It is a common point of confusion whether shellfish are considered fish. Many people use the terms interchangeably or group them together under the umbrella of “seafood.” This widespread misunderstanding often stems from culinary classifications rather than biological distinctions. Understanding the scientific differences between these aquatic animals reveals that despite living in water, shellfish are not fish.
Understanding Biological Classifications
Biologically, animal classification relies on structural and evolutionary characteristics. Fish are aquatic vertebrates, possessing a backbone or spinal column. They breathe using gills throughout their lives and move using fins, which are appendages adapted for locomotion in water. Most fish species are also cold-blooded, meaning their body temperature fluctuates with their surrounding aquatic environment.
Shellfish, in contrast, are aquatic invertebrates, meaning they lack a backbone. They fall primarily into two major phyla: Mollusca and Arthropoda. Mollusks are soft-bodied animals often with a hard shell secreted by a mantle. They are unsegmented and typically exhibit bilateral symmetry, often moving with a muscular foot. Arthropods have segmented bodies, jointed appendages, and a rigid external skeleton (exoskeleton) made of chitin. This exoskeleton provides support and protection and must be periodically shed for the animal to grow.
Why the Confusion About Shellfish
The common confusion regarding whether shellfish are fish largely arises from culinary and commercial practices rather than scientific accuracy. In common parlance and the food industry, “seafood” often broadly encompasses any edible marine life, including finfish and shellfish. This culinary grouping merges distinct organisms, leading to the misconception that they are similar or related. Historically, “fish” itself was used more broadly to describe various aquatic animals, including those now classified as invertebrates, further contributing to this linguistic legacy.
Their shared aquatic habitat also contributes to the misunderstanding. Both fish and shellfish inhabit marine and freshwater environments, leading people to associate them because they are harvested from the same places. Despite this shared environment, their internal biology and evolutionary lineages are vastly different. The persistent use of “shellfish” as a compound word that includes “fish” also reinforces a connection that does not exist biologically.
Common Types of Shellfish
Shellfish encompass a diverse array of invertebrate animals, predominantly categorized into crustaceans and mollusks. Crustaceans are arthropods, identifiable by their jointed legs and hard exoskeletons. Common examples include crabs, lobsters, shrimp, and crayfish. These animals often have segmented bodies and appendages specialized for various functions like walking, swimming, or feeding.
Mollusks are another large group of shellfish, characterized by soft bodies often protected by a shell. This category includes bivalves like oysters, clams, mussels, and scallops, which possess two hinged shells. Other well-known mollusks are gastropods, such as snails and abalone, which typically have a single, often coiled shell. Cephalopods, including squid and octopus, are also mollusks, though many lack an external shell, possessing instead a highly developed nervous system.