"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.

Colors of scallops

If you have been reading my latest posts, you will know that I have been focusing on colors in certain minerals. In this present post, I shall discuss colors of certain clams called scallops, which belong to the family Pectinidae. The family is comprised of many genera.  Those belonging to genus Pecten are called pectens (not: pectins, which refers a group of colloidal substances used to jell various foods or cosmetics). 

Pectinids are especially abundant as fossils, if found in the right environment. Their geologic time range is Triassic to modern day. Along the west coast of the USA, fossil pectinids are  common in sediments of Miocene and Pliocene age (20 million to about 3 million years ago). Today, the strong muscle that holds the two valves together is a popular seafood delicacy. 

The two valves of a pectinid are referred to as the left and right valves. 

In addition to the primary (radial) ribs, important morphologic parts of a pectinid are the auricles ("ears") along the hinge line. As shown above, on the right valve, the anterior auricle is the most elongate. On some pectinids a byssal notch is located directly below the elongate anterior auricle. This notch is where the byssus (a bundle of hairlike material used for attachment) exits the shell.


Today, there are approximately 250 species of pectinids. Some have shells which are bright orange, yellow, red, blue, purple, brown, white, or combinations thereof. This color variation comes mainly from heredity although environment can play a role (e.g., some muddy-bottom pectinids have darker color than sandy-bottom pectinids.

The four colored specimens (each one is about 5 cm width) shown above are of species of the pectinid Chlamys, and they show some of the variation in the color. Some people think that they are hand painted, but the colors of these specimens occur naturally. Chlamys lives by having its byssus attached to a rock, shell, or other hard surface.

These are the right (= the specimen on the left side of this image) and left valves of a modern specimen of Chlamys hastata (2.7 cm wide), dredged from 210 feet depth, at the shelf break just south of Long Beach, southern California.

Nodipecten subnodosus, 15 cm wide, exterior and interior of a right valve. This species is most commonly found in modern waters in the Gulf of California, Mexico.

Nodipecten has been observed anchored by a byssus to hard substrates, but they have been observed also to have the ability to swim for short distances. At least temporarily, therefore, they are not anchored by a byssus. Thus, its byssal notch is not well developed. Nodipecten swims by pulsatory clapping together of their valves. This swimming action is exhausting and cannot be sustained for very long; it is normally used only to escape from predators (e.g., sea stars).

Patinopecten caurinus (Gould, 1850), 16.5 cm wide, exterior of a large right valve modern-day specimen from Skagway, Alaska. Patinopecten has a well developed byssal notch, therefore, it lived by attaching to hard surfaces. 

Patinopecten caurinus, 16 cm wide, exterior of a large right valve fossil specimen from a late Pliocene (3 million years old) bed found between Goleta and Santa Barbara, California. If you look carefully, you can see the minute daily growth lines of this shell.