Fish do not belong to a single order. The word “fish” covers an enormous range of animals spread across dozens of distinct orders within multiple classes, from the familiar perch and bass in Perciformes to the bizarre deep-sea anglerfish of Lophiiformes. How scientists sort all these creatures into orders has shifted dramatically in recent decades, and the classification you find today looks quite different from what appeared in textbooks a generation ago.
Why There Is No Single “Fish Order”
The confusion is understandable. We talk about fish as if they are one coherent group, but biologically they are far more diverse than mammals, birds, or reptiles. There are roughly 35,000 described species of living fish, and they occupy almost every aquatic habitat on Earth, from Antarctic ice shelves to desert springs and the deepest ocean trenches. These species are organized into classes (the broadest grouping below phylum), then into orders within those classes, then into families, genera, and species.
The three main classes of living fish are ray-finned fishes (Actinopterygii), cartilaginous fishes (Chondrichthyes, which includes sharks and rays), and lobe-finned fishes (Sarcopterygii, a tiny group that includes lungfish and coelacanths). Each class contains its own set of orders. Ray-finned fishes alone account for the vast majority of fish species and are divided into more than 40 recognized orders. Cartilaginous fishes have around a dozen orders. Lobe-finned fishes, by contrast, have just a handful of living representatives.
The Ray-Finned Fishes and Their Major Orders
If you have ever caught a fish, kept one in an aquarium, or eaten one for dinner, it was almost certainly a ray-finned fish. This group dominates freshwater and saltwater environments worldwide. A landmark molecular study assembled DNA data from over 1,400 bony fish taxa across all traditionally recognized orders, producing the most comprehensive family tree of fishes to date and redefining how many of these orders relate to one another.1PubMed Central. The tree of life and a new classification of bony fishes
A few of the most species-rich ray-finned orders give a sense of the range:
- Perciformes: Often called the perch-like fishes, this has historically been the largest vertebrate order, containing thousands of species including bass, cichlids, groupers, and tuna. It has also been the most problematic, because researchers long used it as a catch-all for species they could not place elsewhere. Molecular work has split many former members into their own orders.
- Cypriniformes: The minnows, carps, loaches, and suckers. This order dominates freshwater ecosystems around the world and is one of the most species-rich groups of fish overall.
- Siluriformes: The catfishes, found on every continent except Antarctica. They range from tiny parasitic species to enormous river-dwelling giants.
- Clupeiformes: The herrings, sardines, and anchovies. These small, schooling fish are hugely important both ecologically and commercially.
- Gadiformes: The cods and their relatives, including haddock and pollock. These cold-water fish support some of the world’s biggest fisheries.
These orders have distinct life-history strategies. A comparative study of Clupeiformes, Gadiformes, Perciformes, and Pleuronectiformes found that each order follows different patterns of growth, reproduction, and population compensation, with herring-like fishes (Clupeiformes) showing much less efficient recovery at low population sizes than the other groups.2ICES Journal of Marine Science. A comparative approach to life-history strategies and tactics among four orders of teleost fish That difference matters for fisheries management: not all fish bounce back from overfishing at the same rate, and the order they belong to is part of the reason.
What Makes Cypriniformes So Successful
Cypriniformes deserve a closer look because they illustrate how a single evolutionary innovation can drive an entire order’s success. Cypriniform fishes share a structure called the Weberian apparatus, a set of modified vertebral bones and ribs that relays sound pressure from the gas bladder to the inner ear. This gives them dramatically better hearing than most other fish. The Weberian apparatus is not unique to Cypriniformes; it unites them with catfishes and several other orders in a larger group called the Otophysi, which together make up the majority of freshwater fish species worldwide.3PubMed. Morphological variation in the Weberian apparatus of Cypriniformes
Cypriniforms also have unusual feeding structures. Their pharyngeal jaws, located in the throat, are enlarged and press against a bony pad on the skull, letting them crush hard food items. They also have a palatal organ on the roof of the mouth that helps process detrital food, and a unique bone called the kinethmoid that allows their upper jaw to protrude in a distinctive way.4PubMed. Using zebrafish to investigate cypriniform evolutionary novelties: functional development and evolutionary diversification of the kinethmoid If you have ever watched a carp vacuum food off the bottom of a pond, you were seeing these adaptations in action.
