Blister Pearls

A pearl is a calcareous object produced within the soft tissue (mantle) of an oyster (family Pteriidae). The sheen or luster (iridescence) of a pearl, referred to as nacre or “mother-of-pearl,” is caused by the intergrowth of microscopic crystals of aragonite (calcium carbonate) and conchiolin (a protein). Both are formed by the mantle tissue (soft part) of an oyster. As I have mentioned on some of my previous posts, nautiloids and some gastropods can make nacre. Nacre is relatively soft and it ranges from 2.5 to 4.5 on the Mohs scale. A copper penny has a hardness of 3.5 and a knife blade has a hardness of about 5.5. 

As a “side note”: aragonite can form two ways: inorganically or organically. If it is inorganically formed, then it is a mineral. If it is organically formed, then it is a biomineral. Only biogenic aragonite has the nacreous sheen.

The purpose of this blog post is to focus on the blister pearls or “half pearls,” which are the result of part of the growth of the pearl against the inside of the shell, rather than totally within mantle tissue. Blister pearls can be as big as 10 mm diameter. Blister pearls are not valuable, but they can still have beautiful iridescence.

This image shows several blister pearls (each one about 7 mm diameter) on a cut-out portion (6 cm diameter) of a pearl-oyster shell (genus, species, and provenance unknown). 

Pearls can be formed by freshwater (river) oysters or by marine oysters. In some cases, other types of bivalves, both fossil and modern-day, are known to produce pearls. These fossil pearls are usually not preserved very well and have been recrystallized to calcite, with a loss of the sheen or luster.

All pearls, whether they occur in oysters living in shallow-marine waters or in freshwater rivers, are formed in response to an irritant (e.g., a grain of sand becomes embedded, by chance, into the mantle). The mantle then secretes nacre around the irritant. Pearls that are cultured (note: they are not simulants) in aqua-labs have a tiny piece (machine rounded) of another oyster shell artificially introduced (“seeded”) into the mantle of the host oyster. Several species of the saltwater oyster Pinctada and, to a much lesser extent Pteria species, are the main pearl-producing oysters.

Two views (exterior and interior) of a right-valve (7 cm height) of the saltwater oyster Pinctada sterna (provenance unknown), with an unusually large (2 cm diameter) blister pearl. This species ranges from southern California to Peru and can be as much as 10 cm in height. The posterior wing on at the top of the left side of the first image is missing.

"Hammer Oysters"

Today, in tropical oceans, there are approximately eight species of unusually shaped bivalves (clams) called “hammer oysters.” That name is because their shells have a “T” shape and can somewhat resemble oysters. “Hammer oysters” live predominantly in coarse sands on reef flats or in crevices of coral rocks in warm shallow-marine waters. They belong to the family Malleidae Lamarck, 1818, which has a geologic range from Jurassic to Recent. In this post, I am focusing on the malleids that belong to genus Malleus Lamarck, 1799, which is known only from the recent record in the Indo Pacific, Australia, and the Caribbean. The name Malleus is the Latin word for hammer.

Malleus albus Lamarck, 1819: right-valve exterior of a late-juvenile specimen [height 162 mm, width 27 mm] (an adult specimen is twice this size). Shallow water on rock flats; common; Indo-Pacific waters.

The attached small shell about half-way down on the valve is a cemented oyster.

This is the left valve of the same specimen.

Same specimen but showing the interior of each valve (left valve on the left side of image, right valve on the right side of image).

The next series of images shows Malleus malleus Lamarck, 1819: adult specimen [height 95 mm, width 175 mm]. Indo-Pacific region. Shallow water; common; Indo Pacific waters.

Right-valve exterior.

Left-valve exterior.

Interior of both valves (right on bottom, left on top)

Internally, the shell of Malleus has a prominent, circular to shortly oblique (V-shaped) depression (ligamental pit), near the top center of the hinge. This depression is where a leather-like, strong ligament helps to hold the two valves together.

The image above is from the same right valve shown immediately above.

The hinge line of Malleus has also a byssal notch just anterior to the ligamental pit. This notch is where byssal threads emanate from the hinge, and these strong threads are used for attachment of the shell to the substrate.

There is a wide and prominent nacreous ("mother of pearl) area below the hinge line, but this area does not continue too far. Along the posterior side of this nacreous area is the adductor muscle scar (elliptical shape). This strong muscle does much to keep the valves together during life.

The characteristic narrow projections (“wings”) along the hinge of Malleus serve as stabilizers, allowing the bivalves to stay in place in the sands, and not be easily dislodged by the swirling currents. Some species of Malleus can lay unattached (free) on the sand, and others can be wedged in crevices. Malleus is unlike true oysters, which are attached (cemented) to the substrate.