Scallops are a popular seafood choice, but many mistakenly believe they are a type of fish. Scallops are distinct marine animals, and understanding their true biological identity clarifies why they are not fish. This article explores what scallops are, highlighting their unique features and the fundamental differences that set them apart from fish.
Scallops: Not Fish, But Molluscs
Scallops belong to the phylum Mollusca, a diverse group of invertebrates that includes snails, clams, and octopuses. They are classified under the class Bivalvia, possessing two hinged shells that enclose their soft bodies. These shells are held together by a strong adductor muscle, which is the edible part commonly consumed. Unlike fish, which are vertebrates, scallops are invertebrates, lacking a backbone or internal skeleton.
Key Differences Between Scallops and Fish
Fish are vertebrates, possessing an internal skeletal system with a backbone, fins, and scales. Scallops, as bivalve molluscs, are invertebrates and lack these features. They have a two-part shell, a soft mantle that lines the shell, and a muscular foot.
Fish use gills, typically covered by an operculum, to extract oxygen from water. While scallops also use gills for respiration, their gill structures are housed within their mantle cavity and function primarily for filter-feeding, pulling water over them to capture small food particles. This integrated process is distinct from the specialized respiratory mechanisms found in fish.
Distinctive Features of Scallops
Scallops exhibit several interesting characteristics that make them stand out among marine life. One notable feature is the presence of numerous simple eyes, often bright blue, arranged along the edge of their mantle. A scallop can have anywhere from 50 to 200 of these eyes, which can detect light, darkness, and motion in their surroundings. These eyes contain complex structures, including a lens and a retina, enabling them to perceive changes in their environment.
Scallops are also unique among most bivalves for their ability to move actively. While many bivalves are sessile, remaining attached to a surface, most scallops can “swim” by rapidly clapping their shells together. This action expels water, creating a jet propulsion that allows them to move in short, quick bursts, typically to escape predators like sea stars. This swimming behavior, combined with their free-living nature, sets them apart from many other bivalve species.