Fish are classified into three major living groups based on their skeletal makeup and jaw structure: jawless fish, cartilaginous fish, and bony fish. That three-part framework has held up remarkably well since the early days of comparative anatomy, but the details inside each group have been scrambled repeatedly as molecular tools have reshaped our understanding of how fish species relate to one another. The word “fish” itself is not even a proper evolutionary grouping, since the lobe-finned fish that gave rise to all land vertebrates are technically closer relatives of humans than they are of, say, a tuna.
The Three Living Pillars
The broadest cut divides living fish into jawless species (lampreys and hagfish), cartilaginous species (sharks, rays, skates, and chimaeras), and bony species (everything else, from goldfish to seahorses to lungfish). Jawless fish are the most ancient branch. Cartilaginous fish have jaws but build their skeletons from cartilage rather than bone. Bony fish have mineralized skeletons and make up the overwhelming majority of fish diversity.
Within bony fish, a further split separates ray-finned fish from lobe-finned fish. Ray-finned fish account for roughly half of all living vertebrate species. Lobe-finned fish, by contrast, include only a handful of living species: two species of coelacanth and a few species of lungfish. Despite their small numbers, lobe-finned fish occupy one of the most important positions in vertebrate evolution, sitting on the branch that eventually led to amphibians, reptiles, birds, and mammals.
Jawless Fish and the Deepest Roots
Lampreys and hagfish are the only surviving jawless vertebrates, and together they form a group called the cyclostomes. Despite looking quite different from each other on the outside, developmental studies have shown that hagfish and lamprey embryos pass through a stage where their craniofacial structures closely resemble one another, hinting at a shared ancestral body plan that later diverges dramatically between the two lineages.1PubMed. Evo-devo studies of cyclostomes and the origin and evolution of jawed vertebrates
The fossil record once suggested that an entire parade of armored jawless fish, collectively called “ostracoderms,” were ancestors of modern lampreys and hagfish. Current phylogenetic work tells a different story. Most ostracoderms now appear to be jawless relatives of jawed vertebrates rather than relatives of cyclostomes. They had extensively bony external skeletons, often covered with tissue types also seen in jawed fish, and some groups even had a partially calcified internal skeleton. They lacked vertically biting jaws, but in many other respects they were already on the path toward the jawed condition.2Zoological Science. Early Jawless Vertebrates and Cyclostome Origins This reclassification matters because it means the evolutionary history of those fossil jawless fish has little to tell us about where lampreys and hagfish came from.
Cartilaginous Fish
Sharks, rays, skates, and chimaeras share a skeleton made entirely of cartilage. This group, the Chondrichthyes, numbers around 970 living species and splits into two subgroups: the elasmobranchs (sharks, skates, and rays) and the holocephalans (chimaeras).3Molecular Biology and Evolution. Evolutionary Origin and Phylogeny of the Modern Holocephalans (Chondrichthyes: Chimaeriformes): A Mitogenomic Perspective Chimaeras are sometimes called “ghost sharks” or “ratfish” and live mostly in deep water, so they are far less familiar to most people than their elasmobranch cousins.
Even within the elasmobranchs, the differences between sharks and rays run deeper than body shape. CT scans of cartilaginous skeletons have revealed that certain structures of the hyoid arch (the set of cartilages that support the jaw and gills) are clearly visible in sharks but reduced or absent in batoids, the group that includes rays and skates. The pelvic girdle also differs in shape between the two groups, reinforcing the idea that sharks and rays diverged from each other a long time ago.4PubMed Central. Segmentations of the cartilaginous skeletons of chondrichthyan fishes by the use of state-of-the-art computed tomography
The Rise of Jaws and the Earliest Jawed Fish
The evolution of jaws was one of the most consequential innovations in vertebrate history, and the earliest jawed fish were a now-extinct group called placoderms. Placoderms were armored fish that dominated oceans and freshwaters during the Devonian period, roughly 420 to 360 million years ago. Their fossils reveal a surprising range of feeding strategies. One particularly striking example is Alienacanthus malkowskii, an arthrodire placoderm whose lower jaw was twice as long as its skull, representing the oldest known case of extreme jaw elongation in any vertebrate.5PubMed Central. Extreme lower jaw elongation in a placoderm reflects high disparity and modularity in early vertebrate evolution
Another extinct group, the acanthodians (often called “spiny sharks” though they are not true sharks), have been the subject of long-running debate about where they belong on the vertebrate tree. Cladistic analyses have presented arguments for placing acanthodians as relatives of the bony fish rather than the cartilaginous fish, which reshuffles assumptions about what the common ancestor of jawed vertebrates looked like.6PubMed. Heads and Tails: A Chordate Phylogeny
Ray-Finned Fish and the Teleost Explosion
