Since 1870, the largest pearls have been found mainly in a very large species of pearl-oyster, Margaritifera maxima, obtained off the north and west coasts of Australia, among the Sulu Islands, and elsewhere in the Malay Archipelago. In the fisheries for this species, the mother-of-pearl is the principal object sought, and the pearls are obtained incidentally. It is the largest of all the members of this family, reaching in exceptional cases twelve or thirteen inches in diameter, and weighing upward of twelve pounds; while the Ceylon oyster rarely exceeds four ounces in weight. So marked is this difference, that the Australian species is often designated the “mother-of-pearl oyster,” and the Ceylon species the “pearl-oyster.” Jameson notes that it differs from the Margaritifera margaritifera, its nearest competitor in size, in its much longer hinge, its shape, its lesser convexity, and in its color and markings. As described by him, the color ranges from pale yellowish brown to deep brown, with traces of radial markings of dark brown, green, or red in the umbonal area. In its marginal region, the shell is marked by a series of circumferential lines about one third of a millimeter apart.
Several geographical varieties of this species are recognized in the mother-of-pearl trade, differing principally in the coloring of the interior surface. The chief commercial varieties are “Sydney” or “Queensland,” “Port Darwin,” “West Australian,” “New Guinea,” “Manila,” “Macassar,” and “Mergui.” The nacre of those from the Australian coast is almost uniformly silvery white. That of the “Manila shell” is characterized by a broad golden border surrounding the silvery white nacre. The “Macassar shell” lacks the golden border of the “Manila shell,” and is similar in its uniform whiteness to the “Sydney shell,” but its iridescence is much greater.
The Margaritifera carcharium, from Sharks Bay, on the coast of Australia, yields yellow pearls and small quantities of mother-of-pearl. This species is small—three or four inches in diameter. The color is grayish or greenish yellow, with several somewhat indistinct radial bands of brownish green. The nacre has a yellowish green tint, with a margin of pale yellow, with brown markings.
In the West Indies and on the Atlantic coast of tropical America, especially the coast of Venezuela, occurs the Margaritifera radiata. This species is quite small, and seems to be closely allied to the Ceylon oyster. Like the latter, the nacreous interior is rich and brilliant, but owing to its small size, the shell is wholly valueless as mother-of-pearl. The principal and almost the only fishery for this species is on the Venezuelan coast, in the vicinity of Margarita Island, the islands of Cubagua, and Coche.
The coast of Japan yields the Margaritifera martensi, which occurs among the numerous islands in the southern part of the empire, but does not extend beyond 40° north latitude. This species is likewise small, and closely resembles the pearl-oyster of Ceylon, from which it differs principally in coloration. As noted by Jameson, brown and white predominate in the exterior coloring, and the interior of the lip is marbled with yellow ocher and chocolate brown, instead of pink, as in the Ceylon shell.
There are numerous other species of pearl-oysters, but they are of slight economic importance, and do not support fisheries of value.
As only a small percentage of the individual mollusks contain pearls, it follows that vast quantities are destroyed without any return whatever, and handling them merely adds to the expense of the industry, as well as reduces the resources of the reefs. This could be obviated if it were possible, without opening them, to determine the individual mollusks containing pearls.
Among the several methods proposed for this purpose, especially interesting is the use of X-rays, which was suggested by Raphael Dubois of Lyons, France, in 1901.[[82]] The shells of some pearl-oysters—those of Ceylon and of Venezuela for instance—are relatively thin, and it was thought that by the means of the rays the presence of pearls could be ascertained, and non-pearl-bearers could be saved from opening, and be returned to the reefs without injury. Although the calcareous shell partly interrupts the radiations, it is not difficult to recognize the presence of large pearls.
The theory has never been found practical in application, owing largely to the rough and irregular exterior of the shell and the small size of the pearls. The presence of the larger pearls may be ascertained by this method; but it is exceedingly probable that a very large percentage of the small ones, and especially the seed-pearls, would be overlooked. Furthermore, if in their sixth year oysters contain no pearls, the probability of appearance therein later is very small, and little benefit would result from their return to the water. As to saving the trouble of opening the non-pearl-bearing mollusks, labor in the pearling regions is usually inexpensive, and this cost is far more than offset by the reasonable certainty of securing practically all the small as well as the large pearls by the present method of operation. Owing to the greater thickness and the economic value of the large pearl-oysters—as those of Australia or of Mexico, for instance—the application of X-rays to them is obviously impractical. However, when pearl-oyster culture becomes a highly developed industry, with personal ownership in those mollusks returned to the water, some method such as this might be of great value.
Pearls are yielded by various species of Unionidæ or Naiades occurring in the rivers of America, Scotland, Saxony, Bavaria, Norway, Sweden, Russia, France, China, etc. These mollusks exist exclusively in the fresh-water streams, lakes, and ponds, and quickly die when submerged in salt water. The Unionidæ are of particular interest in America, as it is here that this group is most abundant, and nearly every stream east of the Rocky Mountains contains more or less of them. The Mississippi basin abounds in Unios, or “clams,” as they are known to the fishermen of that region, and furnishes about 400 of the 1000 recognized species of this important family.