The term “fish” is commonly used to describe any cold-blooded, aquatic animal with fins, gills, and a backbone. However, this simple definition masks a complex scientific reality. Biologically, the group we call “fish” is not a single, unified taxonomic unit like mammals or birds. These animals represent several distinct and ancient evolutionary branches, requiring a look at biological classification to trace their separate lineages.
Placement within the Animal Kingdom
All animals commonly referred to as fish belong to the large Phylum Chordata, which groups organisms that possess a notochord, a flexible rod-like structure, at some stage of development. Chordates also feature a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. This phylum encompasses a vast array of life, including both invertebrates and all vertebrates.
Within the Chordata, fish are placed into the Subphylum Vertebrata, which includes all animals with a skull and a vertebral column, or backbone. The notochord present in earlier development is replaced or supplemented by this segmented bony or cartilaginous structure in vertebrates. This foundational grouping establishes the shared ancestry and basic body plan for fishes, amphibians, reptiles, birds, and mammals. Vertebrates arose about 500 million years ago, with the first fish-like creatures marking this evolutionary advancement.
The Three Distinct Evolutionary Lineages
The animals we recognize as fish are separated into three major, distinct lineages that diverged early in vertebrate evolution. These three groups are the jawless fish (Superclass Agnatha), the cartilaginous fish (Class Chondrichthyes), and the bony fish (Superclass Osteichthyes). The development of the hinged jaw was a significant evolutionary event, separating the jawless forms from the other two groups.
Jawless fish, such as lampreys and hagfish, represent the most ancient surviving lineage of vertebrates, lacking the hinged jaw that characterizes all other fish. The other two lineages are grouped together as jawed vertebrates, or Gnathostomata, a classification that includes all subsequent vertebrate life. This split occurred in the Silurian period, over 400 million years ago, demonstrating the deep evolutionary division among “fishes.” The bony fish later split into the ray-finned fish and the lobe-finned fish, the latter of which gave rise to all land-dwelling vertebrates.
Key Features Differentiating the Groups
The jawless fish, or Agnatha, are characterized by a lack of paired fins and true jaws. Instead, they possess a round, sucking mouth often equipped with rasping teeth, as seen in lampreys. Their skeleton is entirely cartilaginous, and they retain a persistent notochord throughout their lives.
The cartilaginous fish, Chondrichthyes, which include sharks, rays, and chimaeras, possess skeletons made entirely of cartilage, though it may be hardened by mineral deposits. They are jawed predators that have specialized, tooth-like placoid scales embedded in their skin. They also lack an operculum, the bony flap that covers the gills in other fish, instead having five to seven exposed gill slits.
Bony fish, or Osteichthyes, are the most diverse of the three groups, encompassing over 32,000 species, including salmon, tuna, and goldfish. Their primary distinguishing feature is a skeleton composed of hardened bone tissue. Unlike cartilaginous fish, bony fish have a swim bladder, an internal gas-filled organ that helps regulate buoyancy in the water column. They also possess a hard, protective bony plate called the operculum that covers and protects their gills.
Why “Fish” is Not a Single Scientific Class
The three groups of fish are so different that the term “fish” is not considered a valid Class in modern scientific classification, or taxonomy. In the system of grouping organisms by common ancestry, known as cladistics, a true group must include a common ancestor and all of its descendants. The traditional grouping of “fish” fails this test because it excludes a significant descendant group: the tetrapods, which are all four-limbed vertebrates like amphibians, reptiles, birds, and mammals.
Bony fish, specifically the lobe-finned fish, share a more recent common ancestor with all land-dwelling vertebrates than they do with cartilaginous fish or jawless fish. Therefore, the term “fish” is classified as a paraphyletic group. This means it describes an evolutionary grade rather than a single, complete branch of the tree of life.