Stiff Pen Shell (also known as the Sawtooth Pen Shell)
Phylum: Mollusca, Class: Bivalvia, Order: Pterioida, Family: Pinnidae, Genus: Atrina, Species: Atrina rigida · Pinnidae (Pen Shell family) · Bivalve (two-shelled), whole shell, fan/wedge shape
Species
Atrina rigida
Shell Type
Bivalve (two-shelled), whole shell, fan/wedge shape
Family
Pinnidae (Pen Shell family)
Size
Approximately 8-10 inches in length, which is large and typical for a mature specimen of this species. Adults generally range from 6 to 12 inches.
Color & Pattern
Dark brownish-black to olive exterior with a rough, weathered texture. The interior surface often shows a pearly, iridescent nacre near the pointed anterior end. The colors are natural and weathered, showing typical marine fouling and organic encrustations.
Rarity
Common in suitable habitats, though intact, large specimens found washed ashore can be somewhat uncommon due to the fragile nature of the thin shell.
Habitat
Shallow subtidal zones, seagrass beds, and sandy-mud flats, typically embedded vertically in the substrate anchored by strong byssus threads. Found in marine environments from the intertidal zone down to about 30 meters.
Geographic Range
Western Atlantic Ocean, including the coast of North Carolina, Florida, the Gulf of Mexico, the Caribbean Sea, and down to South America.
Description
A large, triangular or wedge-shaped bivalve with a pointed anterior and a broad, rounded posterior margin. The shell is relatively thin and fragile, characterized by prominent radial ribs and sometimes scale-like spines along the exterior surface. The texture is rough and often covered in organic debris or epibionts.
Key Features
Elongated, wedge-like fan shape tapering to a sharp point at the anterior end; thin and brittle shell structure; radial ribs running toward the wide posterior margin; lack of a strong hinge compared to typical clams.
Collector Value
Low to moderate commercial value, mostly valued by regional shell collectors as a striking display piece due to its large size and unique fan shape. Usually sells for a modest price in shell shops.
Condition Notes
Fair to Good condition for a beach or shallow water find. The shell is intact with minor edge wear, some surface encrustation, and weathering. The brittle nature of pen shells makes finding complete large specimens a notable feat.
Interesting Facts
Pen shells get their name from an old practice where people used the silky byssus threads—which the animal uses to anchor itself—to weave a fine cloth known as sea silk. They also frequently host symbiotic pea crabs inside their mantle cavity.
Ecological Role
Pen shells play an important ecological role by stabilizing soft-bottom marine substrates with their large size and byssus threads, creating microhabitats for various small organisms, algae, and encrusting invertebrates. They filter large volumes of water, contributing to water clarity.
Similar Species
Amber Pen Shell (Atrina serrata), which is generally lighter in color with more delicate, translucent walls and prominent spines; Noble Pen Shell (Pinna nobilis) found in the Mediterranean.
Beach Finding Tips
Look for large fan shapes sticking up out of sandbars or shallow grass flats during extreme low tides. After storms, whole or partial specimens frequently wash up on Gulf and Atlantic beaches.