Nucleus implantation (grafting)
Nucleus implantation, also called grafting or seeding, is the surgical implantation of a shell nucleus and tissue graft (a small piece of mantle tissue) into the pearl oyster, thus starting the development of a cultured pearl. Natural pearls occur when a foreign body, such as a boring worm or grain of sand, irritates the pearl oyster. Nacre is secreted around the foreign body by the mantle tissue to protect the pearl oyster’s tissues.
Grafting is a means of imitating this natural process so that a cultured pearl can be grown. Grafting is a surgical procedure that should only be done by a qualified seeding technician. Almost anyone can learn from a technician how to perform the grafting operation, but it requires years of practice and dedication to perfect this skill and produce high quality pearls.
Guidelines for obtaining good grafting services
- The skill of the grafting technician affects pearl quality and farm operating costs.
- Good technicians are in high demand and prefer to work with experienced, reliable farmers.
- If a technician is confident of the skill of the farmer, he will work for a share of the harvest. Otherwise, a technician will demand a cash payment of $3-$4 for each pearl oyster he grafts.
- In both cases, the farmer usually pays the airfare, lodging, food and expenses of a grafting technician while he works on the farm.
- The farmer must negotiate the best deal he can, since the cost of hiring a technician is one of the major farm operating costs.
- It is preferable to have the technician work for a share of the harvest, since this motivates the technician to return and also if the technician’s earnings depends on the harvest of good quality pearls, he or she will be much more motivated to graft carefully and give the farmer helpful advice.
Beware of technicians who graft too rapidly.
- It is common practice for technicians working on a cash payment basis to simply graft as many pearl oysters as possible without regard for suitability of the pearl oyster to produce a good pearl.
- Careful grafting takes time, therefore, if a technician grafts more than 300 pearl oysters in a regular 8-hour workday, it may be an indication that he is working too fast.
- Traits to look for when hiring a technician are extensive experience, skill and a good reputation.
- Since grafting services are expensive, be sure and select a qualified technician. It takes several years for a grafting technician to be able to reliably produce good quality pearls. \
- The success of a technician is measured by several factors: mortalities after grafting, the number of pearl oysters that reject the nucleus and ultimately, the number of round Grade A pearls produced.
- Always ask for letters of refer for both you (the farmer) and technician to sign.
Technicians can be good sources of technical advice.
- Each seeding technician has his own methods and it is imperative that you follow his instructions
during the grafting process to guarantee success.
- Several months before the technician is scheduled to arrive, ask him for instructions on how to prepare the pearl oysters and farm for the grafting procedure.
- Try and learn as much as possible from the visiting technician about how to operate a farm. Even the best technician cannot produce good pearls if farm management is poor before or after grafting.
- While many technicians also buy pearls, they may not offer the best prices. Technicians will often make a farmer an offer to buy the pearls immediately after a harvest. In many cases, the price offered is below the market price, but you may feel pressured to accept the offer either to please the technician or because you do not feel confident that you can sell the pearls elsewhere.
- Resist the temptation to sell the pearls immediately. You will usually be able to obtain a better price from wholesaler pearl buyers or jewelers.
- If you want to sell to your technician, you should try to have the pearls graded and the price set by an impartial professional.
The grafting process
- Preparation for grafting The ideal pearl oyster size for grafting is about 4.5-6 in (12-15 cm) in length, or about the size of a woman’s hand.
- These animals will be 1½-2 years of age. This age is ideal because the pearl oysters are still growing quickly and will produce nacre at a more rapid speed.
- The nacre produced by young pearl oysters also tends to have more luster than that produced by older animals. Older and larger pearl oysters can be grafted, but are less likely to produce high-quality pearls.
- Pearl oysters gathered from the reef need special preparation for grafting.
- Pearl oysters collected as spat are superior to adults collected from the reef for pearl production. The reasons for this are not completely understood; it may be that animals farmed from a very young age are simply more accustomed to the sometimes stressful conditions of a farm. Also, pearl oysters collected from the reef are likely to be older.
- If you must use pearl oysters taken as adults from the reef, you should prepare these carefully for grafting. These animals should have been hung on lines or in baskets for at least 6 months before seeding begins.
- Pearl oysters must be cleaned on a monthly basis before grafting; a final cleaning is done just before grafting. Cleaning keeps the pearl oysters healthy and in good condition. The cleaning done just before grafting should be a light and gentle cleaning so that the animals are not shocked. The technician may ask for you to perform pre-grafting conditioning.
- Technicians have different methods of conditioning the pearl oysters, but it is common for the technician to ask the farmer to condition the pearl oysters about 6 weeks before grafting.
- Conditioning is a means of inducing the pearl oysters to spawn, thus ridding the gonad of most of the egg or sperm, which can interfere with grafting. If grafting takes places just after the normal annual spawning, this may not be necessary.
- Conditioning is done by raising the lines to the surface of the water for about 4 hours during the heat of the day. The increased temperature and reduction in water pressure makes the pearl oysters spawn. After spawning, they are returned to their usual depth. It is important that this is not done immediately before grafting since conditioning weakens the pearl oysters and they need time to recover.
- A special farm platform or building where the technician can work is needed. Grafting is usually conducted on a covered platform built over the water. This platform must be sturdily built so that it does not shake while the technician is working. It should also be built in an area that is deep enough to hang chaplets of pearl oysters.
