Two sister species
The class Bivalvia brings together molluscs whose transversally-flattened body lies protected between two lateral, calcified valves. Families con be designated according to characteristics either of the shell or of the body. In the latter case, the degree of complexity of the gills serves as criterion for classification.
The marine pearl oysters belong to two genere of the family Pteridae: Pterida ond Pinctada. The interior of both oysters’ shells has a nacreous layer which allows for the production of nacre. Both types of oysters are also producers of iridescent perls.
The genus Pinctada comprises fewer than ten scientifically recognized species, out of a hundred that have been described. Thus, there was a time in the past when one author mistokenly attributed 1O different names to oysters found in the Red Sea, which were actually of the some species!

 

Classification of bivalve molluscs

This confusion arose becouse morphologicol characteristics of the shell were taken into occount, and these often vary with the animal’s age and with environmental conditions. Even the anatomical criteria – form of the anal process, of the adductor muscle and of the larval shell- can vary among individuals. It was in 1901 when Jameson, while reviewing the pinctadas in the British Museum of Natural History, arrived at a first classification of all the species. He reduced the number to 27, and was the first to make a distinction between Pinctoda maxima and Pirictoda margaritifera. ln 1954 Hynd produced a key to all Australian species; and Ranson, in 1961, extended the examination to numerous examples from all the oceans of the world. Research would still be needed today to establish the biological criteria for diagnosis of all the pearl oyster species in the world.
Two Pinctodas produce pearls in southern seas, Pinctoda maxima and Pinctoda margaritifera. They are easy to distinguish because their shell is much larger and thicker than that of other species, and their hinge has no teeth. Consequently, the faster nacre-formation in the shells of these two pinctcidines allows for the production of larger pearls in varying colors. The shell of Pinctada margaritifera, dark and slightly more convex, is not much different from that of Pinctada maxima.
Of more significance is the byssus which persists in the adult blak-lip pinctada, which enables it to became fixed to blocks of dead coral, whereas white-lip pinctadas stay unattached as adults becouse they have lost their byssus.

Classification of bivalve molluscs II

Representatives of the genus Pinctada have existed in tropical seas since the Tertiary era (30-20 millions years BP). The zone of distribution of Pinctada margaritifera is much broader than those of other pinctadas. It encompasses almost the whole inter-tropical zone of the Indo-Pacific Ocean, the Persian Gulf, the Red Sea, Lower Califomia and the Gulf of Mexico. Pinctada margaritifera’s distribution is tied with that of coral reefs around scattered tropical islands. It is cultivated especially in French Polynesia, in the Tuamotu-Gambier archipelago, where it produces the black variety known as the Tahiti pearl.
The geographic dispersal of Pinctada maxima is restricted to the Indo-Malaysian region. lts area extends from the Asian continent (from Burma to the south of Japan) up to the north coast of Australia, with greater concentration in Malaysia, Indonesia and the Philippines. We are going to follow the long course of pearl cultivation in the Philippine archipelago. The success of the white and the golden pearl of the southern seas is due to the imagination and courage of men who learned, with great patience, how to draw from these slow~working molluscs their beautiful gifts from the sea.


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