Farming Techniques and Methods for Freshwater Mussel Pearl Cultivation

Part One: Artificial Breeding of the Freshwater Pearl Mussel (Triangle Sail Mussel)
　　
China has more than 100 species of freshwater mussels, but only the triangle sail mussel (Hyriopsis cumingii), the wrinkled comb mussel (Cristaria plicata), and the toothless mussel (Anodonta) are suitable for pearl cultivation. At present, the triangle sail mussel is the most widely used in local production and has the highest value for pearl cultivation. Japan's endemic species, the pond butterfly mussel (Hyriopsis schlegelii), belongs to the same genus but a different species from China's triangle sail mussel. Its main quality indicators are superior to those of the triangle sail mussel, and its pearl quality is high. Japan mainly relies on the pond butterfly mussel for freshwater pearl cultivation. At present, it has already been introduced locally for trial breeding.
I. Main Biological Characteristics of the Freshwater Pearl Mussel
Mussels belong to the phylum Mollusca, class Bivalvia (Lamellibranchia), subclass Palaeoheterodonta, order Eulamellibranchia, family Unionidae. Freshwater mussels are benthic animals, living year-round at the bottom of rivers, lakes, and ponds. The triangle sail mussel prefers habitats with clean water quality, a pH of 7-8, slight water flow, and sandy-mud bottoms. Mussels typically feed on bacteria in the water, phytoplankton, protozoa, rotifers, small cladocerans, small copepods, and organic detritus. Freshwater mussels create a water current by the beating of cilia on their gills and obtain food by filtering through the gills. The suitable water temperature for growth is 15-35°C, and the optimal water temperature for reproduction is 18-28°C. Growth is inhibited when the water temperature is above 35°C, and they enter a semi-hibernation state below 10°C. The lower limit of adaptation to low dissolved oxygen is 3 mg/L. When the water temperature reaches 18°C (mid to late April), the gonads of the triangle sail mussel begin to mature; in May-June they carry eggs and release glochidia. The egg-carrying capacity is 400,000-500,000 eggs per mussel, and during the breeding season glochidia are released 5-8 times. In artificial mussel cultivation, glochidia can be collected multiple times. Although mussels have a high egg-carrying capacity, due to the complex metamorphic development, the survival rate to the glochidium stage is only about 10%. After leaving the mother mussel, the glochidia attach to fish gills and body surfaces by means of byssal threads and shell hooks when they encounter fish, temporarily living as parasites. The larvae develop and metamorphose by absorbing nutrients from the fish body, and finally break out of the cyst and fall to the bottom to live freely as benthos. The juvenile mussel develops the stomach, adductor muscles, intestines, foot, and other organs possessed by adults, and has already taken on the mussel form. The byssal threads and hooks unique to the glochidia disappear.
II. Artificial Breeding of the Freshwater Pearl Mussel (Triangle Sail Mussel)
(I) Selection and Cultivation of Parent Mussels
1. Selection of parent mussels. Parent mussels should preferably be 4-5 years old and have already spawned. The mussel shape should be regular, the shell color glossy, the shell substance thick and solid, the nacre layer colorful and beautiful, the mussel body robust and plump, the outer gills intact and uninjured, with forceful water spraying and strong shell-closing ability. The female-to-male ratio should be 1:1 or 2:1, and parent mussels should preferably come from different water systems to avoid inbreeding. Identifying male and female is very important for selecting and pairing parents. Besides the difference in gonad color, sexually mature mussels can also be identified by their external appearance and gill filament characteristics. Female mussels have relatively swollen shells, greater shell width, a more rounded and blunt posterior margin, and more densely spaced gill filaments on the outer gills; male mussels have smaller shell width, a slightly pointed posterior end, and more sparsely spaced gill filaments on the outer gills.
2. Requirements for Parent Mussel Cultivation Ponds
(1) The pond area should be more than 1,000 sq m, well-ventilated and sunny, with conditions for slow water flow.
(2) The water color should preferably be yellow-green, with a transparency of about 30 cm and a water depth of 1.5 m, ensuring the water is fertile and lively with abundant natural food.
(3) The bottom silt is generally about 10 cm; 10-15 days before stocking, clear the pond with quicklime.
3. Parent mussel cultivation. After the Spring Festival each year, begin selecting parent mussels, mark each shell with ♀ or ♂, then hang them in the pond alternately by female (♀) and male (♂) at a spacing of 30-35 cm, which facilitates natural fertilization and increases the fertilization rate. After the parent mussels are hung in the water, careful observation and strengthened management are needed. Hang about 600 mussels per mu.
(II) Artificial Seed Collection
After the fertilized eggs develop into glochidia in the outer gill cavity of the female mussel, they must parasitize the gill filaments or fin rays of fish to absorb nutrients and complete metamorphosis. Artificial seed collection is the artificial induction of glochidia to parasitize fish bodies.
