Rock Scallop Shell, Crassadoma gigantea
Rock Scallop Shell, Crassadoma gigantea. Shell collected off the beach in the greater San Diego area, San Diego, California, October 2018. Size: 20 cm (7.9 inches) x 20 cm (7.9 inches). Collection, photograph and identification courtesy of Bob Hillis, Ivins, Utah.
Rock Scallop, Crassadoma gigantea. Underwater photograph of a Rock Scallop, in the greater San Diego area, San Diego, California, July 2018. Photograph and identification courtesy of Bob Hillis, Ivins, Utah.
Phylogeny: The Rock Scallop Shell, Crassadoma gigantea (J.E. Gray, 1825), is a member of the Pectinidae Family of Scallop Shells. The genus Crassadoma is one of sixty-four genera in this family, and this is the only species in this genus. They are also known as the Giant Rock Scallop Shell and the Purple-hinged Rock Scallop Shell and in Mexico as Ostión Gigante de Roca and Vieira de Roca. The genus name Crassadoma comes from the Latin words meaning “thick house”, referring to their thick shells.
Description: Rock Scallop Shells are large and heavy with the right valve adjusting in shape to match the surface contour of the rock to which it attaches. The left valve has a rough exterior surface and rounded ribs. The shell may have a reddish, gray, or brown exterior. The interior is white, with a purple region near the hinge. Live specimens have a bright orange mantle. The Rock Scallop Shells reach a maximum of 22.8 cm (9.0 inches) in length and 19.5 cm (7.7 inches) in height.
Habitat and Distribution: Rock Scallops live more like oysters than scallops. They attach themselves permanently to rocks and other hard surfaces via their right valve. They are found from the lower intertidal zone to depths up to 76 m (250 feet). They are a temperate to tropical Eastern Pacific species that are found in all Mexican waters of the Pacific Ocean, included around the Revillagigedo Islands, with the exception that they are absent from the Sea of Cortez.
Ecology and Behavior: Rock Scallops are suspension feeders that feed primarily on diatoms, other plankton, and suspended organic matter. In turn they are preyed upon by crabs, gastropods and sea stars. They are gonochoric and reproduce sexually, through broadcast spawning, with external fertilization. The eggs hatch into planktonic larvae (veligers) and form their shells at around two months. They are free swimming (like most scallops) until they reach 5 cm (6 months) at which time they attach themselves to the rock. Before they attach permanently to the rock they can temporarily attach by byssal threads. They have life-spans of up to 50 years. They host a variety of epibionts on their shells such as algae, coralline algae, bryozoans, and tube dwelling polychaetes. Most of these species benefit their host by providing camouflage in exchange for a place to live. Some species of epibionts such as the Boring Sponge, Cliona celata, are parasites, which damage the host scallop. From a conservation perspective the Rock Scallop has not been formally evaluated. However, they are fairly common with a relatively wide distribution and should be considered to be of Least Concern. Rock Scallops are edible and are the target of commercial, recreational, and artisanal fisheries. They are also farmed. Over harvesting has severely disrupted population levels in localized areas.
Synonyms: Crassedoma giganteum, Hinnita poulsoni, Hinnites crassa, Hinnites giganteus, Hinnites multirugosus, Lima gigantean, Pecten (Chlamys) multirugosus, and Pecten (Chlamys) multirugosus crassiplicatus.