Fish Classification: Key Orders and Families of Fishes

Fish Classification: Major Orders and Their Characteristics

Coelacanthiformes (coelacanths) and Ceratodontiformes (lungfish) are among the most ancient lineages. Lepidosireniformes include the South American lungfish.

Ancient and Primitive Fish Orders

• Polypteriformes: bichirs with fleshy pectoral fins and spiracles; their flap-like fins are crowned with thorns, and their strong jaw structure is more similar to tetrapods than to teleosts. They are found in freshwater habitats in Africa.
• Acipenseriformes: sturgeons (family Polyodontidae and others).
• Semionotiformes: one family, Lepisosteidae (gars), with 7 species distributed in 2 genera (Atractosteus and Lepisosteus); Lepisosteus has ganoid scales.
• Amiiformes: scales with 45 to 50 rays in the dorsal fin; the dorsal fin extends from the middle region to the base of the tail; the caudal fin has one lobe and is heterocercal.

Teleost Fish Orders

• Osteoglossiformes: families Pantodontidae, Notopteridae (featherbacks, knifefishes; 8 species) and Mormyridae.
• Elopiformes: the dorsal fin has only soft rays, and the maxillae are toothed.
• Anguilliformes (eels): cycloid scales; 4 suborders, 19 families, about 110 genera and 400 species; mostly predators, with leptocephali larvae.
• Saccopharyngiformes: includes Eurypharynx pelecanoides; operculum and ribs present; swim bladder or bladder present.
• Clupeiformes (herrings, sardines): caudal fin homocercal, physostomous, with spines in the fins.
• Cypriniformes (carps, minnows): one dorsal fin, pharyngeal teeth in the throat.
• Esociformes: teleost fish such as pike.
• Osmeriformes: 14 families and 240 species; the maxilla forms part of the upper mouth; scales absent or with radii; anadromous species.

Deep-Sea and Specialized Orders

• Stomiiformes: 4 families, 5 genera and 320 species of deep-sea fish.
• