World Aquaculture Magazine - June 2021

64 JUNE 2021 • WORLD AQUACULTURE • WWW.WA S .ORG in multiple layers, resulting in a pearl (Fig. 2). China, Japan, Australia, Indonesia, French Polynesia, Cook Islands, Philippines, India, Sri Lanka, Myanmar, Thailand, Malaysia and Mexico have utilized a large variety of molluscan species to produce lustrous pearls to generate farm income in aquaculture (Gervis and Sims 1992, Southgate et al . 2008). China, being the main producer of cultured pearls in the world, is the leading producer in fresh water while Japan leads production of marine pearls. Pinctada maxima is the major oyster species and yields the largest and most-prized cultured pearls. Among the different species of freshwater mussels, Hyriopsis cumingii is the most important species for pearl production, followed by Cristaria plicata . India is home to around 5,000 molluscan species (Zoological Survey of India 2003) and it harbors many freshwater species that are distributed across different geographical ecosystems viz. the northeastern, western, central and southern states. However, production of pearls at a large scale is being practised with three species in the Unionidae family: Lamellidens marginalis, L. corrianus and Parreysia corrugata (Janakiram 2003, Saurabh et al . 2014). These P earls, often referred as the queen of gems has a rich cultural significance and tremendous market demand (Fig. 1). Recently pearl culture has emerged as a lucrative aquaculture enterprise in countries with natural mussel resources. Previously pearls were collected fromwild resources, but in recent years technological advancements in pearl culture have been studied and methods standardized, allowing the development of pearl culture technologies in molluscan bivalves of freshwater and marine origin. The basic mechanism behind the formation of pearls is an activation of mussel defence mechanisms, where some kind of irritant induces pearl mussels to secrete the lustrous nacre that covers the foreign particle and gradually develops layer upon layer, forming a gleaming pearl (Alagarswami 1987). Not all molluscan bivalves can produce lustrous pearls but species that possesses a nacreous secretion underneath its shell are accredited as pearl-producing candidate species (Zhu et al . 2019). In pearl farming, the defence response is mimicked wherein a nucleus is inserted into the bivalves and the entry of the nucleus stimulates nacre secretion from the epithelial tissues of the same as a defensive response. In due course, the nacre deposits on the nucleus Freshwater Pearl Culture: an Emerging Avenue for Aqua Farmers Sweta Pradhan, Sonal Suman, Bindu R. Pillai, S. K. Swain and S. Saurabh FIGURE 2. Freshwater pearls, indicating variation in size and color. FIGURE 1. Designer pearls.