A young rock scallop (Crassadoma gigantea) was noticed filtering plankton from the surrounding ocean water, and this long lived scallop can reach 50 years of age. Once older this rock scallop blends extremely well into its environment as surrounding life continues to attach to the shells/valves. A rock scallop has a round shell, radiating shell ridges with spines, and an orange mantle edged with eyes. This scallop is the second largest in the world, and is also the heaviest scallop due to its very thick valves. An empty rock scallop shell can be identified by having a purple stain close to the hinge. The rock scallop can grow to 9.8 inches (25 cm) across, with a range from southern Alaska to northern Mexico.
References:
Fretwell, Kelly and Starzomski, Brian. “Rock Scallop, Crassadama gigantea” Biodiversity of the Central Coast, University of Victoria, 2014, https://www.centralcoastbiodiversity.org/rock-scallop-bull-crassodoma-gigantea.html
Jensen, Gregory with Gotshall, Daniel and Flores-Miller, Rebecca. Beneath Pacific Tides: Subtidal Invertebrates of the West Coast. Bremerton: MolaMarine, 2018.