Science, Tech, Math › Animals & Nature Stiff Pen Shell (Atrina rigida) Share Flipboard Email Print nikkigensert/Getty Images Animals & Nature Marine Life Marine Life Profiles Marine Habitat Profiles Sharks Key Terms Amphibians Birds Habitat Profiles Mammals Reptiles Insects Forestry Dinosaurs Evolution View More By Jennifer Kennedy Jennifer Kennedy Marine Science Expert M.S., Resource Administration and Management, University of New Hampshire B.S., Natural Resources, Cornell University Jennifer Kennedy, M.S., is an environmental educator specializing in marine life. She serves as the executive director of the Blue Ocean Society for Marine Conservation. Learn about our Editorial Process Updated on March 10, 2019 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. Description 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. Classification 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. Feeding 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. Sources 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 Cite this Article Format mla apa chicago Your Citation Kennedy, Jennifer. "Stiff Pen Shell (Atrina rigida)." ThoughtCo, Jul. 31, 2021, thoughtco.com/stiff-pen-shell-atrina-rigida-2291843. Kennedy, Jennifer. (2021, July 31). Stiff Pen Shell (Atrina rigida). Retrieved from https://www.thoughtco.com/stiff-pen-shell-atrina-rigida-2291843 Kennedy, Jennifer. "Stiff Pen Shell (Atrina rigida)." ThoughtCo. https://www.thoughtco.com/stiff-pen-shell-atrina-rigida-2291843 (accessed June 3, 2023). copy citation Featured Video