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The Genus Pteria Scoppoli 1777

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  • The Genus Pteria Scoppoli 1777

    The family Pteriidae is composed of three or four genera, and pearl oysters belonging to the Pinctada and Pteria genera are -of course- the most important from our point of view.

    Taxonomy

    An old name for this species is Magnavicula, which means “Large Wing”, with the “new” -Pteria- name originating after the Greek word “Ptero” which translates into “wing”, hence the name of “Winged oysters”.

    These species are identified by a more elongate shape than that of Pinctada species, thus their shells are longer (anteroposterior) than wider (dorsoventrally). The shells are usually thin (when compared with the larger Pinctada oysters), and unequal, with the left valve being more convex than the right valve. The posterior shell margin is straight to slightly concave. Shell length exceeds height in smaller specimens but is equal to or shorter than height in larger specimens. The byssal notch, located immediately ventral to the anterior auricles, is either broadly rounded or vertically elongated and narrow. The posterior ear is usually much more prolonged (unless this part of the shell has been broken) especially in young specimens, are narrow and gradually taper, at the end and deeply sinuated. In some individuals, however, the posterior auricles are noticeably short.
    Click image for larger version

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    The exterior surfaces of the shells are usually dark brown to black with variants of lighter shades of brown or green (often co-occurring with light brown), with or without numerous, thin, faint radial rays of a slightly lighter shade than the background color. In lighter colored specimens, darker colored stripes are occasionally present. The shell’s surface is smooth. The periostracum is light brown and often persists to the adult stage forming small, straight or curving downward, narrow, fluted to tubular shaped in younger specimens, tapering scales projecting from the shell’s surface.
    Inner shell of Pteria genus pearl oysters



    The prismatic margin is wider in the right valve. The anterior auricles are subtriangular in outline and variable in size. The inner prismatic margin is solid black or brown. Their nacre is highly iridescent, silvery-gray, and even coppery-gold, often with a strong lavender or green tint, more pronounced along the nacreous border. The posterior adductor muscle scar is oval, adjacent to the anteriorly positioned, smaller, rounded, posterior pedo-byssal retractor muscle scar. The hinge teeth are represented by the weakly to obsolete sub-umbonal anterior tooth in the left valve and a submarginal posterior ridge in the right with complementary sockets in the opposing valves.

    Anatomy

    Although nucleated cultured pearls can be produced from these species, it is acknowledged that this is much more difficult to achieve than with the Pinctada pearl oysters, this due to considerable morphological differences between these two genera. Only in recent years has a steady and successful production of round pearls from Pteria sterna been reported from Mexico, and for Pteria penguin in Hainan (China) and Indonesia, although not commercially in these two countries. This is an important development and one that may someday help in the establishment of “winged oysters” for actual cultured pearl production and not just Mabe pearls.

    According to Muramatsu (1992), the main reason for the difficulty of seeding this genus of pearl oyster is due to its “masculine” anatomy, where “there is no room in the (inner body) structure to cover the pearl softly as there is in the Akoya oyster (Pinctada)”. McLaurin-Moreno has stated that the inner body of the Rainbow Lip oyster will be almost devoid of gonadic tissue (especially during summer) and is also “crossed” by several muscle fibers that allow the body to strongly contract, allowing for the cultured pearls to become dislodged or expelled from the animal’s body.
    Internal Anatomy of the Rainbow Lip Pearl Oyster Pteria sterna


    The shape of the abductor muscle is different from that of pearl oysters of genus Pinctada, being much rounder or “kidney shaped”. The “**** flap” (also used by many to distinguish between Pinctada species) in the winged pearl oysters is also much diminished in size and of less prominence and the foot is much larger and useable than that found in genus Pinctada (where it is usually limited to a small appendage), giving testimony to these mollusk affinities to living on fan corals and under stronger water currents.

