Actinopterygii, is a major taxonomic class (or subclass) of fish, known as the "ray-finned fishes," whose diverse number of species includes about half of all known living vertebrates and 96 percent of all fish species. The actinopterygians include the most familiar fish, such as sturgeons, gars, eels, carp, herrings, anchovies, catfishes, goldfishes, piranhas, oarfish, seahorses, bass, cichlids, pickerel, salmon, and trout.
The ray-finned fishes are so called because they possess lepidotrichia or "fin rays," their fins being webs of skin supported by bony or horny spines ("rays"), as opposed to the fleshy, lobed fins characteristic of the Sarcopterygii, which together with the actinopterygians comprise the superclass Osteichthyes, or bony fish. The actinopterygian fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the link or connection between these fins and the internal skeleton (e.g., pelvic and pectoral girdles).
As a group, the ray-finned fish play a huge role not only in aquatic ecosystems, both marine and freshwater, where they serve as both prey and predator, but also in diverse areas of human livelihood, from commerce to recreation, aesthetics, recreation, and nutrition. These fish provide essential nutrition for millions of people, are viewed in aquariums and underwater, are sought through sports fishing, and play a fundamental role in food chains, ensuring healthy ecosystems and controlling prey populations, including insects (Jonna 2004). Beyond this, their extraordinary diversity and geographic range—ubiquitous throughout fresh water and marine environments from the deep sea to the highest mountain streams (with some species even venturing outside of water) and with a spectacular array of colors, body forms, and behaviors—has added greatly to the human enjoyment of nature.
Actinopterygii (the plural form of Actinopterygius) is commonly placed as a class of vertebrates (ITIS 2017; Jonna 2004; Nelson 1994), generally with the parent taxon Osteichthyes (the bony fish) listed as a superclass. In some classification schemes, however, Osteichthyes is listed not as a superclass but as a class, in which case Actinopterygii is listed as a subclass. Alternatively, Nelson (1994), while using Actinopterygii as a class, chooses not to use Osteichthyes as a formal taxon at all because it "is clearly not a monophyletic group."
With such a great multitude of species, the Actinopterygians' characteristics tend to be spread over a considerable range. Many, but not all, of the Actinopterygians, for example, have scales, which may be either the more primitive ganoid form(diamond-shaped, shiny, hard, and multilayered), or the more advanced cycloid or ctenoid forms, which overlap a bit like roof tiles from head to tail to reduce drag. Cycloid scales have smooth edges and ctenoid have rough edges. Other characteristics include nostrils set relatively high up on the head with internal nostrils absent, spiracle (a hole behind the eye through which some cartilaginous fish pump water to the gills) usually absent, pectoral radial bones attached to the scapulo-coracoid skeletal complex (except in Polypteriformes), interopercle and branchiostegal rays (bone-like infrastructure) usually present, and bony gular plate (protecting the throat and lower jaw) usually absent (Nelson 1994). This group is considered to be monophyletic. Extant species can range in size from Paedocypris, at 7.9 millimeters (0.3 inches), to the massive ocean sunfish, at 2,300 kilograms (5,100 lb), and the long-bodied aarfish, to at least 11 meters (36.1 feet).
Most bony fish belong to the Actinopterygii; there are only eight living species of lobe finned fish (class Sacopterygii), including the lungfish and coelacanths. Nelson (1994) and Jonna (2004) recognize 42 orders, 431 families, over 4,000 genera, and about 24,000 species of ray-finned fish. This is about half the number of species of known extant vertebrates. About 42 percent of the species of bony fish are known only or almost only from freshwater (Nelson 1994). However, species of fish are not only being continually discovered, but also some are believed to be becoming extinct faster than they can be discovered (Jonna 2004).
Actinopterygians are generally classified into two groups—the Chondrostei and the Neopterygii (Jonna 2004; Nelson 1994; ITIS 2018). The Chondrostei include paddlefishes, sturgeons, and bichirs (ITIS 2017). The Neopterygii include Amiiformes (bowfin), Semionotiformes or Lepisosteiformes (gars), and Teleostei (modern bony fishes). Most fish today fit into the Teleostei, with about 23,000 of the 24,000 actinopterygians being teleosts.
Other classifications of the Actinopterygians exist. For example, in addition to Chondrostei and Neopterygii, Lundberg (2006) also lists a taxon of Actinopterygii known as Cladistia, comprised of the bichirs, reedfishes, Polypteriformes, and Polypteridae.
The ray-finned fish are extraordinarily diverse in terms of body form, color, habitat, behavior, and so forth. They live in almost all types of habitats with the exception of land that is constantly dry (and some species spend a considerable amount of time outside of water), including the depths of the ocean to 7,000 meters, subterranean caves, desert springs and ephemeral pools, high altitude lakes, and polar seas, and including temperatures from -1.8°C to nearly 40°C (28.8°F to nearly 104°F), salinities from 0 to 90 parts per million, pH levels from four to above ten, and dissolved oxygen levels down to zero (Jonna 2004). Actinopterygians may swim, walk, fly, or be immobile, they feed on nearly all types of organic matter, they exhibit a huge variety of colors, and they have different types of sensory systems, including vision, hearing, chemoreception, electroreception, lateral line sensation, and so forth (Jonna 2004). The electric eel and various other fish can produce electric organ discharges (EODs), which may be low voltage for electrolocation and high voltage to stun prey or offer protection.