1. Selection of host fish. Host fish should be gentle in temperament and slow-swimming; otherwise, the glochidia will drop off prematurely and fail to complete metamorphosis. They should be robust, free of disease and injury, with intact fin rays; otherwise, the host fish may die before the glochidia complete metamorphosis. In addition, they should be easy to obtain. In actual production, yellow catfish (Pelteobagrus fulvidraco) and bighead carp (Aristichthys nobilis) are usually chosen as host fish. Yellow catfish prefer quiet, are afraid of disturbance, have strong tolerance, and a high rate of glochidia attachment (one yellow catfish 15 cm in body length can host 200-300 glochidia); bighead carp are gentle in temperament and easy to raise and manage, and are also a relatively ideal host fish. After capture, host fish need short-term temporary holding to allow mud and sand in the body surface, fins, and gills to be expelled while recovering strength. The temporary holding density should not be too high, generally 150-200 fish per cubic meter of water. During temporary holding, keep the environment as quiet as possible so the fish are not disturbed. If the temporary holding period is long, feed a moderate amount of food daily; for yellow catfish, feeding earthworms or mussel meat is best, while for bighead carp, mainly fertilize to cultivate water quality.
2. Identification of glochidium maturity. The sign of glochidium maturity is that they have detached from the egg membrane, and the byssal threads and hooks have attachment ability. The identification methods are as follows:
(1) Open the mussel with a shell opener; first observe the gill color. The outer gills of the triangle sail mussel are plump and appear brownish-yellow or a mixture of purplish-black and grayish-yellow; if egg granules are clearly visible from the outside of the gills, it indicates the mother mussel is mature.
(2) Take eggs for observation. When pricking the outer gills with a needle, if the byssal threads of the glochidia stick to each other and form threads, it indicates the glochidia have broken through the membrane and are well matured.
(3) Pick out eggs with a needle and place them on a glass slide; the two valves of the eggs can be seen slightly opening and closing; under a microscope, most or all have broken through the membrane, and the two valves open and close continuously, indicating the glochidia are mature.
When examining, samples should be taken from the near-front part of the gill lamellae. If most eggs are mature, indicating that more than half of the glochidia have broken through the membrane, the standard for seed collection has been reached. The examination of the mother mussel should be short and the movements gentle. Too long a time or excessive stimulation will cause the mother mussel to abort.
3. Methods of Artificial Seed Collection
(1) Still-water seed collection method. After stimulating mature mother mussels by drying in the shade or sun, place them in a seed collection basin with water just enough to submerge the shells. Soon the glochidia will be seen being released; at this point immediately introduce the host fish and appropriately stir the water in the basin to increase the attachment rate and raise the dissolved oxygen content. After 10-20 minutes, move the host fish into a flowing-water pond. The duration of seed collection is related to temperature; when the temperature is high, the glochidia are more active and the seed collection time should be shorter; conversely, it can be longer.
(2) Flowing-water seed collection method. Continuously spray water onto the container holding the mother mussels and host fish so that the water in the container keeps flowing and renewing, maintaining fresh water quality and sufficient oxygen. After one night, the glochidia released by the mother mussels will have parasitized the fish bodies. Then promptly separate and culture the fish with collected seeds. This method has a high loss rate of glochidia.
(3) Net cage seed collection method. In places with a wide water surface, set up small net cages made of 39-mesh polyethylene cloth (1.2×0.8×0.6 m), placing 10-20 mother mussels and 400-500 host fish in each cage. The mother mussels naturally release glochidia for 2-3 hours, and about 200 glochidia per fish will parasitize the gills and fins of the fish, completing one round of seed collection. Then disperse the host fish into sparse culture.
(4) Direct pond seed collection method. The area is 200-300 sq m, water depth 0.5 m, hard mud bottom, sufficient water source, fresh water quality, moderate fertility, no wild miscellaneous fish in the pond, with inlet and outlet provided and filtered with mesh cloth to prevent harmful organisms from entering. During the breeding season, select mature mother mussels, hang one mussel per sq m and stock 20-30 host fish. After the mother mussels have basically released their glochidia, remove the mother mussels. The host fish should be fed and carefully managed, and the pond water kept fertile, lively, and fresh so that the host fish grow well and the development of the glochidia is ensured.
4. Precautions for Seed Collection
(1) When selecting mature mother mussels, movements should be light and quick to prevent abortion.
(2) Control the parasitic density; generally 150-200 glochidia per fish. If the number is too great, nutrition will be inadequate, and in severe cases it will cause the death of the host fish.
(3) The water flow should be appropriate, not too strong or too rapid, to avoid loss of glochidia.
5. Cultivation of Host Fish
After glochidia parasitize the fish body, they must undergo a certain parasitic period before completing metamorphosis. The parasitic period is related to water temperature, as shown in Table 10-1.
　　
Relationship between the parasitic period of triangle sail mussel glochidia and water temperature:
Average water temperature 18-19°C: parasitic period 14-16 days; 20-21°C: parasitic period 12-13 days; 23-24°C: parasitic period 10-12 days; 26-28°C: parasitic period 6-8 days; 30-35°C: parasitic period 3-5 days.
Fish parasitized by glochidia should be cultured with flowing water and appropriately fed and fertilized. As long as they are raised properly, each host fish can yield more than 100 juvenile mussels.
　　
Part Two: Cultivation of Juvenile and Young Triangle Sail Mussels
　　
The cultivation of juvenile and young triangle sail mussels (small mussels) is the foundational work of pearl farming production. With the rapid development of pearl production, the demand for small mussels is growing. How to rapidly cultivate large quantities of high-quality small mussels has become an urgent task for pearl farmers.