    Habitat & Ecology

    Most Winged pearl oysters are found in the Caribbean-Pacific and Indo-Pacific. The two major pearl-mollusk regions are known as the Western (Caribbean-Pacific) and Eastern (Indo-Pacific). In both regions, all precise locations for Pteria have water temperatures that rarely fall below 16 degrees Celsius (68 degrees Fahrenheit) and will rarely exceed 30 Celsius (86 F). Individuals of Pteria attach to a wide variety of substrata, including rocks, fan corals (hydroids), and even other pearl oysters, within 2 to 25 m of depth. They have a higher affinity for colder waters or require highly productive waters rich in oxygen to survive in more tropical areas. They are known to be found individually when attached to fan corals or in vast numbers when attached to each other over sandy-gravely bottoms.

    The three most recognized species of this genus -due to their use in fisheries or aquaculture- are:
    1. “Mabe-gai” or “Penguin Winged pearl oyster” (Pt. penguin). This species is distributed from the Red Sea and the Arabian Gulf throughout the tropical eastern Indo-Pacific to southern Japan (Okinawa to Honshu) and includes Southeast Asia, the Philippines, Queensland round to northwest Australia, Thursday Island, southern China, and Taiwan. This species is farmed in Okinawa, Hong Kong, Australia, Thailand, and the Philippines. It is the largest species of the genus, the largest, oldest individuals attaining a size of up to
    2. “Concha Nácar” or “Atlantic Winged Pearl oyster” (Pt. colymbus), found in the eastern Atlantic region, from the coast of North Carolina (United States) and all the way down to Brazil. It is also the smallest of this group, with maximum size of 8 cm (3.1 inches). It was fished alongside it Akoya-cousin (Pinctada imbricata) during the initial American fisheries at the time of the European discovery of the Continent. There are no cultured farms with this species at present, but it is a fast-growing mollusk that can reach a “seeding” size of 70 mm (2.7 inches) in just 12 months, so it has some untapped potential in aquaculture.
    3. “Concha Nácar” or “Rainbow Lip pearl oyster” (Pt. sterna), which is found from the Baja California peninsula in Mexico (and may become an invasive species in southern California, USA, during “El Niño” years) and all the way down to Perú. This species sustained the largest pearl fisheries in the world from this genus. There are only two farms of this species, one in Guaymas and a small one in La Paz, both in Mexico.

    The Pearls of Pteria

    Most species are small to medium sized, with the largest species being the “Mabe-gai” pearl oyster (Pt. penguin), thus their pearls have rarely been able to compete in size with those of genus Pinctada, but many sources mention that some species are highly prolific in their capacity to produce natural pearls, with the medium-sized “Rainbow Lipped Oyster” (Pt. sterna) being one of these.

    Some noteworthy and historical pearls originated from the American " Rainbow Lipped Oyster” (Pt. sterna) that was fished from Mexico's Baja California and Gulf of California (Sea of Cortez), Panama and all the way down to Ecuador. Included are the famous Marie Antoinette’s pearl necklace and the “Rainbow Parure” jewelry set.
    Click image for larger version

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    “Rainbow Parure” jewelry set, elaborated in the 19th Century in Great Britain for a private customer. Was exhibited at the Museum of Islamic Art in Doha, Qatar. Most of the pearls have been identified as belonging to Pt. sterna (except for the larger central pearl in the necklace and a few more (lighter colored) that were identified as originating from the Panamic Black Lip oyster (Pinctada mazatlanica). Photo courtesy of Enrique Arizmendi of “Perlas del Mar de Cortez”.

    In the case of cultured pearls, the “rainbow lip” species can produce vastly colorful pearls that sometimes resemble Tahitian pearls and even Fiji pearls. Pearls from the “Mabe-gai” pearl oysters have proven to be far less colorful and more “traditional” looking but, nonetheless are quite unique due to their origin and quantities produced.
    Pteria penguin cultured pearl necklace and Xrays



    Fluorescence under UV
    One unique trait to this genus is its ability to display a light pink to blood-red fluorescence under long-wave Ultraviolet (UV) light, this due to the presence of porphyrins (proteins that are bound to metallic ions), the deeper color corresponding to the shell or pearl’s darker color. This is in direct contrast with pearls from other species, which -for the most part- display a blue-green fluorescence.

    You can see this unique trait in this video:

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