerable size but a large one requires 7 years. Cultured pearls are genuine pearls but less valuable than uncultured pearls which can be identified by experts. Imitation pearls are beads coated with an iridescent substance called pearl essence that is obtained from the scals of fish.