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The pen shell (Atrina rigida) has two thin-walled, fan-shaped shells and is one of the largest bivalves (two shells) in the world, reaching lengths up to 10 inches (250 millimeters). In Florida, they are common in shallow waters near seagrass beds and are usually found buried in the sand with only the upper portion of the shell exposed. The tissues inside are a bright orange, although tissue of other Atrina species can be white or beige. Pen shells are found throughout Florida waters and are often observed washed up on local beaches after periods of high winds and wave action. The adductor muscle inside the pen shell is edible, similar to scallops, and is preyed upon heavily by starfish and horse conchs. After a predator has consumed the animals, smaller invertebrates and fish use the buried shells as refuges.

Pen shells are edible, but consumers should follow the Florida Department of Health seafood safety guidelines and only consume shellfish collected from areas open to harvesting, which can be found on Florida Department of Agriculture and Consumer Services website.