In most of the inshore waters of Australasia pearls may be secured; the fisheries are most extensive on the northern coast of Australia, in the Sulu Archipelago, and about the Dutch East Indies. Tuamotu Archipelago, Gambier, Fiji, and Penrhyn are prominent in the South Pacific Ocean.
In the seas of Europe few pearls have been found, but the rivers have yielded many; and although the resources have been greatly impaired, many beautiful gems are yet found there.
South America contributes the important reefs on the coast of Venezuela—the land of unrest and revolutions, whose fisheries were first exploited by Columbus. Other South American countries in which pearls are collected are Panama, Ecuador, Peru, etc. In North America, pearls are found in the pearl-oyster of the Gulf of California, the abalone of the Pacific coast, the queen conch of the Gulf of Mexico, and in the Unios of most of the rivers, especially those of the Mississippi Valley.
Since pearly concretions partake of the characteristics of the shell within which they are formed, it follows that practically all species of mollusks whose shells have a well-developed nacreous lining yield pearls to a greater or less extent. But the number of these species is relatively small. They belong chiefly to the Margaritiferæ, or pearl-oyster family of the sea, and to the Unionidæ, or family of fresh-water mussels. Pearls occur also in some univalves, but not so abundantly as in bivalves of the families mentioned. Broadly stated, we may hope to find pearls within any mollusk whose shell possesses a nacreous surface; and it is useless to search for them in shells whose interior is dull and opaque, such as the edible oyster for instance.
The great bulk of the pearls on the market, and likewise those of the highest quality, are from the Margaritiferæ, which are widely distributed about tropical waters. Although these mollusks are spoken of as pearl-oysters, they are not related in any way to the edible oysters (Ostrea) of America and Europe.[[79]] The flesh is fat and glutinous, and so rank in flavor as to be almost unfit for food, although eaten at times by the poorer fishermen in lieu of better fare. The origin of the name is doubtless due to the fact that in the somewhat circular form of the shell they resemble oysters rather than the elongated mussels of Europe, to which they are more nearly related in anatomy. Also in that—like their namesakes—they are monomyarian, having only one adductor muscle.
The two valves or sides of the pearl-oyster shell are nearly similar in shape and almost equal in size; whereas in the edible oysters one valve is thin and somewhat flat, while the other is thicker, larger, and highly convex. In the latter, also, the hinge, or umbo, is an angular beak; but in the pearl-oysters the umbo is prolonged by so-called ears or wings into a straight line the length of which is nearly equal to the breadth of the shell.
The byssus, or bunch of fibers, by which pearl-oysters attach themselves to the bottom indicates their relationship to the mussels. The possession of a small foot and somewhat extended migratory powers—at least in the first years of growth—also distinguish them from the sedentary edible oysters. But from an economic point of view, the principal difference is the possession of a thick, nacreous, interior lining in the shells of pearl-oysters, which is wholly lacking in the edible species. Like their namesakes, the pearl-oysters are exceedingly fertile, a single specimen numbering its annual increase by millions.
Commercially considered, the pearl-oysters are roughly divisible into two groups, (1) those fished exclusively for the pearls which they contain, and (2) those whose shells are so thick as to give them sufficient value to warrant their capture independently of the yield of pearls. The best examples of the first group are the pearl-oysters of Ceylon and of Venezuela, and to a less extent those of the Persian Gulf, the coast of Japan, and of Sharks Bay, on the Australian coast. Of the second group, the pearl-oysters of Torres Straits and of the Malay Archipelago are the most prominent members. Between these two groups are the many species and varieties whose shells and pearls are more evenly divided with respect to value, including those of Mexico, Panama, the Red Sea, the South Sea islands, etc.
Some conchologists recognize a large number of species of Margaritiferæ, while other authorities consider many of these as local variations of the same species. There is much difference in the size, color, and markings of the shells in different localities, owing to varying geographical and physical conditions. The distinction of species and the nomenclature herein adopted are those of Dr. H. L. Jameson, who has recently revised and rearranged the collection of shells belonging to this family in the British Museum of Natural History,[[80]] and to whom we are indebted for descriptive notes relative to several of the species.
The greatest pearl-producer in the family of pearl-oysters is the Margaritifera vulgaris of the Gulf of Manaar and the Persian Gulf, and to a much less extent of the Red Sea. It occurs in various other inshore waters of the Indian Ocean, and about the Malay Archipelago and the coast of Australia and New Guinea, although it is not the principal pearl-oyster of those waters. An interesting account of its immigration into the Mediterranean Sea through the Suez Canal was given by Vassel in 1896.[[81]]