Mikimoto, Mise, and Nishikawa perfected the methods of saltwater perliculture in the Akova Pearl Oyster (Pinctada fucata) during the early 1900s, leading to a thriving Japanese industry. This guide provides the history of Akoya pearls and how they are produced.
At their pre-World War IIheight in 1938, approximately 350 pearl farms were active in Japan, covering 5,200 underwater hectares and producing over 10 million cultured Akoya pearls. The original technique is still in use in Japan, with modifications and enhancements intended to increase the yield. Nearly all active perliculture ventures worldwide use a version of the basic Japanese procedure. We will use that procedure as the starting point in our discussion of modern perliculture.
To begin, one first needs a reliable source of adult clams. Japanese perliculturists originally employed female divers, called ama, to collect wild adults for nucleation. The site of much of this activity was Honshu Island's Ago Bay, where wild populations of Akoya Pearl Oysters once flourished naturally on the rocky bottom.
Because dependence on gathering mature wild clams risks depleting the resource very quickly, aquaculture methods have been developed to provide a reliable supply of new clams. The first step was deploying spat collectors, submerged tangled materials (e.g., branches, old fishing nets, ropes, or earlier, stones)32 on which free-swimming pearl oyster larvae can settle. These were already in general use by the time Mikimoto began his experiments." The spat could be raised in controlled environments until of sufficient size for nucleation. The next step was the full domestication of the pearl oyster, managing all aspects of pearl oyster reproduction, spawning, and settling under laboratory control.
Aquaculture has the advantage of allowing the farmer to select parent clams for their desirable characteristics, and thus produce a greater number of the next generation with those features. The Japanese have done just that with the Akoya Pearl Oyster. Although its nacre and Akoya pearls were originally yellowish green, like those of the closely related Ceylon Pearl Oyster, aquaculture has created "improved" Akoya with whiter nacre. The disadvantages of aquaculture include the cost of additional facilities, increased labor, and algal food for the young pearl oysters." The more serious loss is that of genetic diversity in the pearl oyster population.
In Japanese pearl farm hatcheries, adults are induced to spawn, and fertilization occurs, in large aquaculture tanks. The planktonic larvae are maintained and fed for fifteen to twenty-five days, until they are ready to settle and metamorphose into juvenile clams. At this point, spat collectors are introduced to the tank, providing convenient hard surfaces for settlement. The juveniles are plucked off the spat collectors and packed into baskets for transport out into the bay to be suspended from floating lines. There they remain for a "grow out" period originally lasting about three years, and more recently reduced to two years."
During the long "grow out" period, water quality in the bay must be monitored closely for any change in temperature, relative amounts of planktonic food, or dangerously toxic red-tide organisms that might adversely affect the health of the crop. The hanging baskets provide protection against predators such as starfish, crabs, and fish. but also provide additional surface on which algae. barnacles, anemones, and cementing worms can settle. These so-called fouling organisms on the outer surfaces of the shells can cause deformities and prevent the shells from closing; on the baskets they can interfere with water circulation." Periodic cleaning of the baskets and shells is therefore necessary." Assuming that all goes well, in two years' time, the juveniles-now known as -mother oysters"-have grown large enough to be returned to the pearl farm.
Up to this point, perliculture is similar to most other aquaculture ventures. Now, however, the pearl oysters undergo a process that is unique to pediculture: grafting (also called nucleation or seeding), meaning the implantation of a pearl nucleus, with or without a shell bead, into a pearl oyster. Skilled grafters (still mainly Japanese in most countries) are experienced, highly paid technical staff whose work results tend to be carefully tracked in terms of the percentage of quality pearls produced"'
The pearl oysters must first be prepared for grafting. It is the natural response of a bivalve to close tightly when removed from water or handled. Pearl oysters are therefore placed in a water bath, sometimes of slightly higher temperature or with a mild anesthetic, until they relax and gape. This step must be carefully controlled-over-relaxation can kill the animal. Once it is gaping, a wooden or plastic wedge is inserted into the shell, holding the valves apart, a technique adapted from early freshwater perliculture experiments." The pearl oyster can then he removed from the water and placed into the grafting clamp without closing when the wedge is removed. But time is now critical-too long in this position can also be fatal.
