THE MOLLUSK OF NATURAL PEARLS FROM AUSTRALIAN PINCTADA MAXIMA
P. maxima (described in Jameson, 1901) is the largest species of the Pinctada genus and indeed the largest of the “pearl oysters,” reaching sizes that may exceed 40 cm. The species has an extraordinary life span of up to 40 years or longer. It occupies a wide-ranging area of the Pacific, from Burma to the Solomon Islands, with Australian, Papua New Guinean, and Philippine waters the traditional habitats. Indeed, it may still have prolific shell beds in these areas. The range extends from Hainan, off the coast of China, down to the eastern and western coasts of Australia. The mollusk lives at depths of up to 90 meters, but growth rates are optimized if the depth is limited to 30–40 meters.
P. maxima have a light beige color externally, though variants do occur, and radial markings are absent. Internally, the nacre is thick and has a high luster, with the outer border having a gold or silver band, the reason why P. maxima is popularly known as the golden- or silver-lipped pearl oyster. The left valve is convex and the right valve only slightly so. Pea crabs, Pinnotheres villosulus, live in symbiotic harmony with some 85% of Pinctada maxima, both wild and hatchery-grown (figure 11). Such close associations between various mollusks and pea crabs are common. Upon opening P. maxima, one is often treated to the extraordinary sight of a small crab scurrying around within the mantle cavity, as if the lower portion were a bed on which to lay its weary head while the upper portion holds the comforting blankets to its shell cradle.
As natural pearls may form within P. maxima as the result of some trauma to the mantle, it is interesting to speculate on the possible role of intruding crustaceans in producing these magnificent wonders. The animal certainly does wander in the region of the gills (which filter water and exchange oxygen), and by all accounts this appears to be the area of the mollusk where most natural pearls form. Figure 12 shows this position to be typically within the mantle
and in front of the gills, close to the widest point of the adductor muscle.
Natural blister pearls that encase dead pea crabs inside the shell have been noted on several occasions, not only in P. maxima but also in other shells (Edwards, 1913; Hedegaard, 1996; PearlMan, 2011; figure 13). There have also been reports of “pearlfish” (slim, eel-shaped marine fishes of the Carapidae family) and other cohabiters of this wonderfully protective pearl shell dying inside P. maxima and providing the basis for the formation of other incredibly interesting blister pearls (Smith, 2003; Hochstrasser, 2011).
A supreme example examined by one of the authors (KS) in recent years is shown in figure 14. Here a blister pearl–encrusted pearlfish is attached to the shell, not far from where the heart and gonad would have been in the living mollusk. This attests to the symbiotic harmony of the fish living within the protective valves of the P. maxima. As one ponders the lengthy life cycle of this mollusk and considers many decades of examining microradiographs of the natural pearls produced within its slender and near transparent mantle, it is surprising to find there is still debate over what initiates the growth of a natural pearl. It is clear that within the valves, life is not motionless. Apart from invading life forms, another potential trigger is the tremendous amount of ocean floor debris that likely finds its way over the mantle and onto the mollusk’s gills.
There is no convenient single initiator but rather a wealth of possibilities that make the growth process even more intriguing. Of the hundreds of thousands of microradiographs examined by the authors, very few definitively show what caused a particular pearl’s formation. Two spectacular examples that come to mind appear in figures 15–17.
In figures 16 and 17, the Pectinidae shell is extraordinarily clear. The owner understood the uniqueness of the pearl and stored it safely in his collection, which has allowed us to reexamine the specimen several times as imaging technology has improved. The images in figure 17 were obtained via X-ray computed micro-tomography and further manipulated to obtain the vividly detailed images presented here.
With these two pearls in particular, plus a few others we have documented that are not quite as spectacular, we were particularly lucky to have obtained them from reliable sources. In recent years, a variety of foreign bodies, including natural pearls and even shells, have been artificially inserted into cultured pearl sacs (produced from a graft of mantle tissue, or from mantle damage due to human handling) to further coat them with nacre. These practices, by
deceiving gem laboratories and consequently the industry, have placed a question mark over all natural pearls.
OTHER P. MAXIMA PRODUCTS
There is little if anything wasted by those who farm P. maxima. The mollusk provides us with not only pearls, both natural and cultured, but also very high quality mother-of-pearl and an edible delicacy. Pearl shell (figures 18 and 19) is used today, as it has been for the last two centuries, in the manufacture of luxury utensils, as inlays in jewelry and furniture, and in various art forms. In fact, the value of the shell fished in toward the end of the 19th century often exceeded that of the natural pearls (table 1).
Today, with the main use of the oyster (both wild and hatchery) being the production of large South Sea cultured pearls, the shell has a lower proportional value. Nevertheless, it remains an important element in the value stream of pearling companies. It may be appropriate to quote Kornitzer again, for never have the writer’s words been bettered in any works concerning this great bivalve: A shell it was, as large as a soup-plate, no more. A brilliantly nacreous thing with a natural polish, smooth as a mirror and reflecting not only my still youthful features, but also, it seemed, some of the things the future promised to hold for me. How interesting, and how foolish, to believe that one can see into the future at the magic touch of some alien thing and vaguely guess one’s destiny in a waking dream!