I. Collection and Cultivation of Juvenile Mussels
The juvenile mussels that drop off the fish body are 0.2-0.3 cm long, white and transparent, with a very long foot and cilia that beat continuously, allowing forward and backward movement. Because they are tiny and have poor viability, they are extremely closely related to factors such as dissolved oxygen, water flow, food, and harmful organisms. Therefore, management and protection must be strengthened to ensure the survival and normal growth of juvenile mussels.
(I) Collection and Cultivation in Flowing-Water Ponds
1. The flowing-water pond consists of three parts: a storage pond, a sedimentation pond, and a cultivation pond. The cultivation pond can be 2 m×1 m×0.2 m, with each pond having an inlet and an outlet to ensure smooth water flow, and the inlet and outlet should be screened with fine mesh to prevent harmful organisms from entering with the water.
2. The stocking density in flowing-water ponds is 200 parasitized bighead carp of 10 cm body length per sq m or 30-50 yellow catfish of 15-20 cm.
3. If upon examination there are no more white spots on the gills and fin rays of the host fish, it indicates the juvenile mussels have completely detached; at this point remove the host fish, otherwise the fish will eat the juvenile mussels.
4. After removing the host fish, the cultivation pond should continue to maintain continuous water flow, slightly slower in the early stage and faster later, with a flow velocity of 2-3 m/min and a flow rate of 0.5-3.0 tons/hour. At the same time, promptly remove the silt at the bottom of the pond to prevent silt accumulation from turning foul and suffocating the juvenile mussels.
5. The cultivation pond should avoid direct sunlight and prevent the growth of green mud algae that entangle the juvenile mussels.
6. When the collection density is 20,000-30,000 juvenile mussels per sq m, after about 30 days of cultivation, when the shell length reaches 0.5 cm, they can be separated for sparse cultivation.
(II) Collection and Cultivation in Net Cages
1. The net cage for collecting juvenile mussels is made of 39-mesh polyethylene netting, with a layer of plastic film laid on the bottom of the cage, and 1 cm of mud spread on top of it. The net cage area is generally 1-2 sq m. The net cage should preferably be placed in water with slight flow. The host fish are cultured in the net cage.
2. Stock 150 bighead carp of 10 cm or 30-40 yellow catfish of 15-20 cm per sq m. During the cultivation period, frequently scrub off the attachments on the net cage to ensure water exchange inside and outside the cage. After about 10 days of cultivation, the glochidia can detach on their own. At this point remove the fish and continue cultivating the juvenile mussels.
3. The stocking density of juvenile mussels is 10,000-15,000 per sq m. After 50-60 days of cultivation, when the shell length reaches 0.8-1.0 cm, they are separated for sparse cultivation.
II. Cultivation of Young Triangle Sail Mussels
Generally, mussels with a body length of 1-9 cm are called young mussels; those with a body length of 10 cm or more are called adult mussels. Methods for cultivating young mussels:
(I) Mud Pond Flowing-Water Cultivation Method
1. Pond Site Selection
(1) Sufficient water source, good water quality, long sunlight duration, and convenient management.
(2) Located at the lower end near a relatively large pond (storage pond), with a certain water level drop, able to regularly introduce fertile pond water to cultivate mussels. The ratio of mud pond area to water-supply pond area is 1:5.
2. Pond Construction Requirements. The mud pond is designed with parallel water supply, using culverts to connect the mud pond with the pond, or a branch culvert to lead pond water into the main inlet ditch and then separately into the mud ponds, so that fertile pond water continuously flows into the mussel cultivation mud ponds. Taking surface water from the pond is best. The mud pond is rectangular, 10-20 m long, 1.5-2.0 m wide, 0.5-0.6 m deep, with a flat bottom covered with 3 cm of thin fertile mud. Both ends of the mud pond have an inlet and an outlet, with the outlet 20 cm above the pond bottom, and the inlet and outlet filtered with mesh cloth. Before stocking young mussels, disinfect each sq m of mud pond with 200 g of quicklime.
3. Stocking Density and Time. Stocking time is generally in mid to late June; if possible, the earlier the better. Stock about 300 juvenile and young mussels of 0.8-1.0 cm body length per sq m (150,000-200,000 per mu). By early September the small mussels can reach 7-9 cm, with a survival rate of over 95%, and by mid-September pearl implantation can be carried out. When stocking, gently and evenly scatter the young mussels into the mud pond by hand.
4. Feeding and Management
(1) Water quality management. The water source for the mud pond mussel cultivation comes from the pond (storage pond). When the pond water becomes too lean, promptly apply fertilizer; when the water is too fertile, inject an appropriate amount of fresh water, keeping the pond water transparency at 25-35 cm.
(2) Fertilization and pest control in the storage pond. Organic fertilizer is suitable for fertilizing the storage pond; chemical fertilizers should not be used. It is strictly forbidden to splash insecticide over the entire storage pond to prevent and treat parasitic diseases of cultured fish; medicated feed should be used instead, because insecticides can easily cause shock in small mussels, affect their growth, and in severe cases can cause their death.
(3) Disinfection of the storage pond water. Every half month, apply 15 kg of quicklime per mu, mix with water, remove the residue, and splash evenly, maintaining a pH of 7-8.
(4) Frequently remove harmful organisms.
(5) Maintain flowing water throughout the cultivation period; the flow velocity can be lower in the early stage and gradually increased with the growth of the mussels. The flow velocity is generally 5-8 m/min. The water depth of the mud pond should be maintained at about 35 cm.
