Species Description: Valves of Argopecten gibbus are deep and generally equal, with 17-23 (usually 20) deep, radiating ribs (Allen and Costello 1972). The left (upper) valve is somewhat more convex than the right (lower) (Broom 1976). Base color of the shell is creamy white to yellowish, splotched with patches of bright red, maroon, or lavender, with the left valve typically more brightly colored that the right. Wings lateral to the hinge of the shell are equal in length. Calico scallops may reach 2 inches in shell height, and over 3 inches in shell length.
Potentially Misidentified Species: Valve color and shell morphometry are used to distinguish calico scallops from related species. Argopecten gibbus is distinguished by its yellowish to white base color splotched with patches of bright red, maroon, or lavender. It is also slightly convex in the left (upper) valve (Broom 1976). Two other scallops that are documented to occur in the Indian River Lagoon are similar in size and overall shape. The bay scallop, Argopecten irradians, is generally a uniform gray to gray-brown color with distinct convexity of the right (lower) valve. The rough scallop, Aequipecten muscosus, may also be mistaken for the calico scallop. However, the rough scallop has unequal wings and sharp scales on lower surface of the ribs, with shell color ranging into shades of yellow and brown.
Regional Occurrence: The range of the calico scallop extends from Maryland through Florida, the Gulf of Mexico, and south to Brazil, including Bermuda and much of the Caribbean.
IRL Distribution: Argopecten gibbus is not considered common within the Indian River Lagoon. While it is sometimes observed in or around inlet areas, it is primarily found in nearshore and offshore waters where it supports a commercial fishery (Arnold 1995; Moyer and Blake, 1986; Blake and Moyer 1991).
Age, Size, Lifespan: Argopecten gibbus reaches 40 to 60 mm (1.6 to 2.4 inches) shell height and approximately 80 mm (3.2 inches) in shell length. It lives an average of 20 months, with a maximum life expectancy of 24 months (Allen and Costello 1972).
Abundance: Cape Canaveral, Florida has been reported as the enter of abundance in the range of calico scallops, and may be a source of supply larvae to areas north of Florida by transport in Gulf Stream currents (Roe et al. 1971). Scallop grounds offshore of Cape Canaveral are among the world's largest, extending from St. Augustine south to Stuart, Florida. Vast scallop beds occur within this area, some measuring over 2,600 feet in length, and 8,600 feet in width (Allen and Costello 1972).
Roe et al. (1971) reported the abundance of calico scallops varies within and between areas on a seasonal basis.
Locomotion: Scallops are one of only a few Molluscan groups that have the ability to swim actively, especially as a predator avoidance response. Argopecten gibbus, like most scallops, swims horizontally to the substratum by quickly adducting its valves to propel itself on jets of water (Donovan et al 2004). Examinations of shell morphometry in relation to swimming ability in scallops found that the relatively thinner shell and slight left convexity of the left (upper) valve generate more lift during swimming, and thus produce the long range swimming ability and speed of Argopecten gibbus, which may swim as much as 9 body lengths per second (Stanley 1970; Donovan et. al. 2004) Small scallops tend to swim more actively than larger ones.
Reproduction: Argopecten gibbus reach sexual maturity at approximately 4 months of age, or when they reach a size of 19 mm shell height (Miller et al. 1979; Arnold 1995), though Roe et al. (1971) found that sexual maturity correlates more to size rather than age.
Spawning and recruitment in calico scallops occur throughout the year (Allen, 1979), with healthy individuals spawning as much as 3 - 4 times (Roe et al. 1971; Arnold 1995). Maximum reproductive effort occurs from late fall through spring, with fall spawning being somewhat less intense than spring spawning (Moyer and Blake, 1986). Despite its decreased intensity, some evidence suggests that fall spawns may be crucial to maintaining standing stocks of calico scallops in the following year (Moyer and Blake, 1986).
Calico scallops are sequential hermaphrodites, first releasing sperm in to the water column followed by eggs, likely in response to changes in water temperature due to upwelling effects (Arnold 1995).