The earliest known fossil actinopterygiian is Andreolepis hedei, dating back 420 million years (Late Silurian). This microvertebrate has been uncovered in Russia, Sweden, and Estonia (PD 2008). While appearing in the Devonian some 400 million years ago, actinopterygians did not become dominant in freshwaters until the Carboniferous (360 million years ago), when they started to invade the seas (Jonna 2004).
A listing of the different groups is given below, down to the level of orders, arranged in what has been suggested to represent the evolutionary sequence down to the level of order based primarily on the long history of morphological studies. This classification, like any other taxonomy based on phylogenetic research is in a state of flux. Many of these ordinal and higher-level groupings have not been supported in both the recent morphological and molecular literature. Examples of demonstrably paraphyletic or unnatural groups include the Paracanthopterygii, Scorpaeniformes, and Perciformes (Johnson and Wiley 2007). The listing follows Froese and Pauly (2018), with notes when this differs from Nelson (2006) and ITIS (2017).
- Subclass Chondrostei
- Subclass Neopterygii
- Infraclass Holostei
- Order Lepisosteiformes, the gars
- Order Amiiformes, the bowfins
- Infraclass Teleostei
- Superorder Osteoglossomorpha
- Order Osteoglossiformes, the bony-tongued fishes
- Order Hiodontiformes, including the mooneye and goldeye
- Superorder Elopomorpha
- Order Elopiformes, including the ladyfishes and tarpon
- Order Albuliformes, the bonefishes
- Order Notacanthiformes, including the halosaurs and spiny eels
- Order Anguilliformes, the true eels and gulpers
- Order Saccopharyngiformes, including the gulper eel
- Superorder Clupeomorpha
- Superorder Ostariophysi
- Order Gonorynchiformes, including the milkfishes
- Order Cypriniformes, including barbs, carp, danios, goldfishes, loaches, minnows, rasboras
- Order Characiformes, including characins, pencilfishes, hatchetfishes, piranhas, tetras.
- Order Gymnotiformes, including electric eels and knifefishes
- Order Siluriformes, the catfishes
- Superorder Protacanthopterygii
- Superorder Stenopterygii
- Order Ateleopodiformes, the jellynose fish
- Order Stomiiformes, including the bristlemouths and marine hatchetfishes
- Superorder Cyclosquamata
- Order Aulopiformes, including the Bombay duck and lancetfishes
- Superorder Scopelomorpha
- Order Myctophiformes, including the lanternfishes
- Superorder Lampridiomorpha
- Order Lampriformes, including the oarfish, opah and ribbonfishes
- Superorder Polymyxiomorpha
- Order Polymixiiformes, the beardfishes
- Superorder Paracanthopterygii
- Order Percopsiformes, including the cavefishes and trout-perches
- Order Batrachoidiformes, the toadfishes
- Order Lophiiformes, including the anglerfishes
- Order Gadiformes, including cods
- Order Ophidiiformes, including the pearlfishes
- Superorder Acanthopterygii
- Order Mugiliformes, the mullets
- Order Atheriniformes, including silversides and rainbowfishes
- Order Beloniformes, including the flyingfishes
- Order Cetomimiformes, the whalefishes
- Order Cyprinodontiformes, including livebearers, killifishes
- Order Stephanoberyciformes, including the ridgeheads
- Order Beryciformes, including the fangtooths and pineconefishes
- Order Zeiformes, including the dories
- Order Gobiesociformes, the clingfishes
- Order Gasterosteiformes including sticklebacks, pipefishes, seahorses
- Order Syngnathiformes, including the seahorses and pipefishes
- Order Synbranchiformes, including the swamp eels
- Order Tetraodontiformes, including the filefishes and pufferfish
- Order Pleuronectiformes, the flatfishes
- Order Scorpaeniformes, including scorpionfishes and the sculpins
- Order Perciformes 40 percent of all fish including anabantids, Centrarchids (incl. bass and sunfish), Cichlids, gobies, gouramis, mackerel, perches, scats, whiting, wrasses
- Superorder Osteoglossomorpha
- Infraclass Holostei
- In Nelson (2006) and Lundberg (2006), Polypteriformes is placed in its own subclass Cladistia.
- In ITIS (2017), Gobiesociformes is placed as the suborder Gobiesocoidei of the order Perciformes.
- In Nelson (2006) and ITIS (2017), Syngnathiformes is placed as the suborder Syngnathoidei of the order Gasterosteiformes.
- Froese, R., and D. Pauly (eds.). 2018. FishBase Fishbase.org. Retrieved June 25, 2018.
- Integrated Taxonomic Information System (ITIS). 2017. Actinopterygii ITIS Taxonomic Serial No.: 161061. Retrieved June 25, 2018.
- Integrated Taxonomic Information System (ITIS). 2018. Neopterygii ITIS Taxonomic Serial No.: 553120. Retrieved June 25, 2018.
- Johnson, G. D., and E. O. Wiley. 2007. Percomorpha Tree of Life. Retrieved June 25, 2018.
- Jonna, R. 2004. Actinopterygii Animal Diversity Web. Retrieved June 25, 2018.
- Lundberg, J. G. 2006. Actinopterygii. The ray-finned fishes Tree of Life Version 07 August 2006 (temporary). Retrieved June 25, 2018.
- Nelson, J. S. 2006. Fishes of the World. John Wiley & Sons. ISBN 0471250317.
- Nelson, J. S. 1994. Fishes of the World, 3rd ed. New York: John Wiley & Sons. ISBN 0471547131.
- Paleontology Database (PD). 2008. Andreolepis Paleontology Database. Retrieved June 25, 2018.
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