Diving Into the Scientifically Complex World of Pearls

The luster of Queen Victoria’s brooch, Marilyn Monroe’s necklace and Elizabeth Taylor’s pear-shaped gem won’t soon fade. But researchers at the American Museum of Natural History hope their pearl exhibition, running through April 14, also will seed a nugget of scientific wisdom amid the splendor.

Paula Mikkelsen, assistant curator in the museum’s division of invertebrate zoology, said she wants visitors to replace their preconceived notions of uniform white spheres with a new appreciation of the dazzling array of pearl colors, shapes and sizes made by freshwater and marine mollusks around the world.

“The second thing is I want people to be aware that pearls are made by living animals, that in order to either gather them or culture them, we have to be aware of the needs of living animals,” she said. “Despite our industrial progress, we are still at the whim of biological animals to make this pearl.”

Mollusks are one of the most diverse groups of animals on Earth, second only to insects, with 100,000 living species and 530 million years of evolutionary history. The shell-clad animals originated in the oceans, later moving to freshwater and land habitats.

As an example of the immense diversity of mollusks, the exhibition includes the world’s largest known specimen of a giant clam. Collected from Sumatra in the Indian Ocean, the 3-foot bivalve lived embedded in a coral reef, its valves gaping upward.

Neil Landman, the exhibition’s lead curator and a curator in the museum’s division of paleontology, said giant clams reach their immense size because of a unique symbiotic relationship with photosynthetic algae that grow along the clams’ edges. The algae gain a habitat, and the filter-feeding clam derives additional energy from the photosynthetic process. The clam’s light-sensitive frilly mantle readily senses shadows and the clam’s adductor muscles will close the shell in response--but Landman and Mikkelsen dispelled a popular myth by saying the slow process poses no real harm to unwary divers.

“You could probably stick your hand in it for 10 minutes and not be in trouble,” Mikkelsen said.

The display includes a model of the largest pearl ever found, a multimillion-dollar, milky white bauble measuring 9 inches across and weighing 14 pounds. It was collected from a giant clam in the Philippines.

Scientists aren’t sure why only some mollusk species produce pearls. But the variety of shapes, sizes and colors are seemingly endless, from the big orange melo pearls of Vietnam’s baler shells and the blue-green pearls of New Zealand abalone to the smaller iridescent gems from Japan’s Akoya pearl oysters.

Scientists estimate that only one in 10,000 mollusks naturally produces a pearl, most of them irregularly shaped instead of round. (Round pearls appear more commonly in the cultured pearl industry because a perfectly round implanted plug is used to seed the pearl formation.) A pearl forms when an irritant lodges between a mollusk’s shell and the soft mantle lining the shell, either by accident or by human intervention in the case of cultured pearls.

In most pearl-forming mollusks, the mantle secretes the colorless or white mineral aragonite and the protein conchiolin, which can take on an array of colors depending on its organic pigments. The resulting compound, called nacre or mother-of-pearl, coats the intruding particle in microscopic layers until a pearl forms. The crystalline structure of nacre reflects light in a unique way, giving nacreous pearls their high luster, or apparent inner glow. Other pearls have a surface resembling porcelain.

Although scientists say pearling ventures haven’t directly led to any mass mollusk extinctions, history has provided some valuable lessons. In the century after Columbus landed in Venezuela in 1498, millions of highly prized pearls from the region’s harvested Atlantic pearl oysters were shipped to Europe to satisfy the craze, bringing the oyster close to extinction. It has never fully recovered.

The western Indian Ocean offers a more optimistic lesson. Until the cultured pearl industry began in the 1920s, most of the world’s pearls came from the small Ceylon pearl oyster, called bil-bil by Red Sea pearlers. Mikkelsen said the oyster has supported a huge industry for centuries, a remarkable longevity she attributes to rotating areas of harvesting.

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Bryn Nelson is a reporter at Newsday, a Tribune company.