Fish Classification

The classification of fishes has undergone much change over the last few decades, and further changes are expected, partly because so many groups are poorly known. There are many conflicting hypotheses of relationships, some based on conflicting evidence between morphological studies and molecular studies; however, progress is being made and there are many areas of basic agreement. There are about 28 000 living species of fishes recognized as valid (formally described and recognized). This is slightly over half the number of recognized tetrapods. About 11 950 of these species are confined to fresh water. Many new species are described every year, and if we use the same species concept as the above figure reflects, then there may be an estimated 32 500 species alive today (some estimated 4500 are yet to be found and described).

Currently, ichthyologists place living species in about 4500 genera, 515 families and 62 orders. The following summary of the groups is based primarily on a synthesis of the recent literature. Numerous fossil groups of Chondrichthyes, Actinopterygii and Sarcopterygii, many occurring in Canada, are not covered below. About 1200 species of native fishes live in Canada, either in fresh water or marine water over the continental shelf (many more live within Canadian territorial limits in deep water in the Arctic, Atlantic and Pacific). Unless otherwise mentioned, Arctic fishes are combined into Atlantic and Pacific.

Methods and Differences Between Classifications

Most ichthyologists adopt a cladistic methodology of classification, which classifies strictly according to recency of common descent. In this system, the goal is to have each taxonomic unit contain all species sharing a common ancestor. This often results in a very different classification than would result from the approach commonly used up to about 1980. In the older approach, evolutionary or synthetic systematics, the inferred amount of divergence among groups was considered, as was the history of their evolutionary relationships, and a given taxon might not contain all the descendants from the inferred ancestral species.

An example of the difference between these 2 approaches comes from showing hagfishes and lampreys in a classification. In a cladistic system, hagfishes would be placed in a category separate from lampreys and other vertebrates because, relative to hagfishes, lampreys are thought by most ichthyologists using morphological evidence to share a common genealogical branching point with all other vertebrates. Systematists using the evolutionary approach could express the relationship in several ways but might prefer to place hagfishes and lampreys together in the category of jawless fishes, separate from all jawed vertebrates, although recognizing that they are very distantly related.

Differences may also exist in classifications because of ichthyologists using different characters, for example, molecular versus morphological characters. Other differences may also exist among classification systems, depending on whether the worker is a "lumper" or a "splitter." A lumper constructs a classification with categories containing more groups than that of a splitter. There are no rules for the amount of difference necessary to designate various categories. A lumper recognizes one family for salmon, whitefish and grayling; a splitter, 2 or 3 families.

Classification

The following summarizes the major groups of fishes. Fishes and the tetrapods belong to the subphylum Craniata.

• Superclass Myxinomorphi
  
  • Class Myxini

The remaining fishes and the tetrapods belong to the infraphylum Vertebrata.

• Superclass Petromyzontomorphi
  
  • Class Petromyzontida
• Superclass Gnathostomata - Jawed Vertebrates
  
  • Class Placodermi (extinct)
  • Class Chondrichthyes
  • Class Acanthodii (extinct)
  • Class Actinopterygii
  • Class Sarcopterygii

Hagfishes

Class Myxini
One order Myxiniformes, with one family, Myxinidae, and 70 species, 3 in Canada (one in Atlantic and 2 in Pacific). Marine, world-wide, temperate regions. Jawless eel-like scavenger feeders that can produce much slime.

Lampreys

Class Petromyzontida
One order, Petromyzontiformes, with 3 families, and 38 species, 10 in Canada (all in Petromyzontidae). Anadromous and freshwater, cool zones of the world. Jawless eel-like fishes, many of which are parasitic-like.

Superclass Gnathostomata - Jawed Fishes

All other fishes and tetrapods are jawed and belong to the superclass Gnathostomata. Two classes are extinct groups of fishes.

Class Placodermi
An extinct group of fishes with bony plates covering the head, living from the Silurian to the Upper Devonian (443 to 360 million years ago); some are known from Canada.

Class Acanthodii
An extinct group of fishes with large eyes and up to 6 pairs of spines between the pectoral and pelvic fins, earliest known jawed fishes living from the Late Ordovician to the Early Permian (458 to 300 million years ago); well known from Canada. Acanthodians are thought to be most closely related to the actinopterygian-sarcopterygian lineage and in classification are usually placed after the class Chondrichthyes.

Jawed Fishes - Class Chondrichthyes/Cartilaginous Fishes

The living members are in 2 subclasses.

Subclass Holocephali
One order, Chimaeriformes, with 3 families (those in North America in family Chimaeridae), and 33 species (a few species in Canada). All marine.

Subclass Elasmobranchii
There are many species of this subclass in Canada, consisting of sharks (total of 34 families, 106 genera and 403 species) in 9 orders, Heterodontiformes, Orectolobiformes, Lamniformes, Carcharhiniformes, Hexanchiformes, Echinorhiniformes, Squaliformes, Squatiniformes and Pristiophoriformes, and rays (including skates) (total of 17 families, 72 genera, and at least 534 species), in 4 orders, Torpediniformes, Pristiformes, Rajiformes and Myliobatiformes. Almost all marine.

