Rock Oyster     Pododesmus macroschisma (cepio)

Also called a jingle (supposedly from the sound the empty shells make), this red-lipped mollusk lives on hard surfaces, like floats at the Boston Harbor Marina.  But unlike real oysters, the rock oyster uses a tough, calcified, ligament-like attachment or byssus to anchor to the float rather than using its shell.  The byssus pierces the smaller, thin bottom shell through a pear-shaped hole. Interestingly the inner surface of the larger top shell has a green color due to presence of living microscopic algae.  

The shell ranges in size from a couple of inches for the small one in the photograph to around five inches.  The body flesh color is creamy while the mantle fringe is a bright red-orange.  The shell is rather smooth with light radiating ridges but heaver concentric rings.  As the animal grows the shell may become covered with other organisms such as small sponges and calcified serpulid worm tubes.

The rock oyster occurs from the low intertidal to around 300 feet subtidally and ranges from Alaska to Baja California.  Spawning occurs during July and August.  It is eaten by starfish.