Scallops belong to the phylum Mollusca, a classification they share with animals like clams, oysters, and snails. Crustaceans, conversely, are members of the phylum Arthropoda, which includes insects, spiders, and centipedes. This biological divide separates the scallop from true crustaceans like shrimp, crabs, and lobsters.
The Scallop’s True Biological Identity
A scallop is classified within the phylum Mollusca and specifically the class Bivalvia, meaning “two valves” or two shells. The soft body is entirely unsegmented and protected by a pair of hinged shells made of calcium carbonate, which are secreted by the mantle.
A defining feature is the large, central adductor muscle, the part commonly eaten as seafood. This powerful muscle controls the rapid opening and closing of the shells, allowing the animal to swim in short bursts via jet propulsion. Along the mantle’s edge, scallops possess numerous sensory tentacles and up to 200 tiny eyes used to detect movement and light.
Defining the Crustacean Group
Crustaceans are members of the phylum Arthropoda, characterized by jointed legs and external skeletons. They possess a rigid exoskeleton, composed primarily of chitin and often reinforced with calcium carbonate. This hard outer layer provides protection and structural support, and must be periodically shed through molting to allow the animal to grow.
Their bodies are fundamentally segmented, typically organized into three main regions: the head, thorax, and abdomen, often fused into a cephalothorax. The body is covered in jointed appendages, including multiple pairs of legs, antennae, and specialized mouthparts for feeding. Examples like shrimp, lobsters, and crabs rely on these segmented bodies and jointed limbs for movement and grasping food.
Comparing Body Structures
The fundamental differences in body structure clearly demonstrate why a scallop is not a crustacean, starting with their external coverings. The scallop’s defense is its two-part shell, which is a non-living secretion of the mantle, entirely distinct from the crustacean’s segmented, chitinous exoskeleton. The scallop’s shell is a closed box for a soft, unsegmented body, whereas the crustacean’s exoskeleton is a suit of armor with flexible joints between segments.
The method of movement also highlights the biological separation of these groups. Scallops move by clapping their shells together using their adductor muscle, which forces water out to propel them forward. Crustaceans, in contrast, use their jointed, paired appendages for locomotion, such as walking legs on the seabed or specialized swimmerets.