The common assumption that sharks and catfish are closely related is likely due to both being aquatic vertebrates with fins. While all fish share an ancient common ancestry, these two animals belong to fundamentally different groups. Their evolutionary paths diverged hundreds of millions of years ago, making their relationship analogous to that between a lizard and a bird. Understanding the biological distinctions between these two types of fish reveals why they are classified into separate classes.
Evolutionary Separation of Fish Classes
The two major branches of fish evolved separately, creating the distinct classes known today as the cartilaginous fish and the bony fish. This separation occurred very early in the history of jawed vertebrates, with both groups appearing in the fossil record roughly 419 million years ago. This ancient divergence means that a shark is evolutionarily closer to a ray than it is to a catfish. The defining difference between these two groups is the primary material composing their internal skeletons.
Cartilaginous fish, which include sharks, skates, and rays, represent one evolutionary lineage. Bony fish, which include catfish, salmon, and almost all other modern fish, represent the other. This deep split established the foundational anatomical differences seen today. Their distinct evolutionary trajectories resulted in unique adaptations for buoyancy control, respiration, and skin texture.
Key Traits of Cartilaginous Fish (Sharks)
Sharks possess a skeleton composed entirely of cartilage, a strong but flexible tissue, rather than calcified bone. This lighter framework contributes to their buoyancy and maneuverability. Sharks do not possess a gas-filled swim bladder for buoyancy control, instead relying on a large, oil-rich liver and the low density of their cartilage.
The skin of a shark is covered in specialized structures called placoid scales, also known as dermal denticles. These tiny, tooth-like structures reduce drag as the shark moves through the water, giving the skin a rough, sandpaper-like texture. Furthermore, the respiratory system is characterized by a series of five to seven exposed gill slits located on the side of the head.
These gill slits lack a protective bony covering, meaning that many sharks must continuously swim to force water over their gills in a process known as ram ventilation. Their jaw structure is also unique, with the upper jaw not fused to the cranium, allowing for a wide gape and powerful bite.
Key Traits of Bony Fish (Catfish)
Catfish belong to the bony fish group, meaning their internal skeleton is made of hard, calcified bone tissue. This bony structure provides a rigid frame for muscle attachment and protection for internal organs. Unlike a shark’s exposed gill slits, a catfish has its gills protected by a bony plate called an operculum. The operculum allows the fish to actively pump water over its gills without needing to swim constantly.
The fins of a catfish, like those of most bony fish, are supported by flexible, radiating bony spines, a feature that gives the group its name: ray-finned fish. Catfish are notable for their smooth, scaleless skin, which is a specific adaptation within the bony fish group. While most other bony fish have overlapping bony scales, catfish have entirely lost this feature.
A defining characteristic of the catfish is the presence of barbels, which are whisker-like sensory organs located around the mouth. These barbels are densely packed with chemoreceptors, allowing the catfish to effectively “taste” the water and locate food sources in dark or murky environments. This combination of a bony skeleton, an operculum, and specialized sensory barbels places the catfish firmly in the most diverse class of all living vertebrates.