Flatfishes and the Puzzle of Pleuronectiformes
Flatfishes, the group that includes flounder, sole, halibut, and turbot, have one of the most striking body plans in the animal kingdom. As larvae, they swim upright like any other fish. During metamorphosis, one eye migrates to the other side of the head, the body flattens, and the fish settles onto the seafloor to live the rest of its life lying on one side. This arrangement was long considered so bizarre that Charles Darwin himself called it a challenge for the theory of gradual evolution. How could you get from a normal fish to one with both eyes on the same side through small steps?
The answer came from fossils. Two extinct genera from the Eocene, about 50 million years ago, turned out to be the most primitive flatfishes known. These ancient fish had strongly asymmetrical skulls, like modern flatfishes, but their eye migration was incomplete: they still had eyes on opposite sides of the head even as adults, representing a genuine intermediate stage between normal fish and today’s fully asymmetrical species.5PubMed. The evolutionary origin of flatfish asymmetry The discovery confirmed that the dramatic cranial rearrangement of flatfishes evolved gradually rather than in a single leap.
Even the boundaries of this order have proven unstable. Molecular analyses have found that Pleuronectiformes, as traditionally defined, may not be a true natural group. The genus Psettodes, an odd flatfish from the Indo-Pacific and West Africa that retains several primitive features, consistently falls outside the rest of the flatfishes in phylogenetic trees. The remaining flatfishes (Pleuronectoidei) do form a coherent group, estimated to have originated roughly 73 million years ago.6PubMed Central. Are flatfishes (Pleuronectiformes) monophyletic? This is a recurring theme in fish classification: groups that looked unified based on shared body plans sometimes fracture when DNA enters the picture.
Seahorses, Pipefish, and Syngnathiformes
The order Syngnathiformes contains some of the most visually distinctive fish alive. Seahorses, pipefish, pipehorses, and seadragons all belong to the family Syngnathidae within this order, and they share a set of traits you would not expect in a fish: fused jaws, elaborate camouflage, and male pregnancy. About 300 described species make up the family, mostly inhabiting shallow coastal waters in temperate and tropical oceans.7PubMed. Molecular phylogeny and patterns of diversification in syngnathid fishes
Male brooding is the defining feature. During mating, females transfer their eggs to specialized structures on the male’s body. In some species, this is just a simple patch of sticky skin on the belly. In others, like seahorses, it is a fully enclosed pouch where the male provides oxygen, nutrients, and salinity regulation to the developing embryos, functioning somewhat like a uterus. A molecular phylogeny of the family showed that this diversity in brooding structures evolved rapidly alongside the group’s major evolutionary radiation, suggesting that innovations in paternal care drove their diversification.8Journal of Heredity. Male Pregnancy in Seahorses and Pipefishes (Family Syngnathidae): Rapid Diversification of Paternal Brood Pouch Morphology Inferred From a Molecular Phylogeny
Mitochondrial genome analysis places the divergence of syngnathids from other bony fishes at roughly 49 million years ago, during the Eocene.9PubMed Central. Complete mitochondrial genomes of eight seahorses and pipefishes (Syngnathiformes: Syngnathidae): insight into the adaptive radiation of syngnathid fishes Within the family, the fundamental split is between species where males brood on the tail (“Urophori”) and those where males brood on the trunk (“Gastrophori”), a division that researchers have proposed should reorganize the family into just two subfamilies.
Sharks, Rays, and Cartilaginous Fish Orders
Cartilaginous fishes split from the bony fish lineage hundreds of millions of years ago and have their own entirely separate set of orders. The two main groups within cartilaginous fishes are sharks (Selachimorpha) and rays and skates (Batoidea), plus a small group of chimaeras (Holocephali).
