How are lamp shells different from clams and other bivalve mollusks?

Although lamp shells (Brachiopoda) and bivalve mollusks both possess two shells, they are distinct groups of animals that are not closely related.1

The primary differences include:

• Shell Orientation and Symmetry: In lamp shells, the two valves are unequal in size and are positioned on the dorsal (top) and ventral (bottom) sides of the animal. These valves are bilaterally symmetrical, meaning the left and right sides are mirror images of each other. In contrast, bivalve mollusks typically have valves that are right and left mirror images of one another.234
• Anatomy: Lamp shells possess a lophophore, a specialized feeding structure, whereas bivalves collect food using ciliated ctenidia (gills).4
• Attachment: Many lamp shells are attached to the substrate by a fleshy stalk called a pedicle. While some bivalves also attach to surfaces using a byssus, many others are free-living or specialized for burrowing using a muscular foot.34

It is important to recognize that while their superficial resemblance often leads to confusion, their evolutionary histories and internal structures are fundamentally different.1