Ray-finned fish, the Actinopterygii, are the largest group of vertebrates on the planet. Within this group, the teleosts (a massive subgroup that includes perch, salmon, catfish, eels, pufferfish, and thousands of others) are especially diverse. One reason often cited for their success is a whole-genome duplication event that occurred early in teleost history. This duplication effectively doubled the number of genes available for evolution to act on. Recent fossil evidence suggests the duplication happened well before the teleost body plan diversified, pointing to an indirect rather than immediate link between having extra genetic material and generating new species.7PubMed Central. Fossilized cell structures identify an ancient origin for the teleost whole-genome duplication
That genome duplication also appears to have enabled some unique physiological innovations. A distinctive feature of teleost blood is the Root effect, in which hemoglobin dramatically loses its ability to carry oxygen when blood becomes more acidic. Rather than being a liability, this quirk allows teleosts to force-inject oxygen into the swim bladder for buoyancy control and to supply oxygen to the eye’s retina, which in many teleosts lacks blood vessels entirely.8PubMed Central. Whole-Genome Duplication and the Functional Diversification of Teleost Fish Hemoglobins
Not all ray-finned fish are teleosts. A handful of “living fossil” lineages sit outside the teleost radiation, including gars, sturgeons, paddlefish, and bichirs. These fish have retained ancestral features that teleosts lost long ago. Gars, for instance, have exons (protein-coding DNA segments) that evolve between half and a thousand times more slowly than those of any other major vertebrate group. Only sturgeons, turtles, and crocodilians come close to matching that pace of near-standstill molecular change.9Evolution. The genomic signatures of evolutionary stasis That extreme genetic conservatism is why gars look essentially unchanged over tens of millions of years. Their dermal armor, made of thick bony scales, traces its developmental origin to trunk neural crest cells, the same embryonic cell population that built the external armor of the earliest vertebrates hundreds of millions of years ago.10PubMed Central. Ancient vertebrate dermal armor evolved from trunk neural crest
Lobe-Finned Fish and the Path to Land
Coelacanths were once thought to be the closest living fish relatives of land vertebrates, but genomic analysis has settled that debate in favor of lungfish. A phylogenomic study of the coelacanth genome confirmed that lungfish, not coelacanths, hold the title of closest living fish relative to tetrapods (the group that includes amphibians, reptiles, birds, and mammals).11PubMed Central. The African coelacanth genome provides insights into tetrapod evolution The coelacanth is still valuable for understanding the fin-to-limb transition, though, because its protein-coding genes evolve exceptionally slowly, preserving a snapshot of the genomic toolkit that the ancestors of all land vertebrates once carried.
How Molecular Tools Have Rewritten the Tree
Traditional fish classification relied on counting fin rays, measuring body proportions, and comparing bone shapes. Starting in the late 1960s, researchers began using formal methods to group species by shared evolutionary features rather than mere similarity, but many relationships remained stubbornly unresolved, especially among the percomorph fishes, a hyperdiverse group that includes everything from tunas to flatfish to seahorses. Molecular phylogenetics changed the picture dramatically, resulting in major rearrangements of the ray-finned fish family tree.12Bulletin of the Peabody Museum of Natural History. Phylogenetic Classification of Living and Fossil Ray-Finned Fishes (Actinopterygii)
One of the most practical applications of molecular tools is DNA barcoding, which uses a short standardized gene sequence to identify species. This approach has been especially powerful for uncovering “cryptic species,” fish that look nearly identical to the naked eye but turn out to be genetically distinct. In a study of flathead fishes across the Indo-West Pacific, barcoding revealed 65 distinct genetic units among only 48 recognized species. One species alone, Platycephalus indicus, contained eight separate genetic lineages differing by as much as 16% in their barcode sequences.13PubMed. Cryptic diversity in flathead fishes (Scorpaeniformes: Platycephalidae) across the Indo-West Pacific uncovered by DNA barcoding Similar discoveries have turned up in freshwater systems: barcoding of loach species on the Qinghai-Tibet Plateau uncovered two cryptic species that had gone unrecognized among 24 native loaches.14PubMed Central. DNA barcoding reveals cryptic diversity in the underestimated genus Triplophysa (Cypriniformes: Cobitidae, Nemacheilinae) from the northeastern Qinghai-Tibet Plateau In a neotropical river basin, combining morphological measurements with barcoding identified both cryptic species and species that had simply been overlooked by traditional surveys.15PubMed. Integrative taxonomy detects cryptic and overlooked fish species in a neotropical river basin
When Classification Gets Messy
Taxonomy assumes that species are distinct units you can sort into a tree. Hybridization throws a wrench into that assumption. In a disturbed stream ecosystem in Mexico, genomic analysis found 17 early-generation hybrids among three deeply diverged swordtail species (Xiphophorus variatus, X. birchmanni, and X. malinche), with individual fish carrying genomic contributions from all three parent species.16Evolution. Complex hybridization between deeply diverged fish species in a disturbed ecosystem Three-way hybridization between species that diverged millions of years ago is not supposed to happen easily, but habitat disturbance brought them into contact and broke down the reproductive barriers.
