Bay Scallop
Argopecten irradians

Key Distinguishing Markings:

• Shell lightweight, thin, but strong.
• 13 to 22 symmetrically radial ribs originating from central point (umbo) of shell.
• The marginal wings or ears at the umbo are slightly different in size and shape.
• Bottom valve more convex than top valve.
• Color highly variable in this species:
  • Top (left) valve – drab; dark grey, black, or brown; occasionally with yellow, orange, or red highlights; ribs sometimes mottled or concentrically banded.
  • Bottom (right) valve – usually lighter in color than top valve, sometimes pale yellow or white.

• Adults can grow as large as 90 mm (3 ½ in).
• Scallops reach sexual maturity at about 55 mm (2 ¼ in), although age actually determines this.

• Bay scallops are primarily found from Massachusetts to Texas.
• In Maryland, they only occur in the coastal bays behind Ocean City and Assateague Island.

• Bay scallops inhabit the higher salinity (>15-20 ppt) shallows of bays and estuaries along the coast.
• This species is intimately associated with eelgrass beds.
• When a blight wiped out the grass beds in the 1930’s, scallop populations plummeted and completely disappeared in some areas.
• Young scallops attach to blades of eelgrass with thin byssal threads.
• By suspending themselves above the substrate, they are removed from suffocating silt and bottom-marauding predators such as crabs.
• Adult scallops live on the bottom, unlike many other bivalves such as clams which live buried in the sediment.
• Because scallops do not burrow into the bay bottom, adults only have a non-functional foot and do not have siphons.
• They can also be found occasionally on shelly areas such as the remnant oyster shell bars in Chincoteague Bay.

• Scallops are filter-feeders, straining microscopic plants (algae) from the water column.
• The food particles are filtered out by the gills of the scallops and transported to the digestive tract by tiny hair-like cilia.

• Scallops are hermaphrodites, simultaneously having both male and female sex organs.
• Spawning in Maryland occurs during May/June, with a second spawning in September.
• Eggs are released into the water column, where they are fertilized.
• During its spawning period, a one-year old scallop produces an average of 16 million eggs.
• The fertilized eggs develop into swimming larvae, complete with tiny shells.
• Water currents distribute the larvae around the estuary. Mortality is very high during this stage of their life history.
• After about 2 weeks, the larvae drop out of the water column and change into juvenile scallops, a process called metamorphosis.
• During metamorphosis, a foot and gills develop, the swimming organ disappears, and the tiny scallops attach to seagrasses or other objects.
• The foot, along with byssal threads, allows the juveniles to crawl up the grass blades.
• The juveniles will remain attached until they reach a size of 20 – 30 mm (¾ – 1 ¼ in), when they drop to the bottom.
• The foot fails to develop further, becoming a vestigial organ in the adults.
• This is a short-lived species, with a life span of only 12 to 26 months.

• Because of their extreme scarcity in our waters, any bay scallops caught should be returned to their environment.
• With conservation of the bay scallops and the seagrasses in which they live, hopefully their numbers will flourish.
• A mask and snorkel or a clear plexiglass-bottom bucket can be used to observe scallops in their natural habitat.

• Unlike other species which are bound to some substrate either by burrowing or attachment, adult bay scallops are free-living and extremely motile.
• They are capable swimmers for short distances, which they accomplish by jetting water through their valves, generally in response to predators.
• Bay scallops have 18 pairs of blue eyes set in the mantle along the margin of the shell.
• The eyes can detect shadows and movement, which brings about an escape response.
• In an attempt to re-establish a population in Chincoteague Bay, MDNR Shellfish Program planted 1.2 million juvenile bay scallops in protective pens and raised them to reproductive age during1997 and 1998.
• By 2002, live scallops were recorded north of the Ocean City Inlet, possibly for the first time in well over a century.
• Although low densities suggest that the long-term viability of this scallop population is still in doubt, the extraordinarily rapid range expansion is a major step towards their establishment in Maryland.

For more information on bay scallops, please contact Mitchell Tarnowski.