What is a "brachiopod"?

A brachiopod is a marine invertebrate characterized by its bivalve shell, which resembles those of clams or oysters. However, brachiopods are not closely related to mollusks. They have a distinct anatomy, with a stalk or a hinge that attaches them to the substrate. Brachiopods have two shell valves, with the upper valve usually larger and the lower valve smaller. They filter feed by extending a specialized structure called a lophophore, which is lined with ciliated tentacles, to capture plankton from the water. Brachiopods have a long fossil record, dating back over 500 million years, and were once much more diverse than they are today. While they may appear similar to clams, brachiopods have their own unique evolutionary history and ecological niche in marine ecosystems.