A scallop is a marine bivalve mollusk, a type of shellfish characterized by its two hinged, fan-shaped shells. Scallop size is highly variable, depending on its species, age, and environment. Commercially, the most important measurement is the diameter and weight of the single, cylindrical adductor muscle. This firm, white, edible muscle is sold as “scallop meat” and dictates the final commercial size and price.
Understanding Scallop Measurements
The size of a scallop is quantified using two metrics: the biological measurement of the shell and the commercial grading of the muscle. Biologically, size is recorded as the shell height or diameter, measured from the hinge to the shell’s furthest edge. This measurement can range from less than an inch to over six inches across, depending on the species.
Commercially, scallops are graded based on the number of adductor muscles required to make up one pound of meat, a system known as the “count.” This count directly translates to the size of the muscle itself. For example, a grading of “10/20” means it takes between 10 and 20 individual muscles to equal one pound.
The largest scallops are designated with a “U” prefix, which stands for “under.” A grade of “U-10” means there are fewer than 10 scallops per pound, indicating a jumbo-sized muscle. Conversely, a higher count number signifies a much smaller scallop, such as a 60/80 count, which takes 60 to 80 muscles to reach a pound.
Size Ranges of Major Commercial Species
The difference in size seen commercially is primarily due to the species being sold. The three main commercial types are Sea Scallops, Bay Scallops, and Calico Scallops, each possessing a predictable size range.
Atlantic Sea Scallops (Placopecten magellanicus) are the largest and most common variety in North America, harvested from deep, cold waters. A mature Sea Scallop shell can reach a diameter of up to 170 millimeters (nearly seven inches). Their large adductor muscle typically measures 30 to 40 millimeters in diameter, resulting in commercial grades of U-10 or 10/20 count.
Bay Scallops (Argopecten irradians) are significantly smaller, dwelling in shallow, protected waters. Their shell generally grows to a maximum length of about 68 millimeters (2.7 inches). The edible muscle is proportionate to the shell, resulting in a much higher count, often falling into the 60/80 or 80/120 range.
The Calico Scallop (Argopecten gibbus) represents the smallest of the three main commercial species. Found in warmer, open waters, their shells usually reach a maximum width of about 76 millimeters (three inches). They have a short lifespan of 18 to 24 months, which contributes to their small final size. Their small muscle size often relegates them to use in pre-mixed seafood products.
Biological Factors Affecting Maximum Size
Beyond genetic differences, the maximum size an individual scallop attains is governed by several biological and environmental factors. Age is a primary determinant of size, as scallops must live long enough to accumulate mass. While Bay Scallops are short-lived, the larger Sea Scallop can live for 20 years or more, allowing for continuous growth.
Water temperature is a major influence on size and geographical range. Colder water temperatures slow a scallop’s metabolism, which can lead to slower but larger growth in some species. Warmer temperatures increase metabolic demand, which can reduce growth efficiency.
Food availability also plays a direct role, as scallops are filter feeders that rely on microscopic phytoplankton for energy. Areas with high concentrations of suitable food allow for greater energy storage and faster growth rates. The depth of the habitat influences both temperature and food supply, with scallops in shallower water often exhibiting greater growth.