Sharks are divided into two major clades. The Galeomorphii includes the orders most people picture when they think of sharks: Carcharhiniformes (the ground sharks, including hammerheads and reef sharks), Lamniformes (the mackerel sharks, including great whites and makos), Orectolobiformes (carpet sharks, including whale sharks), and Heterodontiformes (bullhead sharks). The Squalimorphii includes the deep-water and more obscure sharks: Squaliformes (dogfish sharks), Hexanchiformes (sixgill and sevengill sharks), Squatiniformes (angel sharks), and Pristiophoriformes (sawsharks). A species-level molecular phylogeny supports this two-group division and confirms that each order is broadly natural, with only minor placement issues for a few species.10PubMed. Shark tales: a molecular species-level phylogeny of sharks (Selachimorpha, Chondrichthyes)
An older hypothesis proposed that rays were actually nested within sharks rather than being a separate group. The molecular evidence has rejected this idea, keeping rays as a distinct lineage. Within the rays and skates, orders include Rajiformes (skates), Myliobatiformes (eagle rays, manta rays, and stingrays), Torpediniformes (electric rays), and several others. Molecular and chromosomal studies have grouped Rajiformes, Myliobatiformes, and the guitarfishes together while finding that electric rays diverge early from the rest of the group.11PubMed. Molecular and karyological aspects of Batoidea (Chondrichthyes, Elasmobranchi) phylogeny Even anatomical details like the branching patterns of arteries in the gills differ among ray orders in ways that track evolutionary relationships.12Scientific Reports. Branching patterns of the afferent branchial arteries and their phylogenetic significance in rays (Batoidea)
Lobe-Finned Fishes and Their Place on the Tree
The lobe-finned fishes (Sarcopterygii) are a tiny group in terms of living species, just a handful of lungfish species and two species of coelacanth, but they hold an outsized place in evolutionary biology. Land-dwelling vertebrates, from frogs to humans, descended from lobe-finned fish ancestors, which means that technically, all tetrapods are nested within this class. When biologists talk about “fish orders” in this group, they mean orders like Ceratodontiformes (the living lungfishes) and Coelacanthiformes (the coelacanths).
A long-running debate has been whether lungfishes or coelacanths are more closely related to land vertebrates. Most molecular studies now favor lungfishes as the closer relative of tetrapods.13PubMed Central. Support for Lungfish as the Closest Relative of Tetrapods by Using Slowly Evolving Ray-Finned Fish as the Outgroup But the picture is not perfectly clean. An earlier study using 28S ribosomal RNA found that lungfishes and coelacanths form their own group and are equally closely related to land vertebrates, which would complicate the story of which specific traits helped the common ancestor colonize land.14PubMed. Evolutionary relationships of the coelacanth, lungfishes, and tetrapods based on the 28S ribosomal RNA gene A large-scale analysis of 44 nuclear genes found that different statistical methods gave different answers, with the data nearly equally supporting several possible tree arrangements, suggesting the three lineages split apart in rapid succession.15Molecular Biology and Evolution. The Phylogenetic Relationship of Tetrapod, Coelacanth, and Lungfish Revealed by the Sequences of Forty-Four Nuclear Genes The current consensus leans toward lungfishes as the closer relative, but the split was rapid enough that the genetic signal is weak.
Deep-Sea and Anglerfish Orders
The deep ocean hosts some of the strangest fish orders. Lophiiformes, the anglerfish order, includes species with bioluminescent lures dangling from their heads, males that permanently fuse to females as tiny parasites, and body forms so unusual that males and females of the same species were sometimes described as different genera before researchers figured out the connection. New species in this order continue to turn up regularly; a recently described deep-sea anglerfish from the Andaman Sea belongs to the genus Himantolophus and was distinguished from its relatives by unique features of its lure structure.16PubMed. A new species of deep-sea anglerfish, genus Himantolophus (Lophiiformes: Himantolophidae) from the Andaman Sea, India
Other notable deep-sea orders include Stomiiformes (dragonfish and hatchetfish, many with photophores running along their bodies), Myctophiformes (lanternfish, among the most abundant vertebrates on Earth by sheer number), and Anguilliformes (eels, which range from shallow reefs to abyssal depths). The classification of deep-sea fish has always been particularly difficult because specimens are rare, often damaged during collection, and may look radically different at different life stages.
