Scallop bivalves (clams) belong to family Pectinidae, a worldwide group consisting of several hundred species, including some large northern kinds fished commercially for their edible adductor muscle. This muscle, which is used for holding the two valves together, is considered to be a delicacy by many people (Abbott, 1985).
Crassadoma gigantea (J.E. Gray, 1825) is a large scallop living in shallow waters from the Aleutian Islands, Alaska to Bahia Magdalena in Baja California Sur, Mexico. In cooler waters, it reaches its largest size: up to 23 cm in diameter and up to 20 pounds in weight). The name is derived from crassus (Latin: thick) and domus (Latin: house). The common name for this clam is the “purple-hinged rock scallop.” The internal ligament of scallops occupies a triangular pit at the center of the hinge. Crassadoma gigantea is the only species of this genus, which is endemic (confined) to this region. No records of it are known from the Bering Sea, Kamchatka, Japan, or elsewhere.
Crassadoma gigantea has as a fossil record of Miocene to Pleistocene age but only in northern California and Baja California Sur. One of its synonyms of this bivalve is Hinnites giganteus, a name no longer used because it is taxonomically out-of-date.
All of the images shown below are of same specimen (9 cm height and 9 cm width) of C. gigantea from Leo Cabrillo Beach, Los Angeles County, southern California.
Exterior of right valve.
End view of right valve showing the juvenile-growth region.
The juvenile part of a valve is called the “chlamys” growth stage. A juvenile lives “free” by attaching its strong byssal threads to the substrate. These threads occur on one side of the hinge, and this area (byssal notch) is usually not preserved in the adult stage. The juvenile shell, furthermore, is commonly preserved on both valves. Eventually, the bivalve becomes an adult and becomes permanently cemented to the substrate. Adult Crassadoma gigantea can attach to rock faces or pilings, as well as inside crevices and under boulders. It ranges in depth from intertidal to 80 m.
Interior of right valve.
Interior of left valve.
Side view of both valves together (articulated), with the right valve (strongly convex) overlying the left valve (nearly flat).
The attached strongly convex right valve, is referred to as the “lower valve,” and the less convex (can be nearly flat) left valve is the “upper valve.” The terms “lower and upper,” however, are relative terms for specimens that commonly attach to vertical surfaces, like steep rock faces or harbor pilings. Permanent attachment to the substrate is rare in pectinids.
Other examples of extant-scallop genera in the world are the pectinids: Pecten, Chlamys, Aequipecten, Amussium, Crytopecten, and Lyropecten. “Wild” scallops have traditionally been harvested using scallop dredges or bottom trawls. In some areas, divers collect the scallops by hand, but this is an expensive process.
On a world-wide basis, in the last few years, there has been a decline in the numbers of “wild” scallop productivity. One reason might be the significant decline in the sharks that prey upon the scallop-eating cownose sting rays. The main diet of these rays is scallops, and without the control offered by the sharks, the sting-ray population is increasing (Milius, 2007; Schmid, 2007).
The simultaneous reduction in sea grasses is another contributing concern. Juvenile scallops (referred to as “spat”) attach themselves to sea grasses before seeking their adult mode of life. Sea grasses provide a sanctuary for the juveniles (Milius, 2007; Schmid, 2007).
Another reason for the decline of “wild” scallop productivity might be the rising acidity of the world’s oceans. At the root of this change is the rising carbon-dioxide levels released into the atmosphere by human activities (pollution) and forest fires. The carbon dioxide is absorbed the oceans, which are showing increasingly higher pH levels. This acidity deprives baby scallops the calcium carbonate they need to grow strong shells. Commercial shell-fishing activities all over the world are at risk, but the Pacific Northwest (especially Washington, British Columbia, and Alaska) are unusually vulnerable because winds that gust south in the summer whip deep water to the surface. These upwellings are even more prone to being acidic, because carbon dioxide tends to be trapped in colder depths (Guilford, 2014).
For more details about C. gigantea, SEE:
Coan, E.V., et al. 2000. Bivalve seashells of western North America. Marine bivalve mollusks from Arctic Alaska to Baja California. Santa Barbara Museum of Natural History Monographs Number 2 Studies in Biodiversity Number 2. 764 pp.
References Cited:
Abbott, R.T. 1985. Seashells of the world. A guide to the better known species. Golden Press, New York. 160 pp.
Guilford, G. 2014. From Our Obsession The Sea. https://qz.com/311345/the-worlds-scallops-and-oysters-are mysteriorly-dying-out/
Milius, S. 2007. Too few jaws: sharks decline let rays overgraze scallops. Science News 171(13):197.
Schmid, R.F. 2007. Shark deaths upset rest of food chain: Shark overfishing may be endangering scallop populations, say scientists. Associated Press/ABC New, May, 29, 2007.
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