(6) Regularly check the growth of the mussels and do good work in preventing harm.
(7) At the end of August, that is, half a month before pearl implantation, when the small mussels have grown to 6-8 cm, they should be gathered up and hung in bags (or boxes) for suspension culture.
(II) Small Net Cage Shallow Suspension Cultivation Method
1. The net cage is made with a frame of 40 cm×40 cm×12 cm using bamboo strips and galvanized wire, with 3×2 netting thread and 2.5 cm mesh polyethylene netting, plus a plastic film lining at the bottom, and the netting is wrapped and sewn onto the frame. A layer of river mud is spread on the film.
2. The small net cages are hung in places with sufficient water source and no pollution, fresh water quality, dissolved oxygen not lower than 4 mg/L, pH 7-8, transparency of 30-40 cm, and frequent slight water flow. Hang them 35 cm below the water surface.
3. Place 80-150 small mussels in each net cage, and 50,000-60,000 can be raised per mu of water surface. Small mussels in the same cage should be of uniform size and evenly stocked.
4. As the mussels grow, promptly add and change the river mud; the thickness of the river mud should be such that the small mussels can stand upright.
5. Prevent wind and waves from washing away the mud in the cages; once discovered, promptly replenish the mud.
6. For suspension culture before implantation, simply tear off the plastic film at the bottom of the net cage and wash away the river mud.
　　
Part Three: Pearl Cultivation Surgical Techniques
　　
The pearl cultivation surgery (nucleus-free) on the triangle sail mussel is the key to determining the success or failure of pearl production, and also the key to determining the quality and yield of pearls, so it must be carried out strictly according to operational requirements.
I. Preparation Before Surgery
(I) Main Surgical Instruments
The following tools are needed to complete pearl cultivation surgery: 1. Shell opener; 2. Wooden surgical frame; 3. Wooden peg; 4. Dropper; 5. Shell-opening knife; 6. Pointed scissors; 7. Sectioning knife; 8. Straight-tipped tweezers; 9. Dissecting tray; 10. Small piece board; 11. Sponge; 12. Opening needle; 13. Piece-delivery needle; 14. Gill-pulling board; 15. Other tools such as buckets, basins, brushes, thermometers, etc.
(II) Disinfection, Disinfectant Solutions, and Maintenance Solutions
1. Disinfection. The instruments used in surgery should be placed in a relatively large steam sterilization cabinet or sterilizer and steamed every morning.
2. Disinfectant solutions. These include drugs such as chlortetracycline, streptomycin, and tetracycline, prepared at a concentration of 0.5-1‰. They mainly prevent bacterial infection of the cell pieces and wounds, putrid pieces, and wound suppuration.
3. Maintenance solutions. The main types include PVP maintenance solution, osmotic pressure balance maintenance solution, energy mixture maintenance solution, cytochrome C maintenance solution, and lecithin maintenance solution. The main functions of using maintenance solutions are:
(1) Maintain the balance of osmotic pressure inside and outside the cell pieces, ensuring the cell pieces do not dry out and die after being removed from the body;
(2) Sterilize and detoxify, avoiding bacterial infection;
(3) Accelerate wound healing and promote nacre secretion;
(4) Maintain the neutral environment required for cell respiration;
(5) Increase the survival rate of the recipient mussels.
II. Selection of Surgical Mussels
Surgical mussels include the piece-producing mussels used to make cell pieces and the pearl-culturing mussels that receive the implanted pieces.
By age, the best surgical mussels are exactly one year old, with those less than one year old next best. By size, those in the 7-10 cm range are generally used. For the above specifications, advancing the artificial breeding time of triangle sail mussels to mid to late April, then using net cages, flushing mussels, or a combination of net cages and flushing mussels for rapid juvenile mussel cultivation, generally by the end of August or early September of the same year the small mussels grow to 7-9 cm, meeting the specifications for surgical mussels, and implantation can begin.
III. Surgical Season
March-June and September-December are the surgical periods, with a suitable water temperature of 10-30°C, and the optimum being 18-25°C. Wound healing of the mussel, formation of the pearl sac, and secretion of nacre are all related to water temperature. Within the water temperature range of 30°C, plankton in the water reproduce quickly, natural food is abundant, the mussel's metabolism is vigorous, the nacre secretion function is strong, wounds heal quickly, and a pearl sac can form 5-7 days after implantation, with nacre secretion beginning after 10 days. Comparing spring and autumn, autumn surgery is better, because in autumn and early winter (September to early December), the water temperature of 15-25°C is most ideal. If the small mussels cannot reach surgical specifications for the time being, they can continue to be cultivated until March-June of the following year. But the implantation surgery must be completed before the end of June. After June, they become two-year-old mussels, and the pearl cultivation effect is poor; if there are many small mussels, it is better not to use them.
IV. Preoperative Temporary Holding of Surgical Mussels
First, 7-10 days before surgery, thoroughly wash the mud from the mussel baskets (cages), select mussels that have reached specifications and place them in mud-free baskets (cages) for temporary holding. The purpose is for the mussels to shed mud early, thicken the mantle membrane, and increase its tension, facilitating production and implantation. Second, the night before surgery, take the mussels back and place them in a clean water pond or basin, using flowing water or changing the water, so that food and waste in the mussel bodies are expelled, which helps make piece production clean and hygienic and prevents wound infection after implantation.