How Classification Has Changed
Fish classification was originally based entirely on physical appearance. Researchers grouped species by shared body shapes, fin structures, scale types, and skeletal features. Starting in the late 1960s, scientists began organizing these groupings based on evolutionary relationships inferred from anatomy, but many branches of the fish family tree remained stubbornly unresolved, especially within the enormous radiation of spiny-rayed fishes called Percomorpha.17Bulletin of the Peabody Museum of Natural History. Phylogenetic Classification of Living and Fossil Ray-Finned Fishes (Actinopterygii)
The arrival of DNA-based methods changed everything. Molecular phylogenetics caused a dramatic reconfiguration of the ray-finned fish family tree. Old groupings were broken apart, new ones were assembled, and the relationships among major lineages were reshuffled. The old Perciformes, for instance, turned out to be a grab-bag of distantly related lineages that happened to look similar. Entire orders were created or dissolved as the genetic data accumulated. The comprehensive molecular phylogeny covering all major bony fish lineages has since become the backbone of modern fish classification.1PubMed Central. The tree of life and a new classification of bony fishes
This process is still ongoing. Fish classification is not settled. New genetic data regularly refines the tree, and researchers do not always agree on where to draw lines between orders. The flatfish case, where one genus keeps falling outside its supposed order, is one example. Another is the debate over whether certain traditional orders should be split or merged based on molecular evidence.
Extinct Fish Orders and the Fossil Record
Living fish represent only a fraction of the orders that have existed. The fossil record contains several entirely extinct classes and orders. Placoderms, armored jawed fish that dominated the Devonian period roughly 360 to 420 million years ago, are the most famous example. For decades, researchers debated whether placoderms formed a single natural group or a loose assembly of early jawed vertebrates scattered along the base of the evolutionary tree. Recent analyses using tip-dated clock methods, which account for rates of evolutionary change over time, have found strong support for placoderm monophyly, meaning they genuinely belong together as a group rather than being a scattered set of primitive lineages.18Oxford Academic. Bayesian Morphological Clock Methods Resurrect Placoderm Monophyly and Reveal Rapid Early Evolution in Jawed Vertebrates
Identifying fossil fish to order, or even to species, can rely on surprisingly specific anatomical details. In catfishes (Siluriformes), researchers have defined morphological features on pectoral and dorsal spines that can distinguish individual species, and these same features have been used to identify fossil catfish specimens from sites in northern Chad dating to the Late Miocene, roughly 7 million years ago.19SpringerLink / Naturwissenschaften. Spine anatomy reveals the diversity of catfish through time: a case study of Synodontis (Siluriformes) Spines and teeth often survive fossilization better than the rest of the skeleton, so ichthyologists have become adept at squeezing taxonomic information out of fragmentary remains.
When Species Do Not Fit Neatly Into Orders
Hybridization is an underappreciated complication in fish classification. Fish hybridize far more readily than most other vertebrates, both in the wild and in aquaculture settings. The offspring frequently have intermediate features that blur the morphological boundaries between species, and sometimes between genera or even families. This creates headaches for taxonomists trying to assign individuals to species or trace evolutionary relationships. Hybridization can also reveal cryptic species, populations that look similar but are genetically distinct, because hybrid zones sometimes form where two such hidden species meet. On the flip side, hybridization occasionally leads to taxonomic inflation, where hybrid populations are mistakenly described as new species.20FishTaxa – Journal of Fish Taxonomy. Exploring the Impact of Hybridization on Fish Taxonomy and Conservation Strategies
This matters beyond academic bookkeeping. Conservation laws and fisheries regulations often hinge on species designations. If a population is classified as a distinct species, it may qualify for legal protection. If it is reclassified as a hybrid swarm or lumped into a broader species, that protection can evaporate. The question “what order is this fish in?” can sound like a purely academic exercise, but the answer sometimes determines whether a population gets managed, protected, or ignored.