In western North America, the problem is even more systemic. Geological forces have fragmented and reconnected rivers over millennia, naturally shuffling fish populations and promoting interbreeding among endemic species. Layer on modern dams, water diversions, and deliberate introductions of non-native fish, and the result is a tangle of historic and recent hybridization that makes it genuinely difficult to draw clean species boundaries for conservation purposes.17PubMed Central. Reticulate evolution as a management challenge: Patterns of admixture with phylogenetic distance in endemic fishes of western North America
Ecological Classification as an Alternative Lens
Evolutionary relationships are not the only way to organize fish diversity. Ecologists often sort fish into functional groups based on what they eat, where they live, or how they move. A study of demersal fish in the Southern Ocean, for example, identified five distinct feeding guilds (krill feeders, benthos feeders, mixed krill-and-amphipod feeders, fish feeders, and benthic shrimp feeders), each distinguished by body shape traits like mouth size and head proportions.18ICES Journal of Marine Science. Morphological traits distinguish feeding guilds in a Southern Ocean demersal fish community In Indian mangrove mudflats, researchers found that body measurements could predict feeding group membership with roughly 78% accuracy when fish were sorted into three broad guilds.19PubMed. Patterns of morphological traits shaping the feeding guilds in the intertidal mudflat fishes of the Indian Sundarbans
Migration behavior offers another functional classification. Fish that travel between salt and fresh water are called diadromous. Within that category, anadromous species (like salmon) breed in fresh water but feed in the ocean, while catadromous species (like many eels) do the reverse. The geographic pattern of these strategies maps neatly onto global productivity: anadromous species dominate at temperate latitudes where oceans are more productive than freshwater systems, while catadromous species are more common in the tropics where freshwater habitats tend to be richer in food than the adjacent ocean.20PubMed. Aquatic productivity and the evolution of diadromous fish migration
Convergent Evolution and the Limits of Appearance
One reason classification by appearance can be misleading is convergent evolution, where unrelated fish independently evolve similar traits in response to similar pressures. Electric organs provide a vivid case. The ability to generate electric fields has evolved independently at least six times in fish, used for communication, navigation, hunting, or defense. Despite millions of years of separate evolution and very different organ shapes, the independently derived electric organs rely on similar developmental pathways and transcription factors.21PubMed Central. Genomic basis for the convergent evolution of electric organs The convergence runs all the way down to the molecular level: in both South American electric knifefish and African electric fish, the same sodium channel gene that normally drives muscle contraction was independently co-opted for exclusive expression in the electric organ, with matching amino acid changes in the regions that control how the channel shuts off.22PubMed Central. Sodium channel genes and the evolution of diversity in communication signals of electric fishes: convergent molecular evolution Even the mitochondrial energy-production genes in these two groups show strikingly parallel evolutionary changes.23PubMed Central. Convergent patterns of evolution of mitochondrial oxidative phosphorylation (OXPHOS) genes in electric fishes
Amphibious behavior tells a similar story of repeated, independent evolution. At least 33 fish families include species that routinely leave the water, representing what appear to be independent evolutionary origins of land emergence. Within a single family, the blennies, a highly amphibious lifestyle may have evolved as many as seven separate times.24PubMed. Repeated evolution of amphibious behavior in fish and its implications for the colonization of novel environments Some amphibious fish, including the basal ray-finned bichir Polypterus senegalus, even use a contralateral stepping gait on land that resembles how four-legged animals walk, an ability that has popped up independently across the fish tree of life.25Journal of Experimental Biology. Patterns and processes in amphibious fish: biomechanics and neural control of fish terrestrial locomotion
Extreme Habitats and What They Reveal
The deep sea has been colonized by teleost lineages multiple times, and genomic work is starting to uncover the genetic toolkit that makes life at extreme depth possible. Genome assemblies from 11 deep-sea fish species revealed a convergent amino acid change in the rtf1 gene across most species living below 3,000 meters. Lab experiments suggest this mutation can affect how efficiently genes are transcribed, which may be an advantage under the crushing pressures and near-freezing temperatures of the abyss. The same study challenged the long-held assumption that the cellular stabilizer TMAO rises in a simple linear relationship with depth.26PubMed. Evolution and genetic adaptation of fishes to the deep sea
Cave environments push adaptation in a different direction. Fish of the genus Sinocyclocheilus in Chinese caves have evolved exaggerated cranial features, including a duckbill-shaped head, a dorsal horn, and a pronounced hump. These are not random degenerations. Computational fluid dynamics simulations show that these structures amplify the pressure signals detected by the lateral-line system by as much as 430%, effectively turning the fish’s head into a more sensitive hydrodynamic antenna for navigating in total darkness.27Europe PMC. Morphological adaptations of cavefish support enhanced hydrodynamic perception for underwater environmental monitoring These extreme specializations are a reminder that body shape, while unreliable as a sole guide to evolutionary relationships, is often an exquisitely precise reflection of the environment a fish inhabits.