While the name "pearl oyster" suggests a close relationship with other types of oysters, pearl oysters are actually a distinct species from edible oysters and have important anatomical and behavioural differences. There are a small number of mollusks capable of producing a pearl and only those mollusks that have shells lined with nacre NAY-kur , the pearlescent substance inside the animal's shell produce the pearls used in the jewellery industry.
Pearl oysters feed on small algae found in the water column. The gills in bivalves are large and tiny hair-like cilia on the gills are used to remove small particles from the water. Both adults and larvae feed on algae and other small organisms. Clear tropical waters contain limited amounts of algae. Therefore, a large amount of water must be filtered daily in order for the pearl oyster to obtain sufficient food. This is the reason importance is placed on not crowding pearl oysters on the farm and for keeping the shells clean from organisms that compete for food.
Pearl oysters are protandric hemaphrodites which means that most are first male, then female. The male phase usually occurs during the first 2 to 3 years of life, with the change to the female phase in later years.
Pearl oysters have been reported to live as long as 25 years. Pearl oysters reproduce by releasing millions of eggs or sperm into the water column where fertilisation occurs randomly. In less than 24 hours, the fertilised egg develops into a trocophore larva, a free-swimming organism. The larvae remain suspended in the water column for 2 to 3 weeks before undergoing metamorphosis, changing into an attached juvenile "spat". Shortly before metamorphosis, the larva develops an enlarged foot and an eye-spot.
The foot remains after metamorphosis and the young spat retains the ability to move about for several months even after it attaches itself to a hard substrate.
Pearl oysters can attach and reattach themselves using the byssus. Sometimes a natural pearl forms when an irritant, such as a fragment of shell becomes lodged inside the mollusk when it is feeding, or a parasite drills through the shell. To protect itself, the mollusk forms a sac around any irritant or invader that managers to get caught up inside its body.
This sac secretes nacre to cover the irritant and, over time, the growing pearls are completely covered with the beautiful iridescent substance we call nacre, or mother-of-pearl. The nacre and sac materials are made by the mollusk's mantle, the layer of tissue cells that surround the body of the mollusk and lines the shell.
The mantle tissue cells that make up the pearl sac are called epithelial ep-uh-THEE-lee-yuhl cells. One commonality all cultured pearls share is the nucleus. Every pearl produced commercially today except naturally forming keshi pearls and pearls from Bahrain will have been nucleated.
The nucleus used in all pearls farmed in saltwater today is a mother-of-pearl bead made from freshwater mussel shells found in North America. This bead is made from an oyster shell that has been cut, rounded, and polished. A nucleus is surgically implanted in the oyster's gonads or mantle lobe together with a small section of mantle tissue. Implanting a bead alone will not stimulate pearl formation.
The epithelial cells - mantle tissue- play a vital role in the pearl formation process. As the oyster recognises the nucleus as an irritant, it forms a sac around the irritant before coating it with smooth layers of nacre. Pearl farms now produce all the cultured pearls used in the jewellery industry today, and, while they are real, genuine pearls formed inside a living oyster, they are produced with a little human intervention.
Saltwater oysters are nucleated by opening the shell a mere 2 to 3 centimetres and making a minute incision in the gonad - the oyster's reproductive organ. The mother of pearl nucleus is inserted into this incision which is then followed with a very small piece of mantle tissue from a donor oyster. The mantle tissue is placed between the mother of pearl bead and the gonad with the side containing epithelial cells facing the nucleus. Typically, saltwater pearls tend to be higher quality, although there are several types of freshwater pearls that are considered high in quality as well.
Freshwater pearls tend to be very irregular in shape, with a puffed rice appearance the most prevalent. Nevertheless, it is each individual pearls merits that determines value more than the source of the pearl. Regardless of the method used to acquire a pearl, the process usually takes several years. Mussels must reach a mature age, which can take up to 3 years, and then be implanted or naturally receive an irritant.
Once the irritant is in place, it can take up to another 3 years for the pearl to reach its full size. Often, the irritant may be rejected, the pearl will be terrifically misshapen, or the oyster may simply die from disease or countless other complications. Imitation pearls are a different story altogether. Layers of nacre coat the irritant, eventually forming an iridescent gem the pearl.
The only difference between naturally developed pearls and cultured pearls is that, with cultured pearls, a pearl farmer embeds an irritant between the shell and the mantle by cutting into the mollusk's tissues. With freshwater pearls, irritants do not need to be introduced. S imply cutting the oyster's soft tissues is enough to begin the pearl-making process. Once the irritant has been planted, how long does it take for a pearl to form? Some pearls can develop in a period of six months.
Larger pearls can take up to four years to develop. This is one of several reasons why larger pearls can yield higher values. Harvesting Pearls Pearl farmers must have immense patience to wait for a pearl inside an oyster shell to develop. When a pearl in an oyster is ready, the harvester opens the shell, extracts the pearl and evaluates it for quality. Some oysters can produce two to three pearls over the course of their lifetime, but only an oyster with pearls of good quality will repeat the process of producing a pearl.
0コメント