<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (March 2013) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (March 2013) (Learn how and when to remove this template message) </Td> </Tr> <P> A cultured pearl is a pearl created by an oyster farmer under controlled conditions . Cultured pearls can be farmed using two very different groups of bivalve mollusk: the freshwater river mussels, and the saltwater pearl oysters . </P> <P> A pearl is formed when the mantle tissue is injured by a parasite, an attack of a fish or another event that damages the external fragile rim of the shell of a mollusk shell bivalve or gastropod . In response, the mantle tissue of the mollusk secretes nacre into the pearl sac, a cyst that forms during the healing process . Chemically speaking, this is calcium carbonate and a fibrous protein called conchiolin . As the nacre builds up in layers of minute aragonite tablets, it fills the growing pearl sac and eventually forms a pearl . </P>

Name the shellfish which is cultured to obtain pearl