Embryology: Eggs develop into free-swimming trochophore larvae within 48 hours of fertilization. The larval stage persists 14-16 days before settlement, typically on hard substrata. Most commonly, settlement occurs on disarticulated shells from previous generations of scallops (Arnold 1995). Juvenile scallops attach to substrata via byssal threads and will remain attached until they reach 2.5 cm (1 inch) in shell length in approximately 3 months (Allen 1979).
Argopecten gibbus that settle in spring generally reach a size of 30 to 35 mm shell height by the following fall and are fully able to reproduce. As a result of this rapid growth and early maturation, scallop cohorts may overlap, with many different size classes occupying the same bed.
Salinity: In areas where Argopecten gibbus is most common, salinity typically ranges between 31 - 37 ‰ (Arnold 1995).
Trophic Mode: Calico scallops feed primarily on microalgae, diatoms, bacteria, and organic particulates which are filtered over the gills and passed to the mouth via cilliary tracts.
Habitat: Typical habitat is open marine or saline estuarine waters from the subtidal zone to the continental shelf, in depths of 30 - 1200 feet (Roe et al. 1971; Allen and Costello 1972), with scallop beds typically distributed parallel to the coastline. Roe et al. (1971) found that calico scallop beds north of Cape Canaveral tend to occur in deeper waters than those more to the south.
Typical benthos types for Argopecten gibbus include unconsolidated sediments that may be composed of hard sand, sand with shell hash, quartz sand, and sand-gravel.
Associated Species: Predators of Argopecten gibbus juveniles and adults include sea stars, gastropod mollusks, squid, octopods, crabs, sharks, rays, and several species of bony fishes (Roe et al. 1971; Broom 1976; Arnold 1995; Donovan et al. 2004).
Fisheries Importance: The calico scallop commercial fishery is highly variable and unpredictable (Arnold, 1995) (Figures 1 and 2 below) and is centered entirely in the waters around Cape Canaveral in northern Brevard County. It has sometimes been a high value fishery, with commercial harvests of over $19.5 million in both 1987 and 1988. However, the fishery has been plagued with problems, not only in unpredictable and highly variable recruitment of scallops, but also with parasitic infestations, which have been devastating to commercial harvests. Population crashes in 1989 and 1991 (Figures 1 and 2) have been directly attributed to infestation by a protozoan parasite of the genus Marteilia (Moyer et al 1993). In 1989, Marteilia caused a massive die-off of calico scallops within a 2,500-square-mile area off Brevard County, decreasing the commercial harvest to $6.6 million, less than half of the $19.5 million catch from each of the previous 2 years. This same parasite reappeared in 1991, when it was first detected in apparently healthy scallop beds in January. By February, few adult scallops remained alive. The commercial fishery has never fully recovered; as can be seen by the decreased harvests in the years that followed. Note that there are no commercial landings reported for 1996. In that year, commercial boats switched their fishing efforts primarily to rock shrimp (Arnold 1995).
Within the five-county area encompassing the IRL (Volusia, Brevard, Indian River, St. Lucie and Martin Counties) the commercial catch of the calico scallop, Argopecten gibbus, between 1987 - 2001, accounts for nearly 100% of the statewide total, with a harvest of approximately 38 million pounds, and a value in excess of $55.8 million. This ranks the calico scallop third in both commercial value within the IRL, and in pounds harvested.
Figures 1 and 2 below show the dollar value of the calico scallop fishery to IRL counties by year. Brevard County reaped 100% of the commercial value of calico scallops, due to the richness of the scalloping beds off Cape Canaveral. Note in figure 2 that the first 3 years of data have been removed to show some additional scale detail in the remaining years. Commercial harvests for calico scallops were lowest in 1996 ($0) and 1991 ($38,220), and highest in 1987 and 1988 when catches exceeded 10 million pounds annually and were valued in excess of $19.5 million each year.
|VOLUSIA||BREVARD||INDIAN RIVER||ST. LUCIE||MARTIN||TOTAL|
|YEAR||Value ($)||Value ($)||Value ($)||Value ($)||Value ($)||Value to IRL|
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