Species Description: Argopecten irradians concentricus is a member of the family Pectinidae. The bay scallop is difficult to distinguish from the queen scallop (Chlamys opercularis) and the calico scallop (Argopecten gibbus). For the bay scallop, the shell is right convex (the right valve is more rounded than the left) (Fay et al. 1983). The shell of A. irrradians concentricus is broadly fan shaped with more than 14 radial ribs (Fay et al. 1983, Mikkelsen and Bieler 2008). Shells are symmetric and vary considerably in color. They usually have a molted pattern incorporating dark grey, black, or brown with orange, red, or yellow hues (Fay et al. 1983).
Regional Occurrence: Argopecten irradians concentricus occurs in seagrass beds in Atlantic coastal waters from New Jersey to Florida and in the Gulf of Mexico (Kruczynski 1972, Fay et al. 1983, Arnold et al. 1998, Irlandi et al. 1999).
Age, Size, Lifespan: Adult bay scallops range in size from 55 to 90 mm (Fay et al. 1983). The average life span of Argopecten irradians concentricus is 12 - 18 months (Barber and Blake 1985a). The longest living individuals will live for 26 months (Fay et al. 1983). The larval mortality rate is somewhat high. It can range from 10-50% within the first 8 days of settlement. High water temperatures in the summer months can lead to a shortened lifespan in adults (Blake 2005).
Abundance: The Argopecten irradians concentricus fishery along the east coast of the United States has collapsed since the 1950s (Blake 2005). This species is very sensitive to natural (i.e. hurricanes, red tide events) as well as anthropogenic (i.e. overharvesting and habitat loss) processes (Summerson and Peterson 1990, Blake 2005). The decline in seagrass beds on the west coast of Florida may be a large contributing factor to the loss of bay scallop populations. Studies from several regions suggest that inadequate larval supply does not allow A. irradians concentricus to recover rapidly from depletion events (Arnold et al. 1998). In areas where the bay scallop does thrive, densities can be as high as 24.8 individuals per m2 (Fay et a1. 1983).
Locomotion: Adult Argopecten irradians concentricus swim by "clapping" pulsed expulsions of water from the mantle cavity. Bay scallops exhibit a "zig-zag" swimming pattern by alternating water expulsions between the anterior and posterior gaps in the shell (Fay et a1. 1983).
Reproduction: The bay scallop is a functional hermaphrodite, having both male and female sex organs (Barber and Blake 1985b). It is protandrous, releasing male gametes before the female gametes, to avoid self-fertilization (Fay et a1. 1983). Spawning will not occur until seawater temperatures reach 20°C. Individuals of Argopecten irradians concentricus have one reproductive cycle during their lifetime. Oocyte development begins in August and spawning usually occurs by October (Barber and Blake 1985b).
Embryology: Argopecten irradians concentricus have planktotrophic veliger larvae that appear within 48 hours after fertilization. The pediveliger, with a hinged shell and a fully developed foot stage emerges at approximately 10 days. Settlement and metamorphosis occurs in less than 2 weeks (Fay et a1. 1983). Peak recruitment during the fall in the Suwannee River, Florida region in the mid-1990's was reported to be approximately 20 spat per day. This was similar to what has been previously reported for the North Carolina basins and Long Island, New York (Arnold et al. 1998). The growth of juvenile bay scallops is rapid during the spring months and slows down as water temperatures decrease in the fall (Irlandi et al. 1999).
Temperature: Under laboratory conditions, the optimal temperature for the development of bay scallop embryos is 21.9°C. When seawater temperature dropped below 15°C, or increased to temperatures above 30°C, bay scallop embryos could not survive. Adult Argopecten irradians concentricus exhibit a small increase in its oxygen intake when seawater temperatures increase (Barber and Blake 1985b).
Salinity: Under laboratory conditions, the optimal salinity for the development of bay scallop embryos is 24.4 ppt. Mortality occurred when salinities dropped below 20 ppt or increased above 30 ppt (Tettlebach and Rhodes 1981). Adult Argopecten irradians concentricus exhibit an increase in its ammonia content in response to decreasing salinities (Barber and Blake 1985b).
Trophic Mode: The bay scallop is a filter feeder catching primarily phytoplankton in its gills (Burle and Kirby-Smith 1979, Fay et a1. 1983, Parker 2006).
Associated Species: Argopecten irradians concentricus is the most common host for Pinnotheres (Tumidotheres) maculates (squatter pea crab) in Bogue Sound, North Carolina. This symbiosis causes a decrease in the growth of Argopecten irradians concentricus (Kruczynski 1972, 1973).
Fishery: Bay scallops are fished primarily by small dredges dragged by power boats. In shallow waters, rakes and hand-gathering methods are used. In areas where fishing is allowed, the bay scallop season is usually from autumn to spring, outside of the reproductive periods. In 1981 the dollar value of the bay scallop fishery was $2,427,000 (Fay et al. 1983). In 1995 the Florida Marine Fisheries Commission eliminated all commercial fishing of the bay scallop within state waters and put a cap on recreational fishing in areas near the mouth of the Suwannee River.
Aquaculture: Because of the decline of natural populations, commercial bay scallops are primarily reared in aquaculture facilities. In Tampa Bay, Florida, scallops are maintained in cages suspended in the water column until they are ready to spawn (Blake 2005). The mortality of the settled larvae is usually between 10-50%. Larvae are maintained on a diet of algae grown in sterilized seawater. Settled spat are maintained in re-circulating systems for 30-45 days and then transferred to mesh bags that are hung 1.0 m deep in the bay until maturity. Each year wild caught adults are introduced to aquaculture populations to maintain genetic diversity. Bay scallops are sold to wholesalers dealing in the specialty market for $0.25 - 0.30 each (Blake 2005).
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Irlandi EA, Orlando BA, and WG Ambrose, Jr. 1999. Influence of seagrass habitat patch size on growth and survival of juvenile bay scallops, Argopecten irradians concentricus (Say). Journal of Experimental Marine Biology and Ecology 235:21-43.
ITIS. Integrated Taxonomic Information System. Available online.
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Summerson HC and CH Peterson. 1990. Recruitment failure of the bay scallop Argopecten irradians concentricus, during the first red tide,Ptychodiscus brevis, outbreak recorded in North Carolina. Estuaries 13:322-331.
Tettlebach ST and EW Rhodes. 1981. Combined effects of temperature and salinity on embryos and larvae of the northern bay scallop Argopecten irradians concentricus. Marine Biology 63:249-256.