Some species have highly regular shapes and the same number of ridges on every shell. Other, like Cubitostrea perplicata, are irregular. Different shells have different number of ridges with different shapes. Each shell is disorganized with ridges starting and ending randomly throughout each shell.

When scientists investigated how such patterns were formed, they found an answer in the animals' soft parts. Shells of oysters and other related animals are laid down layer by layer. An organ called the mantle sits where each new layer will be. It lays down the biominerals that become the shell.

Researchers linked the irregular growth of oysters like Cubitostrea to the shape of the mantle. In an oyster like the one in the photograph, the mantle stretched much longer than the edge of the shell it grew on, making the surface ripple and fold like loose cloth. When the mantle started each new layer, it stretched over the old layer of shell. Researchers realized that instead of being purposefully organized, the ridges are just the results of layers after layers of oversized mantle.

References: Checa, Antonio G., and Antonio P. Jimenez-Jimenez. "Rib fabrication in Ostreoidea and Plicatuloidea (Bivalvia, Pteriomorpha) and its evolutionary significant." Zoomorphology 122(2003):145-159.