A scallop is definitively not a crustacean, although both are commonly grouped together and sold as shellfish. Scallops and crustaceans (e.g., shrimp, crab, and lobster) belong to entirely different major divisions of the animal kingdom. Confusion often arises because both possess hard outer coverings and are highly valued marine seafood items. Understanding their classifications reveals profound differences in anatomy and biology.
Scallops: Classification as Bivalve Mollusks
Scallops are classified within the Phylum Mollusca, a diverse group that includes snails, clams, and mussels. They belong to the Class Bivalvia, characterized by two hinged shells (valves) covering the soft body. These fan-shaped shells, often ribbed and symmetrical, are composed of calcium carbonate, acting as a protective covering for the animal.
The unsegmented body features a soft mantle, which is responsible for secreting the shell material. The large, centralized adductor muscle is prized for consumption and is used to rapidly open and close the shells. This powerful muscle allows most scallop species the ability to swim short distances by clapping their valves together. Scallops also possess numerous tiny eyes along the mantle edge, detecting light and movement.
Crustaceans: Defining Characteristics of Arthropods
Crustaceans are members of the Phylum Arthropoda, the largest phylum in the animal kingdom, which also includes insects and spiders. The defining feature of all arthropods is a body structure based on repeated segments.
This segmented body plan is protected by a rigid exoskeleton, composed primarily of chitin. Since this hard outer casing does not grow, crustaceans must periodically shed the structure in a process called molting to increase in size. A hallmark of the crustacean body plan is the presence of specialized, jointed appendages, including multiple pairs of legs, antennae, and mouthparts.
Fundamental Differences Between Scallops and Crustaceans
The fundamental differences between scallops and crustaceans come down to their phylum-level characteristics. The scallop’s shell is a calcareous, two-part structure secreted by the mantle that does not need to be shed for growth. Conversely, a crustacean’s exoskeleton is a single, integrated body covering made of chitin, fused to the tissues, which must be actively molted for growth.
Crustaceans exhibit distinct body segmentation, typically forming a cephalothorax, and possess multiple pairs of jointed legs and antennae. Scallops and other mollusks have an unsegmented, soft body and completely lack the jointed appendages characteristic of arthropods, relying instead on the adductor muscle for movement.
While both are important types of shellfish gathered from the sea, their classification into two different phyla highlights that they are fundamentally different life forms. The scallop is a bivalve mollusk, distinct from the segmented, exoskeleton-bearing crustacean.