It happened in the prosaic London Docks, that staid businesslike place with its background of romance. As the man lifted the pearl shell out of the open case for me to admire its unusual size and weight, I did what probably nine women out of ten would have done in similar circumstances. I eyed myself carefully in the smooth and shining surface. Presently the reflection of my own face seemed gradually to fade, and even as I looked there took shape in my mind the vision of a life oddly governed by the moon-fired stones of my future love.
The vision faded. I stood like a ninny with the shell in my hand. The man nudged me and said, “Trying to drill holes into this shell with your eyes?” “No,” I said apologetically. “I’ve been dreaming. These outlandish things seem to awaken in me the desire to travel, that’s all.” (Kornitzer, 1937). Pearl meat from the P. maxima adductor muscle is a delicacy, particularly in China but also to anyone fortunate enough to experience this gastronomic delight (figure 20). Eaten raw or quickly flash-seared in a hot pan for just a few seconds or slowly braised, it will excite the taste buds of any dissenter. It is estimated that 60% of all pearl meat harvested in Western Australia makes its way to Asian markets after drying and packing. It sells for Aus$100–$150 per kg. The rest is monopolized by top chefs in Sydney and Perth, as well as Broome, which is why very little pearl meat can be found in the shops (Broadfield, 2010).
Chef Matt Stone of Perth says, “What I love about it most is the texture: It’s halfway between a scallop and an abalone. It’s got a bit of chew to it, but not so much as abalone” (Broadfield, 2010). All of the authors who have tasted the meat of P. maxima are in full agreement.
WILD SHELL COLLECTION TODAY
The pearl culturing industry is one of Australia’s most valuable aquaculture industries, with a value estimated at Aus$120–160 million (Hart and Friedman, 2004). Considering the natural as well as manmade challenges, this is truly a significant statistic. Clearly, one important factor behind the industry’s success is the reliance on hatchery-grown mollusks that offer more control over production processes. Interestingly, the Paspaley Pearling Company, whose operations are focused on the waters of the Northern Territory and Western Australia, still fish for wild shell and use them for much of their culturing operations.
To protect the species, the harvesting of mother–of-pearl (MOP) in Western Australia was virtually phased out by the late 1980s, and strict quota controls were placed on sizes suitable for pearl culturing. Hart and Friedman (2004) point out that the fishing for P. maxima targets smaller shell (120–165 mm dorso-ventral measurement, or DVM; see figure 21) that are more suitable for pearl culture, leaving larger (175 mm+) MOP on the pearling grounds. They add that in 2004, the shell were protected by the “gauntlet” strategy adopted by the Fisheries Department, and that “with almost 20 years of protection from fishing mortality, there has been a buildup of MOP on some pearling grounds, leading to proposals to commercialize (again) this component of the fishery.” The quota system has been so effective that the fisheries sector is now the “only remaining significant natural source of large P. maxima MOP left worldwide.” As wild stocks fluctuate, however, historic norms are the most likely outcome.
The wild shell collected by Paspaley are kept separate from their hatchery shell via a strict stock control system that begins the moment a specimen is brought aboard the vessel. Collection of the wild shell occurs mostly off Western Australia’s Eighty Mile Beach (between Broome and Port Hedland in figure 9), but the company also has a quota in Northern Territory waters. Although divers now operate from modern, specially designed vessels (figure 22), the principles are similar to those used in the days of the lugger. With today’s larger ships, up to six divers are pulled along the seabed as the ship plows a slow-moving grid at the surface. Divers are still connected to the vessels by safety lines and air hoses, but they wear modern wetsuits and are not constrained by the hardhat environment once used aboard the luggers.
As they move along the seabed, the divers trail below them a rope basket for the shells (figure 23). Once the basket is full, the diver ascends to a shallower depth where a large storage container awaits. He transfers the shells from his basket and returns down to the seabed to continue collecting. He may repeat this process several times before the dive ends. There is great rivalry between divers, with “scores” being eagerly awaited once back onboard the vessel. While the practice is unquestionably safer now than it was in the days of the luggers, the everyday dangers of such a remote environment remain just as `real today.
It takes a very special type of person to be a diver on a pearling vessel. Spending up to eight hours a day in the deep and unforgiving waters off Western Australia, the diver needs to be adventurous, but calm and to some extent fearless, while maintaining a focused approach to the task. Decompression sickness, sharks, saltwater crocodiles, jellyfish, sea snakes, tangled air lines, and low visibility are just a few of the very real dangers. These dangers are difficult to convey unless the reader is a seafarer with knowledge of Australia’s rugged western coast. Needless to say, few people who lead the pearling life do not know of someone who has been taken by a shark or nearly died following a sting from the thumbnail-size Irukandji jellyfish.
Articles source: Kenneth Scarratt, Peter Bracher, Michael Bracher, Ali Attawi, Ali Safar, Sudarat Saeseaw, Artitaya Homkrajae, and Nicholas Sturman – GEMS & GEMOLOGY, WINTER 2012
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