Atlantic thorny oyster

The Atlantic thorny oyster can grow up to 10 centimetres (3.9 in) in diameter. The valves of the shell are roughly circular and the upper one is decorated with many spiny protuberances up to 5 centimetres (2.0 in) long. When growing in a crevice, the shape of the shell adapts itself to the available space. The color varies but is usually white or cream with orange or purplish areas making it well camouflaged to hide from its predators. The lower valve is flat and is attached to the substrate. When the living animal is lying on the seabed it is usually not visible because of the algae, marine animals and sediment that cover the shell. The flat tree oyster and Lister's tree oyster are often among these epibionts. A diver swimming past may just observe a slight movement on the seabed as the oyster snaps its valves shut. Young animals are much less spiny than adults and resemble members of the genus Chama, the jewelbox clams.