V. Surgical Operation
(I) Piece Production Requirements and Technical Points
1. When cutting the anterior and posterior adductor muscles, the knife must not deviate left or right, to avoid cutting across the mantle and affecting piece production.
2. When trimming off the colored line at the edge of the mantle, it must be trimmed clean, to avoid bone pearls forming from remnants.
3. Tearing the membrane should start from the posterior end of the mantle, making the most of the small pieces near the excurrent siphon; this section of pieces is of high quality and forms good-quality pearls.
4. The torn outer layer tissue strip of the mantle should be placed flat on the glass slide, and the rough parts and uneven width on both sides trimmed flat.
5. Sectioning should be decisive and crisp, with flat cuts. Medium-sized pieces are best, generally 3.5 mm wide × 4.0 mm long, becoming square after contraction.
6. Use tweezers to hold absorbent cotton and wipe away triangular pieces, diamond-shaped pieces, mucus, etc.
7. After sectioning, promptly drip maintenance solution to prevent the cell pieces from drying out and dying.
8. The piece production process should be completed by one person within 2-3 minutes, and promptly used for piece implantation; do not leave them for long, which would make the pieces turn white.
9. Disinfectant solutions, absorbent cotton soaking solutions, cell piece maintenance solutions, etc. should be prepared the same day and used up the same day, strictly controlling each disinfection step.
(II) Piece Implantation Requirements and Technical Points
1. Engrave the number and date on the shell of the selected recipient mussel, with the characters as clear as possible.
2. Hold the piece-delivery needle in the right hand and the opening needle in the left hand, starting from the rear region, occupying 50% of the mussel, and operating in order from left to right and from top to bottom.
3. The implantation depth should be moderate, not piercing through the mantle. If the piece is implanted too deep, shell-attached pearls or string pearls will appear. If implanted too shallow, piece ejection or burnt-head pearls will appear.
4. Implant 30-34 small pieces per pearl-culturing mussel, arranged in a “” shape. The arrangement of implanted pieces goes from the mussel edge inward, generally (6-5-4-2)×2 or (6-5-3-2)×2, and the pieces should be evenly spaced.
5. Rounding is mainly accomplished by gently pushing, pulling, pressing, and squeezing with the opening needle. The roundness of the pearl greatly affects the pearl price.
6. As far as possible, use same-position replacement, that is, pieces from the tail-head position are implanted into the tail-head position, and so on.
7. When opening with the shell opener, it must not be opened too wide, and the peg must not be too large; use a shell-attaching device as much as possible. If the shell opener opens too wide or the peg is too large, it will injure the adductor muscle and cause death; when implanting into young mussels, not using the shell-attaching device will cause the edge shell to break.
8. When implanting pieces, the quality of the small pieces must be strictly controlled. Do not implant triangular pieces, prismatic pieces, dry pieces, whitened pieces, dead pieces, etc.
9. The surgical time for each mussel is required to be completed within 3-5 minutes. After completion, drip disinfectant solution or maintenance solution, or place them in the above medicinal solution for temporary holding, and then promptly suspend them for culture.
　　
　　
Part Four: Cultivation of Pearl-Culturing Mussels
　　
I. Environmental Requirements for Pearl-Culturing Waters
Pearl-culturing mussels live in water. The water environment not only determines whether the pearl-culturing mussels can survive and grow, but also directly affects the yield and quality of cultured pearls. Therefore, the selection of the water environment is very important.
1. The water surface must not be too small; more than 6,000 sq m is required. If the area is too small, the water quality environment changes greatly and is unstable, affecting the growth of pearl-culturing mussels.
2. Water surfaces with a depth of 1.5-4.0 m can be used for pearl culture, 2-3 m is best, and depths below 1 m or above 5 m are unsuitable for farming.
3. The water source must be abundant and free of all kinds of pollution, and the content of toxic substances must not exceed the fishery water quality standard.
4. Flowing water at a certain speed (not exceeding 0.1 m per second) is beneficial to the growth of pearl mussels and the growth of pearls. Production practice has shown that for triangle sail mussels (Hyriopsis cumingii) cultured for pearls in rivers and lakes with flowing water and good conditions, 5-8 g of pearls per mussel can be produced in one year of culture, whereas in still water areas it takes 3 years of culture to produce 5-8 g of pearls per mussel.
5. Sufficient sunlight can directly produce a thermal effect, thereby providing energy for pearl mussels and their food organisms. Light also has a direct effect on the color and luster of pearls. The water area should be kept ventilated so that the water surface ripples, which can increase the dissolved oxygen in the water and also promote the movement of the water body and the transfer of heat.
6. For pearl culture waters, clay soil is best, sandy soil is second, and loess and sandy loam are the worst. The pond bottom should have a suitable amount of mud.
7. A pH value of 7-8 is most suitable for the growth of pearl mussels and the secretion of nacre.
8. As for dissolved oxygen, in general the water body should be greater than 5 mg/L in summer and greater than 3 mg/L in winter.
9. For pearl mussel waters, a transparency of 30 cm is good.
10. The optimal water temperature range for pearl mussels is 32-34°C (for triangle sail mussels).
11. Among the inorganic salts in the water body, calcium ions are the most important, because calcium is the main component of pearls and mussel shells. Usually the calcium content in the water is required to be above 10 mg/L. When the calcium content in the water body is low, it can be solved by applying quicklime.
