sundry the bottom followed by light tilling. Agricultural lime should be applied to absorb excess CO and to supply calcium which is required for moulting. Fertilizers like rice, bran, poultry, and cattle dung are used to increase the fertility of the soil. Prawns are commonly caught in crafts and gears using different types of nets such as cast nets, bag nets, drag nets, trawl nets and barrier nets.
Preservation of prawns is done by peeling and deveining or by cooking and peeling. Figure . Macrobrachium rosenbergii . .
Pearl Culture Pearl is a white, highly shining globular concretion found within the shell of an Oyster. Pearl oysters are sedentary animals. In India it was cultured for the first time in at Thoothukudi. Pearl oysters are found along the coast of Kanyakumari and in the Gulf of Kutch.
High quality pearls are obtained from pearl oysters of Genus Pinctada that can be cultured in the salinity range of ppt in racks, raft and long line methods. Freshwater bivalve Lamellidens is also used in artificial pearl culture. Mostly the pearl oysters inhabit the ridges of rocks or dead corals, forming extensive pearl banks. These pearl beds produce best quality of pearls called as “Lingha Pearl”.
Pearl Formation When a foreign particle accidently enters into the space between mantle and shell of the oyster, it adheres to the mantle. The mantle epithelium encloses it like a sac and starts to secrete concentric layers of nacre around it as a defensive mechanism. Nacre is secreted continuously by the epithelial layer of the mantle and is deposited around the foreign particle and over a period time the formation of repeated layers of calcium carbonate makes the hard and glossy pearl. When the pearl enlarges the oyster dies.
The shell is then carefully opened and the pearls are manually separated and graded (Figure . ). Composition of pearl Pearl comprises of water, organic matter, calcium carbonate and the residue. ( ) Water: - % ( ) Organic matter: .
- . % ( ) Calcium ( ) Residue: . - . % carbonate: % XI Std Biology-Zoology Chapter- the animal