SeaShell News, 3-6-15, Angel Wings, Shelling, Sanibel Island.
“Pholadidae, known as piddocks or angel wings, are a family of bivalve mollusc similar to a clam; however, they are unique in that each side of their shells is divided into 2 or 3 separate sections. Furthermore, one of the piddock’s shells has a set of ridges or “teeth”, which they use to grind away at clay or soft rock and create tubular burrows. The shape of these burrows is due to the rotating motion of the piddock as it grinds the rock to make its home. The piddock stays in the burrow it digs for the entirety of its eight-year lifespan, with only its siphon exposed to take in water that it filters for food. When the piddock dies and leaves an empty tubular burrow, other marine life such as sea anemone, crabs and other molluscs may use the burrow.
The angelwing species Cyrtopleura costata has approximately 26 radiating ribs. Growth lines run horizontally over the surface of the shell. Angelwings have a spoon-shaped brace under the beak of the shell, called the apophysis, where the mollusc’s foot muscles are attached. Cyrtopleura costatapossesses long siphons which protrude from its burrow and circulate water as the source for its food supply. It cannot retract its siphons into the protection of its shell, so the two valves can never shut completely.
The muscles fusing the shell’s valves together are weak, making it rare to find angelwings with both halves still intact. Some shell hunters dig for the living clam, and if dug up, the fragile shell must be placed immediately into a container of water or it will close and shatter. The angelwing’s shell is popular with collectors, as well as a delicious food staple. The angelwing lives offshore and in estuaries, sometimes as much as a metre (three feet) deep in the mud or clay.
Source: Angel Wings.