Stiff Pen Shell (Atrina rigida)

An empty large brown pen shell clam washed up on a beach at the Gulf of Texas

nikkigensert/Getty Images 

The stiff pen shell, or rigid pen shell, is one of the several species of pen shells. These mollusks have a long, triangular, or wedge-shaped shell and attach to rocks or shells in sandy, shallow ocean bottoms.


Stiff pen shells can be up to 12" long and 6.5" wide. They are a brown or purplish-brown color and have 15 or more radiating ribs that fan out across the shell. They may also have erect, tubular spines.

Pen shells may produce black pearls.


  • Kingdom: Animalia
  • Phylum: Mollusca
  • Class: Bivalvia
  • Order: Pterioida
  • Family: Pinnidae
  • Genus: Atrina
  • Species: rigida

Habitat and Distribution

Stiff pen shells live in warmer water from North Carolina to Florida, and also in the Bahamas and West Indies.

They are found on sandy bottoms in shallow water. They attach with their byssal threads, pointed end down.


Pen shells are filter feeders and eat small particles passing through the water.

Conservation and Human Uses

Pen shells have a scallop-like adductor muscle (the muscle that opens and closes the shells) and are edible. They also produce black pearls which may be used in jewelry. Pen shells in the Mediterranean (Mediterranean pen shells) were harvested for their byssal threads, which were woven into an expensive cloth.


  • Gofas, S. 2011. Pinnidae. World Register of Marine Species (Online) Accessed on May 24, 2011
  • Meinkoth, N.A. 1981. National Audubon Society Field Guide to North American Seashore Creatures. Alfred A. Knopf, New York
  • Scott, S. 2011. Pen Shells Provided Ancients Byssal Threads for Fine Sea Silk (Online) Accessed May 24, 2011
mla apa chicago
Your Citation
Kennedy, Jennifer. "Stiff Pen Shell (Atrina rigida)." ThoughtCo, Jul. 31, 2021, Kennedy, Jennifer. (2021, July 31). Stiff Pen Shell (Atrina rigida). Retrieved from Kennedy, Jennifer. "Stiff Pen Shell (Atrina rigida)." ThoughtCo. (accessed June 3, 2023).