12. Nitrogen is a nutrient element essential for the growth of all algae. A high ammonia nitrogen content indicates that the water body is relatively fertile; if it exceeds 2 mg/L, it indicates that the water body is relatively seriously polluted, which affects the growth and survival of mussels; when the ammonia nitrogen content is lower than 0.1 mg/L, it indicates that the water quality is too poor and cannot meet the needs of the reproduction and growth of natural food organisms.
13. The nitrite content should be 0.01-0.10 mg/L. If the content exceeds 0.10 mg/L, slow growth of mussels and pearls or death of mussels will occur.
14. There should be no aquatic vascular plants in the culture waters, to prevent them from competing for nutrients with the food organisms of pearl mussels. Some harmful organisms such as shrimp and crabs should also be removed in time.
II. Selection of Pearl Culture Waters
Whether it is a pond, ditch, harbor, open water, lake, reservoir or river, as long as the environmental conditions of the pearl culture water meet the above requirements, it can be selected as a pearl culture water for raising mussels and culturing pearls.
III. Culture of Pearl Mussels
1. At present, pearl mussels are mostly cultured by hanging and cage culture. Hanging culture methods include single hanging and string hanging, in which more than two mussels are hung on one line. The number in cage culture depends on the size of the cage (net cage, net clip); generally, it is suitable that each mussel can touch the bottom of the cage.
2. For pearl mussels, generally 800-1200 per mu are cultured in ponds; 600-800 per mu in open water. The specific culture density should be increased or decreased appropriately according to water quality fertility, water flow, water depth and other physicochemical and biological factors, as well as the size of the mussels.
3. Pearl mussel culture has become the main production content of the comprehensive development and utilization of local open-water fisheries. Ponds mainly used for pearl culture can be appropriately stocked with grass carp, bream, silver crucian carp, mandarin fish, yellow catfish, etc., and a small number of silver carp and bighead carp can be added, so as to fully tap the production potential of the water body.
IV. Management of Pearl Mussels
1. Management during the temporary holding period after surgery. After surgery, pearl mussels are temporarily held in waters with slight flowing water and clear water quality for 15-30 days of recovery. During this period, after hanging culture, generally do not turn them over at will, do not take them out of the water, and even more do not open the shells for inspection, so as not to affect their physical recovery and the formation of the pearl sac. If the water is too fertile and the transparency is less than 30 cm, measures should be taken to reduce the fertility. One month later, inspect the mussels in the temporary holding water, remove dead mussels promptly, and then hang them again in the pearl culture waters.
2. Strengthen water filling, water changing and aeration. When raising pearl mussels in closed still waters without flowing water, high temperature, shallow water and low pressure often cause oxygen deficiency and death of mussels. Therefore, water must be added and changed in time, or the water quality must be kept fresh and the oxygen content sufficient, so as to avoid slow growth of mussels and pearls under oxygen deficiency or low oxygen conditions.
3. Adjust the hanging depth in time. According to changes in water level, water temperature and season, the hanging depth of pearl mussels should be adjusted in good time. In winter, they should be hung appropriately deep (40-60 cm below the water surface); in spring and autumn, they can be hung shallower (15-20 cm below the water surface); in summer, hang them 20-30 cm below the water surface (water temperature 32-34°C).
4. Do a good job of fertilizing the waters. If the water quality of the pearl culture pond is clear and poor, fertilize in time. The fertilizer can be organic fertilizer or chemical fertilizer, or both. For organic fertilizer, the manure of chickens, ducks and birds is good; before use, it needs to be mixed with 1% quicklime or applied after fermentation and decomposition. Chemical fertilizers are mainly urea and superphosphate (usage ratio 1:2 or 1:3); the two are mixed with water and splashed, achieving small amounts and frequent applications; generally 1 kg of urea and 3 kg of superphosphate are used per mu. Soybean milk (feed and fertilizer) can also be splashed. At the same time, apply calcium fertilizer (quicklime); apply more when the pH value is low, and apply less when the pH value is high. Keep the pond water transparency at about 30 cm.
5. Remove attachments. After pearl mussels have been hung for a period of time, large amounts of attached mud and organisms (such as water algae, freshwater sponges, Zoothamnium, etc.) often grow on the hanging cages, net bags and mussel bodies, affecting the growth of pearl mussels, so they must be cleaned regularly. From April to October, brush and wash once every month.
6. Maintain the culture facilities. During the culture process, if damage to the culture facilities is found, it should be repaired in time to avoid economic losses.
V. Disease Prevention and Control of Pearl Mussels
The causes of disease in pearl mussels are relatively complex, such as inbreeding of mussel species, which reduces culture performance; repeated culture in the same water surface for many years, which reduces the trace elements needed for pearl culture and weakens disease resistance and pearl-producing ability; deterioration of the water environment, pollution of water sources, and invasion by various pathogens; lax disinfection during surgical operations, and weak, thin mussels with poor disease resistance. These are all direct causes leading to frequent occurrence of mussel diseases. It is not easy to detect disease in pearl mussels, and once they become ill, it is very difficult to control and treat. Therefore, mussel diseases should focus on prevention, with treatment as a supplement.
