Freshwater Pearl Culture

Biomineralization

Theoretically any shelled mollusc can produce some sort of a pearl. However only those mollusks which have a pearl lining or nacre on the interior of the shell surface can produce lustrous pearls. It is an abnormal process in the normal biological system of the animal. The mantle with its outer epithelial cells is the tissue responsible for producing pearl nacre. When an external stimulus such as accidental trapping of a hard foreign body or a parasite or a lesion occurs in the outer epithelium of the mantle tissue, it leads foreign body resulting in a pearl. Initially, the epithelial cell of the mantle forms a pearl -sac surrounding the irritant foreign body forming the cellular basis for crystallization of calcium carbonate. This process of pearl formation well known to the scientific community as 'Pearl Sac Theory'. This abnormal response to a foreign body in the normal biological processes that build up the shell in certain mollusks constituted the base for pearl culture operations. The shape of the pearl is governed by the irritant foreign body and its quality by the nature of secretions of the pearl sac. Thus the outer epithelium of the mantle tissue is the ‘key- note' in the "orchestra" of bio-mineralization of a pearl.

Basic steps involved in freshwater pearl culture operations:

In India the common species used in freshwater environment in pearl culture operations are Lamellidens marginalis, L.corrianus, and P.corrugata. A summary of the steps involved are given below:

1. Collection of Mussels
2. Pre-operative conditioning
3. Implantation of Grafits & nuclei
4. Post operative care of Mussels
5. Pond Culture of implanted Mussels
6. Harvest of Mussels & Pearls

Implantation Methods

Methods of freshwater pearl culture vary depending on the surgery done in the internal structure of the pearl mussel and the type of pearl products targeted. A shell bead constitutes the essential input in the pearl culture operations. Certain locally available, inexpensive and bio-compatible acrylic material can be employed as nuclei in fresh water pearl culture. It has been observed that the pearl mussels of the size 8 to 10 cm in shell length and weight of 50gm and above are ideal for pearl culture operation.

Culture Practices

Farming practice of the freshwater pearl culture operation involves six major steps sequentially viz., collection of mussels, pre-operative conditioning, surgery, post-operative care, pond culture and harvesting of pearls.

Collection of mussels

The healthy mussels are collected from the freshwater bodies like pond, river etc. They are collected manually and kept in buckets or containers with water. The ideal mussel size used for pearl culture is over 8 cm in anterior-posterior length.

Pre-operative conditioning

The collected mussels are kept for pre-operative conditioning for 2 to 3 days by keeping them in crowded condition in captivity with aged tap water at a stocking density of 1 mussel/l Pre-operative conditioning helps in weakening of adductor muscles, which helps in easy handling during surgery.

Mussel surgery

Depending on the place of surgery the implantation is of three types viz., mantle cavity, mantle tissue and gonadal implantations. The key materials required during the surgical implantations are beads or nuclei, which are usually made from mollusc shell or other calcareous materials.

Mantle cavity implantation: In this procedure round (4-6 mm diameter) or designed (images of Ganesh, Buddha, etc.) beads are inserted into the mantle cavity region of mussel after opening the two valves (without causing injury to mussels at both ends) of animal and separating carefully the mantles of anterior sides from the shell with the help of surgical set. Implantation could be done in mantle cavities of both the valves. In case of implantation of designed beads care is taken such a way that the design portion faces the mantle. After placing the beads in desired place the gaps created during implantation are closed just by pushing the mantle onto the shell.

Mantle tissue implantation: Here the mussels are divided into two groups; the donor and the recipient mussels. The first step in this procedure is preparation of graft (small pieces of mantle tissue). This is done by preparing a mantle ribbon (a strip of mantle along the ventral side of the mussel) from a donor mussel, which is sacrificed, and cutting that into small pieces (2 x 2 mm). The implantation is done on recipient mussels, which is of two types viz., non-nucleated and nucleated. In the former, only the graft pieces are introduced into the pockets created at the inner side of posterior pallial mantle present at the ventral region of the mussel. In the nucleated method, a graft piece followed by a small nucleus (2 mm dia) is introduced in the pockets. In both the procedures care is taken so that graft or nucleus does not come out of the pocket. Implantations could be done at mantle ribbons of both valves.

Gonadal implantation: This procedure also involves preparation of grafts as described earlier (mantle tissue method). First a cut is made at the edge of the gonad of the mussel. Then a graft is inserted into the gonad followed by nucleus (2-4 mm dia) so that the nucleus and graft should be in close contact. Care is taken such a way that nucleus touches the outer epithelial layer of the graft and the intestine is not cut during the surgery.

Post-operative care

Implanted mussels are kept in post-operative care unit in nylon bags for 10 days with antibiotic treatment and supply of natural food. The units are examined daily with removal of dead mussels and the ones that reject the nucleus.

Pond culture

After post-operative care the implanted mussels are stocked in the ponds. The mussels are kept in nylon bags (1 cm mesh-12 sq. cm) at the rate of 2 mussels per bag and are hung from bamboo or PVC pipes and placed in ponds at 1 m depth. The mussels can be placed at deeper zones (up to 2.0m) during hot summer months to avoid surface heating. The mussels are cultured at stocking density of 20,000 -30,000/ha.

The implantations in pearl culture operations are carried out throughout the year, except during hot summer months (May and June) for minimizing post -operative mussel mortality and rejection rate of implanted graft and nuclei. Ponds (2.5m deep) with clay soil base and slightly alkaline waters are suitable for pearl culture operations. A rectangular shaped pond with proper in - lets and out-lets is ideal for implanted pearl mussel rearing. Ponds without aquatic macrophytes and algal blooms such as Microcystis sp. and Euglena sp. are suitable for pearl culture operations. The ponds are provided with P.V.C tubing (2" dia) platforms (16 x 8 m) as rafts for hanging pearl mussel culture units. The implanted mussels at a density of 50,000 to 75,000/ha are placed in nylon bags (l.0 cm mesh, 12x 14 cm) @ 2 mussel per bag.

