World Aquaculture Magazine - June 2021

WWW.WA S .ORG • WORLD AQUACULTURE • JUNE 2021 65 species inhabit stagnant to slow-moving water such as ponds and reservoirs. Freshwater pearl mussels are filter feeders that preferably feed on green and blue-green algae. ICAR-Central Institute of Freshwater Aquaculture, Bhubaneswar, Odisha is the premier institute of India working on the development of freshwater pearl mussel culture technologies with Lamellidens species. The research institute is also working on the dissemination of freshwater pearl production technology to various stakeholders, farmers and entrepreneurs through training programs to inculcate pearl farming technology. The Pearl Formation Procedure Different aspects of pearl farming in freshwater systems such as preparation of a biocompatible nucleus, pre-operative care of mussels, different methods of surgical implantations to insert the nuclear beads into the bivalve, post-operative care of mussels, pond culture technology, harvest and value addition are included in the pearl farming protocol. Pearl mussels of 35-50 g weight and 8-10 cm long are selected for surgical implantations. Prior to surgery, collected mussels are crowded for 24-48 h to ensure that adductor muscles are relaxed for ease of nucleus implantation. Three surgical methodologies have been standarized and developed for the implantation of nuclear beads into the mussel: the mantle cavity, mantle tissue and gonadal method (Saurabh et al . 2014, Pradhan et al . 2019). The selection of the surgical method is primarlily dependent on the type of pearl that a grower wants to produce. Biocompatible nuclei are prepared of acrylic material or mussel shell powder and the design of the nuclei is based on the opted method of implantation. In the mantle cavity implantation method, designer nuclei are inserted carefully into the umbonal cavity between the outer epithelium of mantle tissue and the nacreous layer of the inner shell. In the mantle tissue method of implantation, pockets are made in the posterior mantle lobe followed by implantation of the nucleus into these pockets along with mantle grafts (mantle tissue) that are obtained from the donor mussel that induce nacre secretion around the nucleus. In this case one can also insert a graft tissue alone, which will give a nacreous pearl. In the gonadal surgery method, gonads are punctured and the round nucleus, sandwiched between the mantle grafts on two sides, is inserted into the incision site (Fig. 3). Following surgical implantations, in the post-operative care module, the mussels carrying nuclear beads are treated with antibiotics to reduce rejection of nucleus and post-implantation mortality. Post-implantationManagement The management of implanted stocks of mussel, which is vital for healing the wound of the implanted mussels, is one of the crucial steps of the post-operative care unit. Mussels in a wet laboratory are carefully observed for 7-10 days. It is important to allow the mussel to open the valve freely to get oxygen for respiration. To get better survival and promote wound healing in implanted mussels at a faster rate, post-operative care units are supplemented with broad-spectrum antibiotics like tetracycline at 1-2 ppm as a prophylactic measure. Mussels in post-operative care are supplemented with green algae for food. Appropriate post-operative care minimizes rejection of nuclei soon after implantation and decreases the chances of irregular pearl formation. Management at the Pond Level In India, freshwater mussel implantation can be done year round, excluding the peak summer months (May-June), to minimize post-operative death and rejection of nucleus beads. Traditional carp culture ponds (2.5 m deep) with a clay soil bottom and slightly alkaline water are conducive to pearl mussel farming. Ponds devoid of aquatic macrophytes and algal blooms like Microcystis and Euglena are ideal for pearl farming. Implanted mussels are placed in nylon bags (30 cm × 13 cm; mesh size 1.5 cm) at two mussels per bag and reared at 1/m 2 of pond surface area (Fig. 4). Ponds are fitted with bamboo poles as rafts for suspending implanted pearl mussels. The most regularly preferred algal species by the freshwater mussel L. marginalis are diatoms, green algae ( Chlorella, Chlorococcum, Scenedesmus , etc.) and blue-green algae ( Spirulina ). A regular health checkup of the cultured mussels must be done every two weeks as there are many chances for mortality of implanted mussels related to internal incision, low availability of food and FIGURE 3. Implantation of a nucleus in freshwater mussels. ( C O N T I N U E D O N P A G E 6 6 )