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Pearl-Producing Mollusks (Molluscs)

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  • Pearl-Producing Mollusks (Mollusks)

    Pearl-Producing Mollusks (Pearl-Producing Bivalves)

    Mollusks represent one of the world's earliest forms of animal life, and date back over 550 million years. Actual pearl-producing mollusks first appeared 530 million years ago, when mollusks developed shells. The scientific discoveries of pearl producing mollusks illustrate the intrinsic and irrefutable value of the world's most historically treasured organic gem.
    Abalone fossil from Mexico - photo courtesy of Baja Pearl Hunter



    Abalone are not bivalves nor part of the "Conchifera" sub-phylum; instead, they belong to the Gastropoda group and yet they also produce "true pearls".


    Scientific Classification of Pearl-Producing Mollusks

    Mollusks encompass the second largest phylum of the animal kingdom (there are over 128,00000 different species). The phylum is divided into two subphyla, only one of which contains species that produce pearls. Conchifera is the name given to the subphylum of mollusks that produces pearls. In this discussion, a pearl-producing mollusk is one that will produce jewelry-quality pearls.



    Which are the Pearl-Producing Mollusks?

    Although any of the mollusks within the Conchifera subphylum can produce pearl-like formations, most of them are not pearl producing mollusks. It is the fourth class of the Mollusca phylum, BIVALVIA, which is the most important for the formation of pearls. Other notable classes that produce pearls are the gastropods (2nd class) and cephalopods (5th class). According to the scientific classification system developed by Carl Von Linne in 1758, each species of mollusk is given a two-part Latin name: the genus and the species.

    Sometimes a third part will describe a sub-species, which may represent regional differences among a species of pearl-producing mollusk. Although the shell of each mollusk is important for identification and critical to pearl formation, it is the inner soft body (and nowadays also its DNA) of the mollusk that scientifically defines the species.


    The Evolution of Pearl-Producing Mollusks

    The figure below illustrates the evolutionary tree for the Phylum Mollusca. Not all pearl producing mollusks belong to a single family or group. Pearl producers are distributed across the evolutionary tree. The boxes that are shaded indicate classes of Conchifera that exhibit the pearl producing compound of nacre. Nacre is the tissue that lines the shell and creates the unique luster of pearls.
    Evolutionary Tree of Pearl Bearing Mollusks


    The widespread presence of nacre indicates one of two points:
    • Nacre is a primitive characteristic that has been lost and regained throughout the mollusks' evolutionary history.
    • The composition of nacre is unique within certain branches, and certain forms of nacre facilitate pearl production.
    If the latter were true, it could explain why certain species of mollusk produce more pearls than others.


    Pearl-Producing Mollusks are Not necessarily Oysters

    Most pearl-producing mollusks are bivalves, meaning their shells have two halves connected by a hinge, like a clam. Although there are approximately 20,000 species of bivalve mollusks, only relatively few of these species are used in pearl culturing to create commercial pearls.

    The term "pearl oyster", commonly used in the pearl trade, is in fact incorrect since pearl producing mollusks are not oysters. Though edible oysters can occasionally produce a "pearl", they are non-nacreous and of no commercial significance. Furthermore, for the safety of our teeth, "pearls" (actually known as "calcium concretions") in edible oysters are undesirable. Commercial edible-oyster farms are in fact shut down if "pearls" persist to form within them.

    Pearl oysters are actually more closely related to "scallops", "pen shells" and "hammer shells" than to oysters, all of these belonging to the Pteriomorpha sub-class of bivalves.


    The Anatomy of Pearl-Producing Mollusks

    The anatomy of a bivalve mollusk facilitates the production of pearls. Unlike a closed snail, or gastropod shell, a bivalve clamshell is open, and water filled, leaving more room for pearls to form.
    Anatomy of a Black-Lip pearl oyster


    Bivalves are also passive filter feeders meaning they maintain an open relationship with the environment by constantly circulating water to supply food. This process is critical for pearl production, since most natural pearls are formed as a reaction to a parasite attack within the shell. The open relationship of the bivalve structure increases the probability of foreign objects and creatures to enter, and the possibility of a natural pearl to develop.

    Gastropods are very Different

    In contrast, gastropods do not rely on water flow for food and are usually quite mobile, some are predators who deliberately hunt for prey and others scrape algae from rocks or graze from seaweeds.

    Gastropods are also able to expel foreign particles before they become a pearl's nucleus through movement. Mobility has been a difficult factor in culturing abalone pearls. Abalone pearls come from large, ear-shaped snails, whose movement often expels inserted nuclei.

    Abalone pearls have beautiful blue-green nacre and display gorgeous rainbow iridescence. Because of their irregular shapes and liquid iridescence, abalone pearls epitomize the beautiful canvas of nacre that creates singular beauty in each pearl.
    Natural Abalone Pearl




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