Lesson2: Pearl Cultivation

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a. Non-Nucleated Cultivation

The non-nucleated pearl cultivation technique is a method that does not rely on an external nucleus to produce pearls and is primarily used for cultivating freshwater pearls. Here are the basic steps for non-nucleated pearl cultivation:

• Selection and Preparation of Host Mussels: Select healthy mussels with excellent genetic traits that are easy to cultivate. Typically, 1-2 year old mussels are chosen, such as the freshwater Triangle Sail Mussel (Hyriopsis cumingii).
• Cutting Epithelium Tissue: Cut small pieces of epithelium tissue from the host mussel’s mantle. The mantle is the part of the mussel that secretes nacre (pearl material) and is rich in cells capable of stimulating the formation of a pearl sac.
• Surgical Insertion: Perform a surgical procedure on the selected adult mussel. Open the mussel and locate a suitable area, usually the region rich in connective tissue. Carefully insert the small epithelium tissue fragment into the connective tissue. Precision is key to minimizing damage to the mussel.
• Pearl Sac Formation: The inserted mantle tissue triggers an inflammatory response within the mussel, and surrounding connective tissue encapsulates it, forming a pearl sac. Cells within the pearl sac begin to secrete nacre, gradually wrapping the tissue fragment and forming a pearl.
• Cultivation Management: After surgery, place the mussels in a well-monitored aquatic environment with appropriate temperature and oxygen levels. Regularly check water quality and provide nutritious feed to ensure the mussels’ health. Periodic inspections of the mussels’ condition and pearl development are necessary, as well as managing potential diseases or parasites.
• Harvesting Pearls: Depending on the pearl growth cycle (typically 1-3 years, depending on the mussel species and cultivation conditions), extract the pearls at the appropriate time. After harvesting, the mussels may rest and be used for the next round of cultivation.
• Characteristics of Non-Nucleated Pearls:
  • Diverse Shapes: Because there is no fixed external nucleus, non-nucleated pearls often form irregular shapes, such as oval, button, flat, or teardrop shapes. However, some high-quality non-nucleated pearls can be nearly round, though one end may be slightly pointed.
  • Higher Yield: Compared to nucleated cultivation, non-nucleated pearl production is more efficient. A single mussel can produce multiple pearls (a single mussel can produce over 20 pearls), increasing overall output.

b. Nucleated Cultivation

The nucleated pearl cultivation technique is widely used for cultivating seawater pearls and some freshwater pearls. It involves inserting an artificially crafted nucleus into the mussel to stimulate nacre secretion around it, forming a pearl. Notably, Akoya pearl cultivation has led to advancements in seawater pearl techniques (including South Sea and Tahitian pearls). Below is the step-by-step process for nucleated pearl cultivation:

• Selection of Pregnant Host Mussels: Select mature, healthy Akoya host mussels (about 1-2 years old) for nucleated pearl cultivation.
• Preparation Work:
  1. Suppressing Mussel Activity: Performing surgery on active mussels can trigger a rejection response, causing mussels to expel the nucleus or become damaged, sometimes even resulting in death. To prevent this, mussel activity is suppressed before the surgery through artificial spawning and reproductive gland suppression techniques.
  2. Nucleus Preparation: The pearl nucleus is typically made by grinding certain types of shells (such as Pigtoe Mussel, Mapleleaf Mussel, or Washboard Mussel) into spheres, which are hard and durable. The quality of the nuclei is crucial, as they need to be firm and resistant to corrosion.It is important to note that the size of the nucleus is limited by the size of the host mussel. If the nucleus is too large, the mussel may struggle to close its shell, potentially leading to death due to the inability to close both valves tightly. For Akoya pearls, the nuclei are made from the shells of American Pigtoe Mussels produced in freshwater environments. The typical size of Akoya nuclei ranges from 2mm to 7.5mm. For Pinctada maxima (Mother of South Sea pearl) and Pinctada margaritifera (Mother of Tahitian pearl), the nuclei usually range from 10mm or larger, sometimes reaching up to 16mm or more.
• Surgical Insertion: Skilled technicians use wooden or plastic openers to gently open the mussel. The nucleus is precisely inserted into the appropriate mantle location (typically in a connective tissue-rich area) using specialized tools. A small piece of epithelium tissue (called a “graft”) is placed around the nucleus. The graft helps stimulate pearl sac formation and influences the pearl’s color. For example, grafts from Pinctada fucata (Mother of Akoya pearl) with yellow pigments often produce yellow pearls, regardless of the host mussel’s natural color. The success rate of producing pearls ranges from 60% to 100%.
• Cultivation Management: After surgery, place the mussels in carefully controlled environments. Monitor seawater temperature and provide shade or cooling measures as necessary. During winter, if the water temperature drops below 10°C, the mussels should be moved to warmer waters (around 13°C). Similarly, if summer temperatures are too high, mussels should be cooled or relocated. Regular cleaning of the mussels is essential to prevent organisms like barnacles or algae from attaching. Anticipate potential environmental issues like red tides or typhoons.
• Harvesting Pearls: Determine the pearl harvesting time based on the pearl’s growth cycle (usually a few months to several years), depending on the desired pearl size. For Akoya pearls, the cultivation period is typically 1-2 years. Pearls harvested after one year are called “annual pearls,” while those cultivated for over a year are called “multi-year pearls.” The harvesting season for Akoya pearls typically runs from mid-November to February when the nacre layers are thinnest, ensuring the best color and luster. Before harvesting, the mussels undergo a final conditioning phase (called 化粧まき in Japanese) to enhance the pearl’s appearance. After the harvest, tools are used to pry the shells open to extract the pearls, followed by cleaning and sorting.