Jawed Fishes - Class Actinopterygii/Ray-finned Fishes

Actinopterygians, the ray-finned fishes, are placed in 3 subclasses, with, of living members, a total of 44 orders, 453 families, and about 26 890 species.

Subclass Cladistia
One order, Polypteriformes, with one family, the bichirs, of 16 species in freshwater Africa.

Subclass Chondrostei
One order of living members, Acipenseriformes, with 2 families, and 25 species of sturgeons (freshwater and anadromous), 5 species in Canada, and 2 of paddlefishes (freshwater).

Subclass Neopterygii
Two orders of living members.

• Lepisosteiformes (gars) with one family of 7 species, 2 in Canada. All usually freshwater.

• Amiiformes (bowfins) with one family of one species, also in Canada. Freshwater.

Jawed Fishes - Division Teleostei

All remaining actinopterygians are placed in the Teleostei, with 4 subdivisions. The Acanthomorpha or spiny-rayed fishes, a branch of the fourth subdivision, contains most of the species of teleosts.

Subdivision Osteoglossomorpha
Two orders of freshwater fishes.

• Hiodontiformes with one family and 2 species (goldeye and mooneye), both in Canada.

• Osteoglossiformes (bonetongues) with 4 families and about 218 species, none in Canada or native in North America.

Subdivision Elopomorpha
Four orders of mostly marine fishes.

• Elopiformes, with 2 families and about 8 species.

• Albuliformes, with 3 families and about 30 species, a few deepwater notacanthids in Canada in Atlantic and Pacific.

• Anguilliformes, with 15 families and about 791 species of eels, many in Canada.

• Saccopharyngiformes, with 4 families and about 28 species.

Subdivision Ostarioclupeomorpha
Also known as Otocephala, with 2 superorders.

Superorder Clupeomorpha - One order, Clupeiformes, with 5 families and 364 species, eg, in Canada several species in Atlantic and Pacific of anchovies, alewife, herrings, menhaden, sardines and shads (also in fresh water in Canada a herring, alewife, shad, and gizzard shad), and elsewhere also pilchards and sprats. Most are marine.

Superorder Ostariophysi - Five orders of mostly freshwater fishes, all except first with modification of first 4 or 5 vertebrae into an apparatus for sound transmission, the Weberian apparatus.

• Gonorynchiformes, 4 families, eg, milkfishes, and 37 species, none in Canada.

• Cypriniformes, 6 families, eg, minnows (50 in Canada plus 4 introductions), suckers (18 in Canada), and loaches, about 3268 species. All freshwater.

• Characiformes, 18 families and at least 1674 species, all freshwater, none in Canada. Popular aquarium fishes.

• Siluriformes, with about 36 families and 2867 species of catfishes, normally up to 4 pairs of barbels on head, freshwater on all continents (8 in Canada plus one probable introduction) except Antarctica, and almost 120 species in oceans.

• Gymnotiformes, with 5 families and about 134 species of American knifefishes, eel-like body and electric organs present, freshwater in southern Mexico, Central America, and South America.

Subdivision Euteleostei
Nine superorders, 28 orders, 346 families, and 17 419 species, both freshwater and marine.

Superorder Protacanthopterygii - Four orders.

• Argentiniformes, 6 families and about 202 species of marine smelts, few in Canada over continental shelf.

• Osmeriformes, 3 families and 88 species of freshwater smelts (including Southern Hemisphere galaxiids), some in Canada.

• Salmoniformes, one family and 66 species, in 3 subfamilies, grayling (one in Canada), whitefishes and ciscoes (15 in Canada), and chars, trouts and salmons (13 plus one introduction in Canada). Freshwater and anadromous.

• Esociformes, 2 families and 10 species, 4 pike, and one mudminnow in Canada. All freshwater.

Superorder Stenopterygii - One order Stomiiformes (dragonfishes), 5 families, and about 391 species, few in Canada mostly in deep water in Atlantic and Pacific. All marine, luminescent organs (photophores) present.

Superorder Ateleopodomorpha - One order Ateleopodiformes, one family, and 12 species, none in Canada. All marine.

Superorder Cyclosquamata - One order Aulopiformes, 15 families, and about 236 species, few in Canada over continental shelf, many in deep water. All marine.

Superorder Scopelomorpha - One order Myctophiformes, 2 families, and about 246 species, many species of lanternfishes occur primarily in deep water off Canada in Arctic, Atlantic, and Pacific. All marine.

Jawed Fishes - Spiny-rayed Fishes

Within subdivision Euteleostei, Division Teleostei, the acanthomorphs or the spiny-rayed fishes. All remaining actinopterygians are placed in this taxon. Many members have true fin spines in the dorsal, anal, and pelvic fins.

Superorder Lampriomorpha - One order Lampriformes, 7 families with about 21 species, in Canada the opah in Atlantic and Pacific and the king-of-the-salmon (a ribbonfish) in the Pacific. All marine.