(I) Prevention of Mussel Diseases
1. Self-breeding and self-raising, and strictly prohibit the purchase of small mussels from outside. Do not purchase, or purchase few, young mussels from other places, so as to cut off the transmission routes of pathogenic bacteria. Avoid inbreeding and enhance the vitality of offspring.
2. Do a good job at the disinfection stage. Strict disinfection is required before and during surgery. Especially in the high-temperature season from May to October, disinfection and disease prevention work should be done even better.
3. Culture waters should be rotated. The same water area should not be used for continuous pearl mussel culture for many years. If conditions permit, rotational culture or intercropping culture can be implemented: after every 4-5 years of culture, rest for 1-2 years, or after 5-6 years of culture, change the culture waters.
4. Isolate diseased mussels in time and remove dead mussels.
5. Improve the ecological environment. Thoroughly remove silt; change water and disinfect regularly; change still-water culture to flowing-water culture; change dense culture to sparse culture; improve fertilization techniques, and so on.
6. Change the implantation time. Change spring and autumn implantation to implantation in the safe period in late autumn.
7. Regularly apply microecological preparations such as photosynthetic bacteria.
(II) Treatment Methods for Common Diseases
1. Bacterial gill rot and foot rot disease. The peak epidemic period is from May to July. You can splash the whole pond with 1 g/cubic m of bleaching powder or 1-4 g/cubic m of Chinese gallnut; in severe cases, soak with 20 g/cubic m of potassium permanganate or 2% salt water for 15 minutes, which can effectively prevent its spread.
2. Saprolegniasis. For each mu per meter of water depth, use 0.5-1.0 kg of salt, 0.5 kg of urea, and 1.0 kg of bleaching powder (containing 30% effective chlorine); dissolve and mix them in water, then splash the whole pond. For severe cases, use continuously for 2-3 days (times).
3. Edema disease. Splash the whole pond with 10 g/cubic m of quicklime every month; in severe cases, soak with 0.1-0.2% chlortetracycline hydrochloride solution for about 15 minutes, and repeat once every other day.
4. Bacterial enteritis. In the epidemic season from April to October, disinfect the whole pond with 1 g/cubic m of bleaching powder or 15 kg/mu of quicklime, and at the same time inject diseased mussels with 1-2 mL of 1-2‰ tetracycline hydrochloride, or inject 1-2 mL of 2‰ chlortetracycline plus calcium gluconate. Soak the mussel bodies in 3% salt water for 5 minutes, and at the same time splash the culture racks with salt water for disinfection.
5. Parasitic gill disease. In the epidemic season from May to July, you can use copper sulfate and ferrous sulfate in a 5:2 ratio at a concentration of 0.7 g/cubic m to splash the whole pond; use 90% crystalline trichlorfon at a concentration of 0.2-0.5 g/cubic m to splash the whole pond; use 1-2% salt water to soak diseased mussels for 5-8 minutes.
6. Mussel plague. At present, there is no good medicine for mussel plague, so the focus should be on disease prevention. In the epidemic season from May to August, diseased mussels can be injected with 0.1-0.2% tetracycline, 1-2 mL per mussel (into the foot), once every other day.
　　
　　
V. Breeding Technology of the Japanese Pond Butterfly Mussel
　　
The pond butterfly mussel is a species endemic to Japan, produced in Lake Biwa in Shiga Prefecture, Japan. It belongs to the same genus as but is a different species from China's triangle sail mussel. According to relevant materials, the main quality indicators of the pond butterfly mussel are superior to those of the triangle sail mussel, and the quality of pearls produced is high. Japan mainly relies on the pond butterfly mussel to cultivate freshwater pearls.
In order to change the variety structure and improve culture efficiency, some units (households) in our city have now introduced the Japanese pond butterfly mussel and have carried out artificial breeding, rearing and pearl culture with success. The relevant techniques are introduced as follows:
I. Morphological Characteristics of the Pond Butterfly Mussel
The external morphology and internal structure of the pond butterfly mussel are very similar to those of the triangle sail mussel, but there are also certain differences. The specific morphological characteristics are:
1. External morphology. The individual is large, the two shells are convex, the body is short and long, the front end is bluntly rounded, the rear end is pointed and long, the dorsal margin extends upward into a triangular wing that is relatively low, the shell is thick, the periostracum is black, and the mussel body is heavy.
2. Internal structure. The hinge area of the dorsal margin of the shell has well-developed hinge teeth and an elastic horny ligament, the nacreous layer is white, clean and bright, the anterior and posterior adductor muscles are strong, the connective tissue of the mantle is well developed, especially the connective tissue of the marginal mantle is even more developed, the visceral mass is large, and the crystalline style is thick and long.
II. Artificial Breeding of the Pond Butterfly Mussel
1. Age at gonadal maturity. Breeder mussels enter the peak breeding period at 4 winters of age, and can even maintain relatively vigorous reproductive capacity up to 6 winters of age.
2. Male and female identification. Identification is based on external appearance and the characteristics of the gill filaments. At the same age, female mussels are slightly larger than male mussels; female mussels have slightly wider growth lines, greater shell width, a relatively rounder and blunter rear end, and more closely spaced outer gill filaments; male mussels have smaller shell width, a narrower and smaller mussel body, a slightly pointed rear end, and more sparsely spaced outer gill filaments.
