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Last updated on April 20, 2014 at 8:28 EDT

Spear Scallop, Chlamys hastata

Chlamys hastata, otherwise known as the spear scallop, spiny scallop, or swimming scallop, is a species of bivalve mollusk belonging to the family Pectinidae. This species can be found on the west coast of North America from the Gulf of Alaska to San Diego, California. A limited amount of these scallops are harvested by divers or by narrow trawls off of the west coast of Canada.

The spiny scallop resides on the seabed in the sublittoral zone between low tide mark and a depth reaching 490 feet, on soft sediment or on rock, especially in areas with a current that is strong. As a filter feeder, this scallop sieves microscopic algae from the water that passes through its gills. Starfish, octopuses, and sea otters prey upon this species. It can detect these predators via smell and sight and can swim away from them by opening and closing its valves. Other organisms often grow on the exterior of its shell and it frequently creates a symbiotic relationship with an encrusting sponge which grows on the upper valve and helps in protection from predators.

This scallop gets its name from the Latin word chlamys, meaning a Greek cloak or a short cape made of wool and worn by a soldier, and hastata meaning “spear-like” from the Latin hasta, meaning a spear or a javelin.

The shell of this scallop is shaped like a fan and has the potential to grow to a height of about 3.5 inches though a more normal adult size is 2.4 inches. It is made up of two valves, each of which is convex and has a small number of broad ribs covered with blunt spines. These radiate from the umbone, which is the rounded protuberance near the hinge, and between them are fine etched striations. The color of the background is white with radial bands of pale purple and the valve on the right, which is normally underneath, is paler than the valve on the left. Beside the hinge are two irregular shell flaps or auricles with the anterior one usually being much larger in size than the other. This provides an attachment for the single strong adductor muscle that functions by closing the shell. On either side of the long hinge there are some small ridges and grooves known as teeth. Their function is to prevent the sideways movement of the valves with regard to each other. Lining the inside of the valves is the mantle, which is a membrane that covers the gut and the other internal organs. It can be seen around the margins of the valves as a thick layer. It’s fringed with a number of short tentacles and there is a row of tiny simple eyes close to each of the valves. The scallop normally lies on its right valve and its exposed, left valve often has a colonial sponge growing on it.

The spiny scallop anchors itself to the substrate with a few byssus threads. It’s not clear what the purpose of this is but they may serve to help orientate the scallop in regards to the current for optimal feeding. They are effortlessly broken when the scallop starts to swim. As a filter feeder, it exposes its mantle by separating its valves and passing water through its gills by ciliary action.

Predators of this organism include starfish, particularly the ochre star and the sunflower star, octopuses, and sea otters.

Image Caption: Chlamys hastata at Ogden Point in Victoria BC. Credit: Daniel Hershman/Wikipedia (CC BY 2.0)

Spear Scallop Chlamys hastata