"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.