3. Breeding season. The breeding season of the pond butterfly mussel is from May to July. Under normal circumstances, the first batch of mature glochidia is released in mid-May.
4. Breeding habits. It ovulates multiple times and matures in stages; each mature batch is released and discharged batch by batch until exhausted. One mother mussel can ovulate 4-5 times during the breeding season.
5. Identification of glochidia maturity. The sign of glochidia maturity is that they have detached from the egg membrane, and the byssus and hooks have attachment ability. During identification, use a speculum to open the mussel; the outer gills can be seen to be plump and yellow (slightly lighter in color than those of the triangle sail mussel). When a needle is inserted into the outer gill, a continuous thin thread can be drawn out. The longer the thread, the better the maturity.
6. Use the flowing-water seedling collection method. After glochidia detachment, remove the host fish, cultivate them in the breeding pond to 1-2 cm, then screen and divide them out of the pond and transfer them to juvenile mussel culture.
III. Juvenile Mussel Culture of the Pond Butterfly Mussel
1. Use small net cages of 3 m × 2 m × 0.7 m to culture juvenile mussels. The mesh size of the net cage is 2.5 cm. The bottom of the cage is lined with net cloth, and the bottom frame is divided into 24 small compartments with small bamboo. The top is fixed and floated with large bamboo. Each net cage holds 8,000 juvenile mussels. By the end of the year, they are divided into boxes; the average individual size is 4 cm, and the average survival rate can reach 86%.
2. Use small net frames of 0.6 m × 0.4 m × 0.15 m, with a mesh size of 2.5 cm, lined with snake-skin bags, and 3 cm of nutrient soil placed inside. Each frame holds 300 juvenile mussels. Cultured until the end of the year, the average individual size can reach more than 6 cm, and the average survival rate can reach 95%.
3. At the end of the year, divide them into boxes. Juvenile mussels below 5 cm are still kept in boxes for culture, and those above 5 cm are hung for culture in round net bags with bamboo rings added inside, with 5 mussels per bag. During hanging culture, the juvenile mussels in the bags must be turned frequently to avoid the appearance of "lotus-leaf edges" on the ventral margin of the mussels. Cultured until the end of the second year, the average individual size grows to 10 cm, and the average survival rate can reach 95%.
IV. Rearing of Preoperative Preparatory Mussels
When the juvenile mussels in the round net bags grow to 10 cm, use double-layer long net bags to divide them into 8 small bags, with 1 mussel per small bag, hung with the ventral margin facing upward. When the preparatory mussels grow to 13 cm, they can be used for surgery.
V. Surgical Implantation of the Pond Butterfly Mussel
Preparatory mussels of 13 cm or more can undergo nucleus surgery. The nuclei used for surgery are round spherical nuclei of 7-8 mm, and 6-10 are inserted per mussel. One month after surgery, the average survival rate can reach 95%, and the average nucleus retention rate can reach 80%.
VI. Quality Comparison Between the Pond Butterfly Mussel and the Triangle Sail Mussel
According to a comparison between the pond butterfly mussel and the triangle sail mussel bred in the same period, cultured in similar water quality, and subjected to the same culture and surgical methods, the pond butterfly mussel has the following excellent traits and characteristics:
1. Although the pearls produced by the local triangle sail mussel are good in quality, the shell distance at the nucleus insertion site is small and the mantle is thin, so large pearls cannot be cultured. Although the winged pearl mussel (Cristaria plicata) has a large shell distance at the nucleus insertion site and a thick mantle, the mantle has too high a water content and easily spits out nuclei, and even if pearls are cultured, the quality is relatively poor. The pond butterfly mussel has both the advantage of good pearl quality of the triangle sail mussel and the advantages of large shell distance and a thick mantle, making it an ideal variety for cultivating large round pearls.
2. The connective tissue of the mantle of the pond butterfly mussel is well developed; the marginal mantle is 1/3 thicker than that of the triangle sail mussel, and the central mantle is 2 times thicker than that of the triangle sail mussel. Therefore, the transplanted cell pieces carry more connective tissue, are easy to survive, and have a strong ability to secrete nacre. The pearls are bright, the pearl layer is thick, and the quality is high, equivalent to pearls from surgical triangle sail mussels cultured for 2 years in the same water quality.
3. The adductor muscle of the pond butterfly mussel is strong, with a diameter 20% larger than that of the triangle sail mussel. Therefore, the shell opening width can exceed 1.0 cm. According to experiments, a 13 cm long mussel can still close normally and survive after being opened to 1.6 cm, which is beneficial to surgical operations.
4. The pond butterfly mussel has a heavy body, a large visceral mass, a thick and long crystalline style, and a high proportion of soft parts in the total weight of the mussel body, indicating that the pond butterfly mussel has good physique, strong vitality, and a high culture survival rate. Through comparison of the survival rates of pond butterfly mussels and triangle sail mussels of the same age, the survival rates of mussels of the current age are similar; starting from the second year of age, the survival rate of the pond butterfly mussel becomes higher and higher, while that of the triangle sail mussel becomes lower and lower.
5. The shell of the pond butterfly mussel is 50% thicker than that of the triangle sail mussel, especially the nacreous layer of the shell is thick, which is beneficial for processing nacre powder and is also an ideal material for making pearl nuclei and carvings.