- The water surrounding the platform must be calm and clear since sediment in the water can interfere with the quality of the seeding. The platform should also be spacious enough that workers can move around without disturbing the technician.
- Technicians usually will request that a special table and chair be provided. Seeding can also be conducted on land, but this is not as convenient.
The grafting procedure
- When the pearl oysters are brought to the platform for seeding, they are opened slightly with special tools. A wooden peg is then inserted to keep the pearl oyster open until the seeding technician works on it. Care should be taken not to leave the pearl oysters open for longer than a few minutes as this can kill it.
- The seeding technician will look for a donor animal from which to take the mantle tissue graft. The mantle tissue of the donor animal determines the color and quality of the cultured pearl, so care should be taken to use only pearl oysters with the most beautiful nacre for this. The donor is then killed and the mantle tissue removed. This strip of mantle tissue is trimmed and cleaned until only a thin strip containing the nacre-producing epithelial cells remains.
- The strip is then cut into tiny squares about 1/8 x 1/8 in (2 x 3 mm) long. These tiny pieces of mantle tissue will be inserted along with the nucleus into the tissues of the pearl oyster. The transplanted mantle “graft” will grow, covering the nucleus with a tough tissue called the “pearl sac.” The inner lining of the pearl sac contains the epithelial cells, which continue secreting nacre onto the nucleus. As the layers of nacre accumulate, a pearl is formed (Figure17).
- The seeding technician will then take the pearl oysters selected for grafting and open them slightly wider. These are placed in a special clamp. After examination, some of these will be rejected as too old, too small or simply unsuitable for seeding. In most cases, the size and shape of the gonad will determine whether the pearl oyster is grafted or not. This is based on the judgment of the grafting technician and is learned after years of practice.
- The technician makes a small cut in the gonad of the pearl oyster. The mantle graft is inserted into the gonad first, followed by the nucleus. The size of the nucleus used depends on the size of the pearl oyster and the condition of the gonad. Generally nuclei range in size from 0.3 in (7 mm) to 1.56 in (14 mm), with the smaller sizes being most commonly used the first time an animal is grafted. A small piece of the donor mantle tissue is inserted last. The pearl oyster is then allowed to close, placed on a chaplet or in a tray and returned to the water. Figure 17.
- Nacre is formed by the epithelial cells of the mantle tissue. The upper diagram shows a natural irritant or a plastic mabe form being covered progressively with layers of nacre deposited by these cells. In the case of a cultured pearl, this process is imitated. The transplanted piece of mantle tissue contains the nacre-producing epithelial cells. The small piece of tissue grows around the nucleus with the epithelial cells forming the inner layer of the pearl sac (from Wada 1973).
- The first 30 days after grafting are a critical period during which the pearl sac forms. The first 30-40 days after grafting are critical because that is when most deaths or nucleus rejections occur.
- During the recuperation period after grafting, the piece of mantle tissue that was implanted along with the nucleus will slowly grow around the nucleus creating the “pearl sac.” Layers of nacre are slowly secreted around the nucleus by cells contained within the pearl sac.
- It usually takes about 1½-2 years to form a pearl with sufficiently thick nacre 0.8-0.12 in or (2-3 mm) to harvest.
- To monitor the grafted pearl oysters, use catch bags. It is common to enclose the newly seeded pearl oyster in a fine-meshed bag called a “catch bag”. Using the catch bag allows you to see if the nucleus is rejected. Catch bags can be used with either chaplets or panels.
- Some technicians may object to using catch bags, you can and should insist on this, as it is the only way to determine how well the technician is doing. Be sure to include this condition in the contract with the technician.
- The catch bags should not be left on for longer than 40 days after grafting because it may interfere with the pearl oysters’ ability to feed. The pearl oysters should be allowed to rest in very calm waters for about 40 days after grafting.
- You may need to have a special line for this if your other farm lines are in an area that experiences swift currents. As always, handle the grafted animals gently when moving them to the line and do not expose them to heat or dryness.
- After 40 days, the pearl oysters can be removed from the catch bags and inspected. Some pearl oysters will die and others will reject the nucleus during the first 40 days. The pearl oysters can be cleaned for the first time after grafting at this point.
- Remove the pearl oysters from the catch bags and carefully inspect the bag for a rejected nucleus. Keeping a record of how many died and how many rejected the nucleus helps you evaluate the performance of the technician.
- Return the pearl oysters that have kept the nucleus to the farm lines. It may be helpful to make a map of your farm or to place plastic tags on the chaplets to help you remember which pearl oysters are grafted. Separate the pearl oysters that have rejected the nucleus and keep these to produce keshi pearls.
- Even though the nucleus is rejected, some of the mantle tissue may remain inside the gonad. The pieces of tissue often form baroque pearls without a nucleus called “keshi.” Farmers usually leave these pearl oysters in the water for 2 years before harvesting. Keshis can be large and attractive and may produce a high proportion of your farm’s revenue.
Article source: The Basic Methods of Pearl Farming, Author: A Layman’s ManualMaria Haws, Ph.D. (Director, Pearl Research and Training Program, Pacific Aquaculture and Coastal Resources Center, University of Hawaii at Hilo, Hilo, HI 96720 USA, Center for Tropical and Subtropical Aquaculture, Publication No. 127, March 2002)
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