English: Pinctada margaritifera (Linnaeus, 1758) - black-lipped pearl oyster shell from the Pacific Basin. (public display, Field Museum of Natural History, Chicago, Illinois, USA)

Bivalves are bilaterally symmetrical molluscs having two calcareous, asymmetrical shells (valves) - they include the clams, oysters, and scallops. In most bivalves, the two shells are mirror images of each other (the major exception is the oysters). They occur in marine, estuarine, and freshwater environments. Bivalves are also known as pelecypods and lamellibranchiates.

Bivalves are sessile, benthic organisms - they occur on or below substrates. Most of them are filter-feeders, using siphons to bring in water, filter the water for tiny particles of food, then expel the used water. The majority of bivalves are infaunal - they burrow into unlithified sediments. In hard substrate environments, some forms make borings, in which the bivalve lives. Some groups are hard substrate encrusters, using a mineral cement to attach to rocks, shells, or wood.

The fossil record of bivalves is Cambrian to Recent. They are especially common in the post-Paleozoic fossil record.

Shown above is the interior of a black-lipped pearl oyster shell (a.k.a. Tahitian pearl oyster). The rainbow-colored iridescence of the shell is nacreous aragonite (CaCO3 - calcium carbonate), also called "mother-of-pearl". This species makes black pearls.

Pearls are spherical to subspherical to irregularly-shaped, biogenic concretions of slightly iridescent, nacreous aragonite. Pearls are principally made by pearl oysters. Natural pearls are scarce. Well-formed, spherical natural pearls are rare. Fossil pearls are known, but are also scarce. Mother-of-pearl is relatively common.

Well-known pearl oysters include Pinctada margaritifera, Pinctada fucata (the Japanese pearl oyster), and Pinctada maxima (the gold-lipped pearl oyster). Mother-of-pearl is well developed in shells of other species such as Pinctada imbricata (Atlantic pearl oyster), Pteria colymbus, Pteria penguin (both are winged pearl oysters), Haliotis spp. (abalones), and Atrina spp. (pen shells).

Natural pearls form when foreign objects, such as sediment grains or other debris, enter a pearl oyster and get embedded in its mantle tissue. The particle is slowly coated with nacreous aragonite, which prevents the particle from causing disease or injury. The end result is a biogenic concretion called a pearl. Natural pearls show a concentric structure through the entire cross-section. Almost all commercially available pearls are semi-natural - they have been cultured. Cultured pearls have been available for many decades. A spherical bead is placed inside a pearl oyster, under its mantle tissue. The bead is slowly coated with nacreous aragonite to produce a cultured pearl, which shows concentric structure only in the outer portions of its cross-section.

Marine pearls can be whitish, pinkish, yellowish, or blackish. Freshwater pearls are also known - natural examples vary from ~spherical to highly irregularly-shaped. Blister pearls are attached to the host mollusc's shell.

Classification: Animalia, Mollusca, Bivalvia, Pteriomorphia, Pterioida, Pteriidae

Locality: unrecorded/undisclosed site in the Tuamotu Islands, French Polynesia, northern South Pacific Ocean

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See info. at:

en.wikipedia.org/wiki/Pinctada_margaritifera