Rough Piddock (also known as the Fluted Piddock or Angel Wing Piddock fragment)
Phylum: Mollusca, Class: Bivalvia, Order: Myida, Family: Pholadidae, Genus: Zirfaea, Species: Zirfaea pilsbryi · Pholadidae (Piddock or Angel Wing family) · Bivalve (fragment); specifically a piece of a boring clam shell with a rectangular-grid surface sculpture.
Species
Zirfaea pilsbryi
Shell Type
Bivalve (fragment); specifically a piece of a boring clam shell with a rectangular-grid surface sculpture.
Family
Pholadidae (Piddock or Angel Wing family)
Size
The fragment appears to be approximately 0.5 to 0.75 inches long. Full adults can reach 5 inches (12-14 cm) in length.
Color & Pattern
Uniformly creamy white to light tan. The color appears natural but weathered. The primary feature is the geometric, decussated (criss-cross) pattern created by radial ribs intersecting with concentric growth lines.
Rarity
Common (as a fragment). Finding a complete, paired set of valves is Uncommon/Rare because the shells are thin, brittle, and lack a hinge ligament, causing them to break apart easily in the surf.
Habitat
Found in the intertidal and subtidal zones. Unlike most clams, they live in burrows they rasp into hard clay, soft rock, or shale using the abrasive front edge of their shells.
Geographic Range
Pacific Coast of North America, ranging from Alaska to Baja California, Mexico. Very common in Southern California, including Coronado.
Description
A worn fragment of a Piddock clam valve. The exterior exhibits a distinct sandpaper-like texture characterized by a file-like grid. This sculpture is more pronounced on the anterior side of the shell, which the animal uses as a biological drill bit to bore into substrates.
Key Features
The distinctive 'waffle' or 'file' sculpture created by crossing ridges; the thin, brittle nature of the shell material; and the slightly curved, rectangular lattice-work ornament.
Collector Value
Minimal ($0-$1). While the full shell is prized by Pacific coast collectors for its 'Angel Wing' shape, fragments like this are ubiquitous on beaches like Coronado and Silver Strand.
Condition Notes
Fair. This is a weathered beach fragment, likely tumbled by waves. It has lost the characteristic 'teeth' found on fresh specimens and lacks the internal apophysis (handle-like structure). Collectibility is low as a specimen, but high as a decorative beach find.
Interesting Facts
The living animal is much larger than its shell and cannot retract fully inside. They are 'boring' clams that spend their entire adult life in a single hole they have carved into the sea floor. Some species of Pholadidae are even bioluminescent.
Ecological Role
As bio-eroders, they help break down hard underwater substrates, creating micro-habitats for other small marine creatures (like crabs and worms) once the clam dies and leaves an empty burrow.
Similar Species
Barnea subtruncata (Pacific Mud Piddock) which has a more elongated, less coarse sculpture, and Cyrtopleura costata (Angel Wing), which is found in the Atlantic.
Beach Finding Tips
Look in the 'crunchy' shell drift lines after a high tide or storm. At Coronado, they are often found near the Hotel del Coronado or further south toward the Naval base where clay deposits occur offshore.
Notes
Coronado beach