SeaShell News, 4-13-15, Atlantic Thorny Oyster, Shelling, Sanibel Island.
The Atlantic thorny oyster can grow 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 colour varies but is usually white or cream with orange or purplish areas making it well camouflaged. 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.
The Atlantic thorny oyster occurs in the western Atlantic Ocean, the Caribbean Sea and Gulf of Mexico where it is found at depths between 9 and 45 metres (30 and 148 ft). Its range extends from North Carolina and Texas southwards to Venezuela and Brazil. It occurs on deep water reefs especially in areas with high sedimentation. It is often lodged in a crevice or concealed under an overhang. It is also a member of the fouling community, being found on sea walls, man made structures and wrecks.
Source: Atlantic Thorny Oyster.