The classification of life, known as taxonomy, attempts to organize the immense diversity of organisms into hierarchical categories. When applied to the vast array of aquatic vertebrates commonly grouped as “fish,” this system encounters a challenge because the term is not a single, unified biological class like Mammalia or Aves. The creatures commonly referred to as fish are actually three distinct evolutionary lineages that diverged hundreds of millions of years ago, each possessing fundamentally different anatomical structures. Understanding the different classes of fish requires analyzing the unique characteristics that separate these three major groups of vertebrates.
The Taxonomic Challenge of Defining Fish
The common grouping of all aquatic, gill-breathing vertebrates as “fish” creates a taxonomic inconsistency because it fails to account for evolutionary history. All vertebrates belong to the Phylum Chordata and the Subphylum Vertebrata, sharing the basic body plan of a backbone and a notochord. However, the traditional concept of a single “fish class” is considered a paraphyletic group in modern biology, meaning it includes a common ancestor and some, but not all, of its descendants.
The term “fish” is paraphyletic because it excludes terrestrial vertebrates (tetrapods). Amphibians, reptiles, birds, and mammals are directly descended from a specific group of ancient lobe-finned fish. To be a truly complete, or monophyletic, group, the classification would have to include these tetrapods. Biologists thus divide aquatic vertebrates into multiple, distinct Classes to accurately reflect these deep evolutionary splits.
The Ancient Lineage: Jawless Fish
The earliest group of vertebrates still surviving today are the jawless fish, formally categorized in the Superclass Cyclostomata (or Infraphylum Agnatha). This lineage has changed little since its emergence over 500 million years ago, predating all jawed vertebrates. Their defining trait is the complete absence of a hinged jaw, which limits their feeding strategies. They also lack paired fins, relying instead on eel-like movements for locomotion.
The two extant groups of jawless fish are the lampreys and the hagfish. Lampreys use a round, sucking mouth lined with horny teeth to attach to other fish as parasites. Hagfish are deep-sea scavengers known for their ability to produce copious amounts of defensive slime when disturbed. They possess a cartilaginous skeleton, though it is more rudimentary than the complex skeletons of sharks.
The Flexible Skeletons: Cartilaginous Fish
The Class Chondrichthyes encompasses the cartilaginous fish, including all sharks, rays, skates, and chimaeras. This group is defined by its endoskeleton being composed entirely of cartilage, a tough, flexible tissue. Although they lack true bone, their skeletons are often strengthened by calcium salts, providing the necessary rigidity for powerful swimming. The largest fish in the world, the whale shark, is a member of this class.
A unique feature of Chondrichthyes is their skin, which is covered in tiny, tooth-like structures called placoid scales, or dermal denticles. These denticles reduce drag and turbulence, contributing to efficient swimming. Unlike bony fish, they lack a swim bladder for buoyancy, forcing them to continuously swim or rest on the seafloor. Many members exhibit specialized reproductive strategies, including internal fertilization via pelvic fins called claspers, resulting in fewer, well-developed offspring.
The Dominant Group: Bony Fish
The Superclass Osteichthyes, or bony fish, represents the largest and most ecologically diverse group of vertebrates on Earth, comprising over 95% of all fish species. Their success stems from a skeleton composed primarily of true bone, which is stronger and more resilient than cartilage. This structural innovation allowed for the evolution of a wide variety of body forms and specialized niches across nearly every aquatic habitat.
Bony fish are divided into two primary subclasses. The vast majority belong to the ray-finned fish (Actinopterygii), whose fins are supported by slender, bony rays connected directly to the skeleton. This group includes nearly all familiar fish, such as tuna and goldfish, and features an operculum, a bony plate that covers and protects the gills.
The smaller but evolutionarily significant subclass is the lobe-finned fish (Sarcopterygii). They possess fleshy, paired fins supported by internal, rod-shaped bones and muscles. This structure resembles the limbs of terrestrial vertebrates, confirming that this lineage, represented today by lungfish and coelacanths, gave rise to all land-dwelling vertebrates. Most bony fish also possess a gas-filled sac called a swim bladder, which allows them to precisely regulate buoyancy without expending constant energy.