Fish vs. Shellfish — What's the Difference?

Fish are vertebrates that live in water and breathe primarily through gills. They typically have streamlined bodies that facilitate efficient swimming. Shellfish, on the other hand, are not true fish and are broadly categorized into two groups: crustaceans and mollusks, which include species like crabs, lobsters, clams, and oysters. These creatures often have hard exoskeletons and do not possess fins.

Fish are covered in scales, which protect their bodies and help in hydrodynamic movement. They also have fins, which are used for propulsion, steering, and stability in the water. Whereas shellfish can have a variety of body structures but are commonly protected by a hard shell or a crusty exoskeleton, and they do not have fins but may have other appendages for mobility.

The diet of fish varies widely; some are strictly herbivores, others are carnivores or omnivores, depending on their species. They can hunt or scavenge, using their acute senses to find food. Shellfish diets also vary, with some filtering plankton from the water, others scavenging the ocean floor, and some being predators, but typically they have less varied diets compared to fish.

Reproduction in fish involves the release of eggs and sperm into the water, often called spawning. Many fish have elaborate mating rituals and can lay thousands of eggs at once. Shellfish, however, have a range of reproductive strategies; for example, oysters can change their gender from male to female and back, while crabs might carry fertilized eggs on their bodies.

Fish habitats are incredibly diverse, ranging from deep oceanic environments to freshwater rivers and lakes. Shellfish also occupy diverse habitats but are often found in specific zones like tidal pools, ocean beds, and sandy shores, adapting to life in fixed locations or burrowing habitats.