The area of the pockets and mesh size in these simple culture units are sufficient for the mussel to open and close their shell valves for feeding and operation. These bags are then tied to the P.V.C floating platform units or bamboo rafts maintained in the culture environment. The ideal hanging depth of the pearl mussel culture units is observed to be 1.5 to 2.0 m deep in the ponds for good survival and growth of mussels. Alternatively, the mussels can also be placed in plastic crates (0.5 x 0.35 x 0.25 m) @ 20 to 25 mussels per crate. The physico- chemical parameters and water level of the ponds are to be monitored throughout the culture period of the mussels. The optimum temperature regime lies between 25°C to 30° c. The fresh water pearl culture pond environment is generally same as employed for the aquaculture of the carps.

The freshwater mussels constitute the benthic invertebrates in pond ecosystem and hence free dispersal on pond bottom should be ideal in terms of growth and survival. It has been observed that mussels when maintained in bottom- set culture units recorded poor survival (27%) as against surface and column-set units (66%). The reason for poor survival in bottom units may be due to the reduced level of primary production and siltation aggravated by the restricted space in the units. Other important parameters are the extent of macrophyte infestation and movement of water in the culture environment. However pond bottom distribution of the operated mussels has certain problems in sampling and harvesting of mussels for pearls.

The ponds are fertilised with organic and inorganic fertiliser periodically to sustain the plankton productivity. Addition of green water (Chlorococcum sp. and Scenedesmus sp.) at regular intervals into the pearl culture ponds as direct mussel feed is observed to be an ideal practice for proper up- keep of the pearl bearing mussel standing crop.

The green feed is developed by 'open culture method' in a series of Ferro- cement tanks (200 liter) arranged all along the pond bundh. The water in the tanks are fertilised as given below:

• Cowdung : 1000kg/ha/yr. (in twelve equal installments)
• Urea : 100kg/ha/yr. (in twelve equal installments)
• Single super phosphate: 100kg/ha/yr.(in twelve equal installments)

Once the fertilizer degrades (7 to 10 days), the green water develops. The algal enriched water is lead into the pearl culture ponds. The mussels by virtue of being mucoid filter feeders can accept a variety of particular organic materials feed. The pearl mussels in captive culture conditions can be maintained on a diet of powdered rice bran and groundnut oil cake (1:1 ratio) at 1% of the weight of the mussels provided on alternate day basis.

Periodical checking of mussels with removal of dead ones and cleaning of bags is required throughout the culture period of 12-18 months.

Food and Feeding

Algae being the predominant component of the first trophic level in aquatic food chain have got much importance in aquaculture systems. Some species of algae belonging to Chlorophyta (green algae), Bacillariophyta (Diatoms) and Cyanophyta (blue green algae) are normally used as feed by the freshwater mussels. The commonly preferred algal species by the freshwater mussel Lamellidens marginalis are diatoms green algae (Chlorella chlorococcum, Scenedesmus etc.) and blue green algae (Spirulina). Culture vessels and tanks of desired capacities are to be selected prior to algal culture. Suitable medium should be prepared well in advance for different species to be cultured.

The pond culture of operated mussels varies from six month or more depending upon the size and number of nuclei implanted, the health of the mussels and the condition of the pond environment. The culture units require periodic cleaning of accumulated silt and other fouling fauna, finding entry in to the units.

Pearl harvest

At the end of the culture period the mussels are harvested. The mussels are either crushed following by sieving to extract pearl or the mussel is individually is sacrificed, or individually pearls are taken out from the pearl sac of the live mussels without sacrificing. The latter method, through difficult, is desirable to prevent depletion stocks of mussels in the natural environment.

In mantle cavity insertion method, the culture period is generally about 6 to 12 months, depending on the size and number of nuclei implanted. In this method the mussel at the end of the culture period is sacrificed. The mussels are opened one by one and the half round or design, shell attached pearls are cut out of the shell valves. Two to five attached pearls are obtained in this method, depending on the number of nuclei inserted. As on the attached side of the pearl, nacre cannot be deposited, it is ground off and cemented with a piece of mother of pearl layer obtained from the shell interior. The success rate is about 60 - 70% of the mussels implanted.

The culture period in mantle tissue implanted mussel is generally from 12 months to 18 months. In this method the mussels after the culture period are carefully and the pearls are removed one by one from the pearl sacs. Four to eight pearls are obtained per mussel depending on the size of the mussel and the number of correct implantations done. The same mussel can again be used for the next operation. Alive graft piece or even a small nucleus (less than 2mm dia) if implanted into the s me pearl sac may result into a pearl in less time. In this method non- nucleated, solid, unattached, and irregular to oval pearls (2 to 3mm size)or round, unattached cultured pearls (2mm dia. Approx.) are obtained. The success rate in this procedure is about 60 to 70% of the mussels implanted. The culture period of gonadal implanted mussel is generally 12 months. The mussel after the culture period are opened carefully and the position of the pearl is felt by touching the area close to the incision scar. By a pair of scissors, fine forceps and needle the pearl formed is carefully removed without cutting or damaging the intestine or other internal tissues. The mussel may also be sacrificed to extract a gonadal pearl. The layers of the gonad are cut open and the pearl is removed easily. In this process regular, round (3.5 - 5.5 mm dia) unattached pearls are obtained.

Source: Central Institute of Freshwater Aquaculture, Bhubaneshwar, Orissa