The term “fish” is often used broadly in everyday language to describe nearly any swimming creature in the water, but this common definition falls short of the precision required in biology. Scientists studying evolutionary relationships grapple with the limitations of the term because the creatures we call fish do not form a single, unified evolutionary group. Understanding the biological answer requires moving beyond simple appearance to examine shared anatomical and ancestral traits. Modern classification methods reveal that the group traditionally known as “fish” is fundamentally flawed from a phylogenetic perspective.
Defining the Biological Criteria of a True Fish
A true fish is defined by a specific combination of physical characteristics that place it within the subphylum Vertebrata. All true fish must possess a backbone, or vertebral column, which is the defining feature of vertebrates. This internal skeleton provides the structural support necessary for complex movement through the water.
These aquatic animals must respire using gills throughout their entire lives to extract oxygen from the water. Unlike land vertebrates that use lungs, this gill-based respiration is a prerequisite for classification as a fish. A fish’s appendages must be paired fins, used for balance, propulsion, and steering, rather than limbs with digits.
Most true fish also have a body covered in scales, which are thin, protective plates that overlap in rows, although some species have lost this trait. While many are cold-blooded, or ectothermic, some active swimmers like tuna and certain sharks can internally maintain a warmer core temperature. The presence of a vertebral column, gills, and fins remains the fundamental biological standard.
Aquatic Animals That Are Not Biologically Fish
Many aquatic creatures that swim and live in the ocean or freshwater environments are often mistakenly labeled as fish, sometimes even carrying the suffix “fish” in their common names. These animals fail to meet the biological criteria of a true fish, usually because they lack the defining vertebrate trait of a backbone. Animals like jellyfish and starfish are invertebrates and belong to entirely different phyla, Cnidaria and Echinodermata, respectively.
Other common “non-fish” examples are marine mammals, such as whales and dolphins, which possess vertebrae but lack gills and fins in the biological sense. They are warm-blooded, breathe air using lungs, and give birth to live young, placing them firmly within the Class Mammalia. Cuttlefish and silverfish also represent misnomers, as the cuttlefish is a mollusk and the silverfish is a type of insect, neither of which are vertebrates.
The Scientific Problem of Paraphyly
In modern evolutionary biology, the traditional grouping of all fish into a single class is considered scientifically invalid because it is a paraphyletic group. A group is defined as paraphyletic when it includes a common ancestor and some, but not all, of that ancestor’s descendants. The problem with the group “fish” is that it excludes a major lineage that evolved directly from a fish ancestor: the tetrapods, which include all amphibians, reptiles, birds, and mammals.
Tetrapods evolved from a specific group of ancient lobe-finned fish (Sarcopterygii). Evolutionarily, a human is more closely related to a lobe-finned fish than that lobe-finned fish is to a shark. The traditional grouping of “fish” carves out the aquatic vertebrates and deliberately leaves out their land-dwelling descendants, which violates the principles of cladistics.
To form a natural, valid evolutionary group, or clade, the classification must include the common ancestor and all of its subsequent descendants. The term “fish” therefore describes an evolutionary “grade”—a collection of organisms sharing a similar level of biological organization—rather than a true branch on the tree of life.
The Three Major Evolutionary Lineages of Aquatic Vertebrates
To address the paraphyly issue, biologists classify the animals commonly called fish into three main evolutionary lineages that accurately reflect their shared ancestry and distinct features.
Jawless Fish (Agnatha)
The most ancient living group is the jawless fish, or Agnatha, which includes hagfish and lampreys. These organisms lack hinged jaws and possess skeletons that are primarily cartilaginous. They represent the earliest divergence in the vertebrate family tree, appearing over 500 million years ago.
Cartilaginous Fish (Chondrichthyes)
The second major lineage is the cartilaginous fish, or Chondrichthyes, which encompasses sharks, rays, and chimaeras. Their defining characteristic is a skeleton made entirely of cartilage, rather than bone. This group evolved from early jawed vertebrates.
Bony Fish (Osteichthyes)
The largest and most diverse group is the bony fish, or Osteichthyes, which accounts for approximately 96% of all living fish species. This lineage is characterized by a skeleton composed of true bone and is further divided into the ray-finned fish (Actinopterygii) and the lobe-finned fish (Sarcopterygii). The lobe-finned fish are particularly significant because they are the direct ancestors of all tetrapods.