A bay scallop, photographed on Rhode Island's shoreline. E. Zabel, courtesy of URI

Bay Scallop (Argopecten irradians)

Color: Exterior shell color ranges from drab gray to yellow or reddish brown. Interior is white, often purplish near the hinge.
Size: 3 inches in diameter.
Habitat: Subtidal zone, eelgrass beds, sandy and muddy bottoms, and offshore in shallow to moderately deep water, such as bays and harbors.
Seasonal appearance: All year.

Description

Unlike most bivalves, which are oblong or oval, the corrugated shell of the bay scallop is almost perfectly circular. The bay scallop has a strong hinge muscle within the shell but does not have a foot for digging or a siphon for water intake. Along the edge, or mantle, of bay scallop shells are 30 to 40 bright blue eyes. Each eye has a lens, retina, cornea, and optic nerve, enabling it to see movements or shadows and to detect predators. Along the edge of the mantle are tentacles containing cells sensitive to chemicals in the water; these cells help the bay scallop react to its environment.

Life History and Behavior

Bay scallops reach maturity when they are one year old, and they spawn in the summer. They grow quickly, rarely living past three years of age. When bay scallops are young, they attach themselves to objects such as eelgrass by means of a byssal thread. This helps them avoid bottom-feeding predators, such as sea stars. As bay scallops grow, they drop to the sediment surface in the vicinity of eelgrass beds and move on to tidal flats to feed at high tide.

The bay scallop is one of the few filter-feeding bivalves that do not live buried in the sand or attached to rocks. Instead they settle and move freely along the bottom sediment surface.

When a bay scallop senses the presence of a predator, it swims away by clapping the two sides of its shell together, forcing water from its mantle cavity and bouncing across the bottom sediment.

Special Notes

• The abductor muscle of the bay scallop makes excellent food, and bay scallops are harvested in Rhode Island using dredges, nets, and rakes.
• In the 1880s, populations were so healthy that Rhode Island was a top supplier in the bay scallop industry. In the 1930s, many of the eelgrass beds in Narragansett Bay began to die off, leading to a severe reduction in the bay scallop population.
• Populations have continued to decline in the Bay in recent years. Previously, one of the richest bay scallop areas was Greenwich Bay. Today, it has no significant eelgrass beds and consequently no significant bay scallop population.
• Efforts are under way by state environmental groups to restore bay scallop populations in areas where they were historically found in the Bay.

Adapted from The Uncommon Guide to Common Life on Narragansett Bay. Save The Bay, 1998.