Pterioida

The Pteriida is an order of bivalve mollusks, characterized by their equivalve shells, which means both valves are equal in size. This group includes well-known species like pearl oysters and fan shells. Pteriida are anisomyarian, meaning they have two adductor muscles, though they often function as monomyarian (one adductor muscle). Their shells are composed of outer simple prismatic calcite and inner nacre layers. These bivalves are typically marine and attach themselves to surfaces using byssal threads or by being cemented.1

Bivalves, in general, are aquatic, filter-feeding organisms. Their flattened, two-valved shells are adapted for burrowing or for attachment to hard substrates. While most bivalves are marine, some inhabit freshwater environments. The classification of bivalves relies on shell features, ligament structure, hinge teeth, and body form.21