Superorder Polymixiomorpha - One order Polymixiiformes, one family with 10 species of beardfishes, in Canada one in Atlantic. All marine.

Superorder Paracanthopterygii - An artificial assemblage of 5 orders.

• Percopsiformes, 3 families and 9 species, one in Canada, trout-perch. All freshwater.

• Gadiformes, 9 families and about 555 species, eg, in Canada the marine grenadiers, hakes, cods, pollock, and walleye pollock, one species confined to fresh water, the circumpolar burbot, including Canada.

• Ophidiiformes, 5 families and about 385 species, eg, in Canada cusk-eels in Atlantic and a viviparous brotula in Pacific and elsewhere pearlfishes.

• Batrachoidiformes, one family and 78 species, toadfishes, one in Canada, the plainfin midshipman in Pacific. Mostly marine.

• Lophiiformes, 18 families and 313 species, anglerfishes, many species with angling device for attracting prey to mouth derived from first ray of spinous dorsal, in Canada in Atlantic a goosefish (called monkfish as a food fish), batfish, frogfish and a gaper, and in deep water in both Atlantic and Pacific several species of ceratioid anglerfishes where miniature males are usually parasitic on the females. All marine.

Superorder Acanthopterygii - The last superorder of actinopterygians contains 13 orders.

• Mugiliformes, one family and about 72 species of mullets, in Canada 2 rarely in Atlantic. Mostly marine.

• Atheriniformes, 6 families and about 312 species of, eg, silversides, in Canada one in Atlantic, one rarely in Pacific, and one in fresh water.

• Beloniformes, 5 families and 227 species of, eg, halfbeaks (in Canada one rarely in Atlantic), flyingfishes (3 rarely in Atlantic and Pacific in Canada), needlefishes (none in Canada), and sauries (in Canada one in Atlantic and one in Pacific). Marine and freshwater.

• Cyprinodontiformes, 10 families and about 1013 species, popular aquarium fishes, eg, killifishes, livebearers, and topminnows, in Canada 3 killifishes in fresh water (one being also brackish in Atlantic) and 2 introduced species into Cave and Basin Hotsprings, Banff National Park.

• Stephanoberyciformes, 9 families and 75 species, in Canada one whalefish rarely in Atlantic and 2 melamphids in Pacific. All marine.

• Beryciformes, 7 families and 144 species, eg, pinecone fishes, roughies, and squirrelfishes, in Canada several species rarely reported in Atlantic. All marine.

• Zeiformes, 6 families and 32 species, eg, dories, in Canada several known in Atlantic and one poorly known in Pacific. All marine.

• Gasterosteiformes, 11 families and 278 species, eg, in Canada pipefishes (one with 3 rare in Atlantic and one in Pacific), seahorses (one rare in Atlantic), tubesnouts (one in Pacific), and sticklebacks (5 as a conservative number, freshwater and marine), and elsewhere also includes cornetfishes and trumpetfishes.

• Synbranchiformes, 3 families, 15 genera and about 99 species, eg, swamp eels, none in Canada. Most freshwater.

• Scorpaeniformes, 26 families and about 1477 species, in Canada many species of scorpionfishes (rockfishes), greenlings, lumpsuckers, poachers, sculpins (Atlantic and Pacific and about 10 in fresh water), searobins (rare in Atlantic), and snailfishes. Most marine, worldwide.

• Perciformes, a very diverse group of 160 families and about 10 033 species, in Canada many freshwater species of temperate basses (moronids, also in Atlantic and introduced in Pacific), sunfishes (includes largemouth bass) and perches (includes darters and walleye), and many primarily marine species of (larvae and adults that stray into Canadian waters) surfperches, gobies, mackerels, tunas, swordfish, billfishes, and butterfishes, and elsewhere also freshwater cichlids and primarily marine butterflyfishes, angelfishes and surgeonfishes.

• Pleuronectiformes, 14 families and 678 species, the flatfishes many in Canada, including some of following: flounders, halibuts, plaice, soles, and turbots. Most marine.

• Tetraodontiformes, 9 families and 357 species, in Canada several species occur rarely in Atlantic, and a mola, the ocean sunfish, occurs in Atlantic and Pacific; worldwide, groups include triggerfishes, filefishes, boxfishes, puffers, and porcupinefishes. Most marine.

Jawed Fishes - Class Sarcopterygii/Lobe-finned Fishes

The lobe-finned fishes and the tetrapods, 2 subclasses, a monophyletic taxon that includes the remaining vertebrates, comprising several fish groups, both recent and fossil, and the tetrapods.

Subclass Coelacanthimorpha
One order Coelacanthiformes, one family, and 2 living species. Not in Canada except in fossil record. Marine.

Subclass Dipnotetrapodomorpha
With many fossil groups and the following with living species.

One order Ceratodontiformes, living lungfishes, 3 families and 6 species in Africa, Australia, and South America. Freshwater.

Infraclass Tetrapoda, the tetrapods, with about 26 734 species of amphibians and amniotes (mammals, reptiles, and birds).