What is Shellfish? Key Differences & Allergy Guide

Shellfish are invertebrates with hard shells, distinct from fish. They include crustaceans like shrimp and mollusks like oysters, vital in seafood.

Shellfish Fish

Fish and shellfish share traits but differ significantly. Though sometimes confused, shellfish aren’t fish. This discussion covers their unique features and importance in seafood. Seafood lovers should note these essential facts for their next dining choice.

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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.

FAQs abut Shellfish & Fish

What makes an animal a shellfish?

An animal is considered a shellfish if it possesses a shell and is classified under the phylum Mollusca, the class Crustacea (phylum Arthropoda), or the phylum Echinodermata. The term “shellfish” is commonly used to refer to the edible species within these groups, particularly those that are commercially fished or raised.

Is calamari a shellfish?

Calamari is included in the list of shellfish, which encompasses molluscs and crustaceans, that have the potential to trigger allergic reactions. Other examples of shellfish that can cause allergies include crayfish and cuttlefish, among others.

Are scallops a fish?

Scallops are not a fish; they belong to the phylum of mollusks, which also includes clams, mussels, oysters, squid, octopus, snails, and sea slugs.

Is crab a type of fish?

Crab is not a type of fish; it is a crustacean. Crabs, along with shrimp and lobsters, belong to a group of animals that have exoskeletons.

What are different characteristics of fish and shellfish?

The different characteristics of fish and shellfish include the presence of fins and a backbone in fish, whereas shellfish do not have bones. Fish use their fins to propel and guide themselves through water, with dorsal fins for balance and paired side fins for propulsion.

How can you identify fresh fish and shellfish?

You can identify fresh fish and shellfish by observing certain characteristics. Fresh fillets will have firm flesh and red blood lines, or red flesh if it is fresh tuna. When pressed, the flesh should spring back. There should be no discoloration, darkening, or drying around the edges of fish fillets. Additionally, the flesh of shrimp, scallops, and lobsters should be clear with a pearl-like color and should have little to no odor.

Is shellfish as healthy as regular fish?

Shellfish is not as healthy as regular fish when it comes to omega-3 content. However, oysters, shrimp, crab, lobster, and mussels contain approximately 25%-50% of the omega-3s per serving compared to the healthiest fatty fish, such as salmon. The amount of micronutrients in shellfish varies depending on the specific type consumed.

What are the 3 types of shellfish?

The three types of shellfish are molluscs, crustaceans, and echinoderms, which are recognized by scientists as aquatic invertebrates.

What to avoid if you have a shellfish allergy?

If you have a shellfish allergy, you should avoid consuming abalone, barnacle, krill, clams (such as cherrystone, littleneck, Pismo, and quahog), crab, crawfish, crayfish (écrevisse), crawdad, lobster, langouste, langoustine, scampi, coral, tomalley, and mollusks.

Is calamari shellfish or fish?

Calamari is considered a type of shellfish, along with other molluscs and crustaceans, that can potentially trigger allergic reactions.

What are people allergic to in shellfish?

People are allergic to tropomyosin (TM), which is the primary allergenic protein found in all types of edible crustaceans and mollusks. TM sensitizes and causes reactions in over 60% of individuals with shellfish allergies, often resulting in severe systemic responses.

Why am I suddenly allergic to shellfish?

Your sudden allergy to shellfish is typically caused by your immune system reacting to a protein called tropomyosin found in the muscles of shellfish. This reaction involves the release of antibodies that stimulate the production of chemicals like histamines, which target the tropomyosin protein.

Why am I allergic to shrimp but not other shellfish?

It is possible for individuals to be allergic to shrimp but not other shellfish due to incomplete cross-reactivity among crustacea. This means that the allergy may be limited to certain crustacea members. It is important for patients to be aware of the potential for inadvertent hidden exposure to the allergen, especially in restaurants where cooking equipment or serving utensils may come into contact with different foods.

Can you be allergic to finned fish but not shellfish?

Finned fish and shellfish are not closely related, so it is possible for someone to have an allergy to finned fish while not being allergic to shellfish. However, it is important to take precautions to prevent cross-contact between fish and shellfish. It is recommended to have a detailed discussion with your allergist to determine the appropriate food restrictions for your specific allergy.

Is salmon and shellfish the same thing?

Salmon and shellfish are not the same thing. Seafood encompasses a wide range of edible freshwater and saltwater sea life. Fish, such as salmon, tuna, cod, haddock, and others, have bones and gills. On the other hand, shellfish, including crab, lobster, shrimp, scallops, and clams, are invertebrates with shells.

Is shellfish healthier than fish?

Shellfish is not as healthy as fish in terms of omega-3 content. However, oysters, shrimp, crab, lobster, and mussels contain approximately 25%-50% of the omega-3s per serving compared to the healthiest fatty fish, such as salmon. The amount of micronutrients present in shellfish varies depending on the specific type consumed.

What is the difference between a crustacean and a fish?

The difference between a crustacean and a fish is that fish are vertebrates with an internal skeleton made of bone or cartilage, while crustaceans, along with clams and snails, are invertebrates. Both fish and crustaceans are aquatic creatures.

Is lobster finfish or shellfish?

Lobster is considered a type of shellfish, along with shrimp and crab. Shellfish, including lobster, are known to be beneficial for brain and heart health. However, it is not uncommon for individuals to have food allergies to shellfish. Throughout history, people from various cultures have been consuming shellfish as a part of their diet for centuries.

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.

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