Pearl-producing mollusks are typically found in warm ocean waters and consist of a variety of species. The most common of these are the pearl oysters, which live in shallow waters where they filter microscopic organisms from the water for food.
These small animals have an outer shell, called a mantle, that covers their body with two halves. The inside of the shell is lined with a layer of calcium carbonate, known as nacre. This is what gives pearls their unique luster and color.
Major types of pearl oysters
(1) Pinctada maxima
(2) Pinctada margaritifera
(3) Pinctada carchariarum
(4) Pinctada albina
All of the above pearl oysters are recorded from Australia.
P. maxima and P. albina are limited in their distribution in the tropical south-west Pacific (Philippines to western and northern Australia). (2) Pinctada margaritifera is widely distributed from the Red Sea throughout the tropical Indo-Pacific.
(5) Pinctada mazatlanica is found in Baja California/Gulf of California, southwards to Peru.
(6) Trochus niloticus is widely distributed throughout the tropical Indo-Pacific, and is collected along the Queensland coast for button-making and nuclei for cultured pearls.
Pinna nobilis is the valid name for the Mediterranean (East Atlantic) species.
Pinna seminuda occurs in the West Atlantic and may be a synonym for Atrina rigida
(7) Pteriapenguin the ‘Black-winged pearl oyster’, is widely distributed throughout the Indo-Pacific from the Red Sea to northern Australia and southern Japan.
(8) Placuna placenta is not a ‘true oyster’, but belongs to the super family Anomiacea (family Anomiidae) commonly called ‘jingle shells’ or ‘saddle oysters’.
(9) Haliotis tubercalata is the valid name for the European (Channel Islands) species.
(10) The Pearly Nautilus (Nautilus pompilius)
(11) Freshwater pearls in Britain (and Europe) may come from the freshwater pearl mussel which is validly named Margaritifera margaritifera
(12) The Chinese freshwater pearl
Pinctada maxima Shells
Structure of an oyster
The pearl oyster is closer in relationship to the scallop than to our edible oyster. It is termed a ‘bivalve mollusk’, ‘bivalve’ meaning two halves or shells, and ‘mollusk’ indicating a soft-bodied animal protected by a hard exterior shell covering.
The group name for such bivalves is Lamellibranchiata, and it includes edible oysters, the edible mussel, cockle, and almost all other marine animals which produce pearls.
The scientific name refers to the characteristic plate-like gills. The soft body of the oyster is enclosed between the two valves which are hinged together along one side of the oyster’s body.
Although the valve has a rounded outline, the hinge or dorsal end is flattened and finished as wings or auricles. The shell is not thick (as it is in mother-of-pearl oysters from Australia), but it is lined with brilliant and iridescent nacre. The external surface is rough, dull, and usually encrusted with other organisms growing on it.
Some authors state that rough waves and rocky sea beds cause a thick protective outer shell layer as a natural defense, whereas oysters from sheltered positions on smooth sea beds have a thinner shell offering less defense.