The next step is literally a surgical operation. Great care is taken not to stress the pearl oyster more than necessary. A small incision is made into the gonad (reproductive organ) of the clam, forming a pocket. The grafter then uses tools resembling dental picks to introduce two ingredients: a bead nucleus and the piece of tissue called the graft. The bead will ultimately form the nucleus of the pearl. The graft will proliferate to form the pearl sack that will secrete layers of nacre onto the bead." It is the nature of these cells that will determine the color and quality of the cultured pearl.
The bead nucleus, usually a perfect white sphere, is produced from the thick shells of freshwater pearl mussels harvested from the Mississippi River watershed of North America. Grafters use the largest practicable beads to produce the largest possible pearls. They must judge carefully, however. A bead that is too large for a given pearl oyster might be rejected or kill the clam. The maximum bead size used for Akoya Pearl Oysters is 9 millimeters. 4'
The graft inserted with the bead is a 2millimeter square of nacre-producing epithelium freshly cut from the frilly mantle edge of another Akoya Pearl Oyster." The grafts are coated with dyed antibiotic to help prevent infection, and for easier visualization, and are kept moist on a wet graft-trimming block-traditionally a block of soft wood such as crepe myrtle or magnolia." The graft must be inserted "right side down" so that the nacre-secreting surface faces the bead nucleus. Incorrect insertion can produce a deformed pearl, or coat the bead with non-nacreous shell material. Circle pearls could be the result of the graft folding during insertion."
One or two sets of bead-plus-graft are inserted in each Akoya Pearl Oyster. Once these nucleus materials are in place, the grafter's task is complete. The incision wound does not require suturing-in a healthy clam under good conditions, it will heal itself. The grafting clamp is released, and the clam closes naturally. Nucleated pearl oysters are then moved into "post-operative" care, consisting of relatively crowded racks or baskets suspended from log rafts close to the laboratory." After this one- to two-week period, any dead clams or other "unsuccessful grafts" are discarded. Those remaining are cleaned and spread into culture racks with ample room between them to grow and feed, after which they are moved into more open waters of the lagoon. In Japan's Ago Bay, the racks hang from culture rafts at 2-3 meters depth."
The culturing period is long and full of apprehensive hope-hope that the clam will survive, that the environment will cooperate, and that the nacre will be deposited evenly to produce a perfect pearl. But this period is not without activity. Although (he pearl oysters now reside in open waters with adequate natural plankton as food, care and maintenance are similar to those used during the "grow out" period. Water quality is monitored, and fouling organisms are cleaned from the pearl oysters and their culture racks with scrapers and high-pressure water jets.
The length of the culture period is critical. If too brief, a thin, inadequate layer of nacre will result, while too long a period risks losing pearl oysters to natural causes and pollution." Akoyas were once held three years until harvest, but like the "grow-out" period, this has been shortened to six months to two years.4'
At the end of the culture period, the pearls are harvested. Harvesting occurs in winter, when the pearl oysters are less active, and nacre production, slowed down by the cold, has reached peak luster.' In Japan, harvesting results in killing the nucleated pearl oysters. Even under the best of circumstances, a remarkably small percentage of the harvested pearls will actually be marketable.
Out of every one thousand nucleated Akoyas in an average Japanese pearl farm, five hundred will die during the culturing period, about two hundred and fifty will produce poor-quality pearls, two hundred will produce saleable pearls of low to medium quality, and only fifty will produce topgrade gem-quality pearls. In comparison, as of the year 2000, pearl yields from the Black-lipped Pearl Oysters in French Polynesia are even lower: of one thousand nucleated pearl oysters, four hundred and forty will die during the culturing period, and two hundred and forty will reject the nucleus. Of the pearls produced from the remaining three hundred and twenty pearl oysters, only eight will be high quality.
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