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What Is The Difference Between Fish And Shellfish?

What Is The Difference Between Fish And Shellfish?

The characteristics of fish and shellfish have a few similarities, but there are some key differences that make them distinctly different. While the terms may be used interchangeably on occasion, shellfish are not fish.

We’ll discuss what these sea creatures are, what distinguishes them from each other and explore their role in the realm of seafood.

If you appreciate seafood, you won’t want to miss these fast facts below before ordering your next meal.

What Exactly are Shellfish?


Shellfish are aquatic invertebrates that are popular for creating healthy and tasty meals and they are real treats for those who love seafood.

Shellfish are best recognized for having a hard shell or exoskeleton, and almost all species of this classification have this feature.

‘Shell’ refers to their hard exoskeleton, while the word ‘fish’ is used to refer to any animal life that lives in the water.

While the name alone seems to imply that shellfish are fish, this isn’t the case. In fact, fish and shellfish are not even closely related!

For this reason, many people and seafood providers will call traditional ‘fish’ the term ‘fin fish’ to distinguish the difference between vertebrate fish and shellfish.

Even they need water to survive, shellfish do not need clean water to grow. However, shellfish are what are known as filter feeders, so they have a system that pups and filters water through their bodies in order to sustain.

That being said, it is extremely important for shellfish to be thoroughly clean and be placed for a period in clean water before being consumed. Many people also have an allergic reaction to shellfish, so they have several consumption risks.

Shellfish are grouped into two categories: crustaceans and mollusks / bivalves. Examples of types of shellfish classified as crustacean include shrimp, prawns, lobster, seahorse, and crabs.

In the mollusks and bivalves category there are mussels, oyster, clam, scallops, conch, squid, snail, octopus and abalone. It is worth noting, that shellfish are one of the most common food allergens.

What Are the Characteristics of Fish?

Salmon fish at market

Fish are cold-blooded aquatic creatures that don’t maintain a constant body temperature. Rather, this is heavily influenced by the environment that they are in.

Therefore, tropical fish that live in warm waters will have a higher body temperature than fish in cold or Arctic climates. This is a trait shared by both shellfish and fin fish.

Beyond the fact that these creatures live in water and rely on thermoregulation, true fish have a whole set of unique qualities and systems. Fish have fins and a backbone, while shellfish have no bones.

Used to help propel and guide them through the water, most fish have several types of fins with dorsal fins (one on the top and bottom of the fish) for balance and paired side fins for propulsion.

Fish also have a tail fin and it is one of the most powerful tools they have to move about in the water.

The surface of most fish is typically covered with overlapping scales that serve as protection from injury and certain types of infections.

Exceptions to this include jawless fish like lampreys, hagfishes and pufferfish that have smooth skin and are free of dermal bone.

Fish secrete a thin covering of mucus atop their scales that helps shield them from infections. The mucus effectively attaches to the scales and serves to immobilize viruses and bacteria to prevent them from entering the fish’s body.

A secondary benefit to this mucus is that it minimizes friction to allow fish to travel more easily through the water.

Fish have unique organs called gills that allow them to breathe underwater, and they are generally located on the side of the fish’s body, just behind the head.

Their gills contain thousands of tiny blood vessels known as capillaries, and water continually pumps across them to filter oxygen from the water and into the fish’s blood stream.

Gills further the breathing process of fish by extracting waste from the fish’s blood like ammonia.

While shellfish tend to spend most of their time bottom feeding on the bottom of the water they occupy, fish need to be mobile to search for food, breed and to spawn.

However, much of their time is spent floating in the water rather than continuously swimming, which is made possible by their swim bladder. This sac holds air, and fish can adjust the air amount to move up and down in the water.

Fun Fact: Members of the ray and shark class don’t have swim bladders.

Are All Fish Considered Seafood?

Seafood is defined as all classes of bony fish and includes tuna, cod, sturgeons, sawfish, pollock, sea bass, perch, sharks, yellowtail, rays and other primitive water-living species in these classifications.

Crustaceans such as crab, lobster, crayfish, clams, conch, mussels, shrimp, mollusks and others amid that classification are also considered seafood.

In North America, the term ‘seafood’ is additionally applied to freshwater organisms that can be consumed by humans. Under this definition, all aquatic life can generally be considered seafood.

Fun Fact: In many European countries, typically only creatures that come from the sea are called seafood.

The Bottom Line: What Is the Difference Between Fish and Shellfish?

Let’s do a quick summary of this brief to reiterate the primary question at hand: What is the difference between fish and shellfish?

The primary difference between the two is that fish have bones, while shellfish are bone-free invertebrates. They also have organs and methods of motion to support their survival.

So, while both shellfish and fin fish fall into the category of seafood, they are biologically different and are placed in different classifications.

Learn More About Shellfish, Fish, and Seafood!

While fish and shellfish are types of seafood, they are very different in design and in how they survive and function in their environment. However, both make tasty dishes (especially with a great seafood seasoning)!

Learn more about fish in this report from the University of Hawaii, and explore more facts about shellfish-including nutrition info-in this Healthline report.

Those who would like to find out more details about the seafood industry can do so with this brief from Britannica.