Scallops are mollusks, belonging to a vast and diverse phylum of invertebrate animals. This classification places them in the same major biological category as an octopus, a garden snail, or a clam, despite their outwardly different appearances. Understanding where the scallop fits requires examining the defining characteristics of this invertebrate group.
Defining the Mollusk Phylum
The Phylum Mollusca is the second-largest phylum in the animal kingdom, encompassing over 85,000 recognized species. All members share fundamental body features, including a soft, unsegmented body often protected by a hard, calcareous shell. The body is divided into three parts: a visceral mass containing the organs, a muscular foot used for locomotion, and a mantle, which is a fold of tissue that secretes the shell.
Most mollusks also possess a specialized feeding organ called a radula, a ribbon of microscopic teeth used for scraping food. While most mollusks are aquatic, inhabiting marine and freshwater environments, some species like land snails and slugs are terrestrial. The phylum showcases variety, from the slow-moving, shelled snail (Gastropoda) to the fast, shell-less squid (Cephalopoda).
Scallops as Bivalves
Scallops are organized into the Class Bivalvia, which translates to “two valves” or two shells. This class includes familiar shellfish such as oysters, mussels, and clams, all sharing the characteristic of having a body laterally compressed and enclosed by a pair of hinged shells. These two valves are connected at the dorsal side by a ligament that forces the shells to open when the internal muscles relax.
Bivalves lack a distinct head and the radula found in other mollusks, relying instead on filter feeding for nutrition. They draw water into their mantle cavity, where specialized gills capture tiny particles of plankton and organic matter. This filtering mechanism is their chief mode of feeding and places them in an ecological role as natural water purifiers.
Distinctive Scallop Anatomy and Function
The scallop is set apart from its bivalve relatives by its powerful adductor muscle and unique mobility. This single, large, cylindrical muscle attaches to the inside of both valves, and it is the only part typically sold and consumed as seafood in North America. The adductor muscle is functionally divided into two parts: a striated portion for rapid, powerful contractions and a smooth portion for sustaining shell closure for long periods.
The rapid contraction of the striated adductor muscle allows the scallop to “swim” by rapidly clapping its shells together, expelling jets of water. This jet propulsion provides brief bursts of erratic movement, primarily as a defense mechanism to escape predators like sea stars. Unlike many stationary bivalves, the scallop is a free-living organism that rests on the seabed and can quickly flee danger.
Another unusual anatomical feature is the presence of numerous small, light-sensitive eyes, which can number up to 200 along the edge of the mantle. These eyes contain a concave mirror structure, rather than a lens like a human eye, which focuses light onto a dual-layered retina. This unique optical system allows the scallop to detect motion and shadows, providing an early warning system for approaching threats.
Commercial Significance and Harvesting
The scallop’s large adductor muscle is prized globally for its sweet flavor and firm texture, making it an economically valuable marine species. The commercial fishery focuses entirely on harvesting this muscle, which is shucked and processed immediately after being caught to prevent spoilage. This is necessary because, unlike clams or oysters, scallops cannot clamp their shells tightly enough to survive long out of the water.
The two main methods for harvesting wild scallops are dredging and diving. Dredging involves towing a heavy metal net across the seabed, which is efficient but can disturb the marine environment and benthic habitats. Diver-caught scallops are collected by hand, a selective method that minimizes habitat damage and yields premium product, though it accounts for a smaller portion of the total catch.
The sustainability of wild scallop populations is managed through strict regulations on harvest size, season, and location. This is complemented by a growing global aquaculture industry that cultivates various species, mainly in Asia.