When is a Pearl Not a Pearl?

E.R. Cross

A letter received from Gregg Swiderski of Park Ridge, Illinois, asked if any Skin Diver readers had found pearls in shells other than pearl oysters. He wrote: "I've enclosed a photo of a pearl found in a Queen Conch shell. The shell was found while diving off the Turks Islands. The pearl is a large one, weighing in at 16.4 carats [not karats]. He concluded his letter by saying he would be eager to hear from other readers who have found such pearls. The large pearl in the photo is slightly irregular and pink in color.
First carats vs. karats. Carat is a unit of weight for precious stones and is equal to 200 milligrams. Karat is a unit of fineness for gold equal to 1/24 part of pure gold in an alloy (i.e. 12 karat gold is 50 percent pure gold).
Pearl production in mollusks starts out as nacre (mother-of-pearl). It is deposited on the shell in the growing processes by the specialty parts of the mantle. Nacre is usually the principle material in the shell it can also produce a pearl. The ratio of nacre to the total mass of the mollusk indicates (to some extent) the chances of a pearl being produced. Since nacre coats and soothes irritants that find their way inside the shell, environmental conditions are also a good indicator of the likelihood of pearls being present in the animal.
The term pearl is loosely applied to the calcareous concretions found in various types of mollusks. A true pearl is specifically the nodule or cyst of nacre that forms apart from the creature's shell. This type of pearl is characterized by its delicate play of luster known as orient. Both freshwater and saltwater oysters can produce true pearls. Some freshwater pearls are famous for their luster, quality and value. The most famous freshwater pearl was the pink queen pearl weighing 231.2 carats. It was produced by a pearl-bearing oyster in the Mississippi.
How often are pearls found in pearl-producing shells? In the pearling areas of the Tuamotu (an archipelago in the South Pacific belonging to France) a pearl is found in about one in a thousand shells. In general the characteristics that make pearls valuable are their size, shape and iridescence.
Because they are filter feeders, bivalves seem to produce pearls more frequently than do gastropods. True pearl-bearing mollusks, both freshwater and saltwater varieties, are erroneously called pearl oysters even though they are not members of the pearl shell (Ostreidae) family. Common helmet shells (Cassis) produce occasional pearls, some of considerable value.
The Killing Fields of Mullosks
Several years ago I maintained saltwater aquariums to support my study of various kinds of mollusks. Primarily I was interested in the relationships between several species of shells frequently found in a spot I regularly dived. Granted, shells (and other marine life kept in aquariums) would not interact exactly as they do in their natural habitat so I kept an assortment of species to study their interactions particularly their food preferences and methods of eating. This included three species of Murex (carnivores), three Cypraea (herbivores) and an assortment of other shells.
The aquariums were stocked at about noon on September 7, following a morning dive. By late evening, a 11.4-inch-long Cymatium pileare began eating a Trachacardium hawaiensis, a bivalve that measures almost two inches long. During this time a Conus textile consumed a small Bursa species. About 6:00 a.m. the next morning, the Cymatium pileare had consumed nearly half of the bivalve. Two days later, the C. Textile killed and ate another Bursa (B. cruentata) in three hours. After nearly a week, C. textile killed and ate a two-inch-long Cypraea maculifera in approximately eight hours. On the same day a Nassa francolinus, a gastropod I had believed to be herbivorous, killed and devoured a small cowry (Cypraea teres) in the same eight-hour period. Two weeks later Comus textile stung and ate a Cymatium pyrum in 16 hours. On September 27, an 111.2-inch-long Nassa francolinus was observed stinging and eating a two-inch-long Cypraea sulcidentata.
Some mollusks are fish eaters; others eat crustacea. An olive shell I had in one of my aquariums was observed catching, killing and devouring a small crab.
Cross Talk welcomes any information about your diving adventures, especially hairy run-ins with bivalves, gastropods and the odd cowry, as they make up one of the more interesting quarters of our diving domain the smaller they are, the more fascinating they become.
P.S.: Ever conscious of Mother Earth, I returned all living species back to their natural environment.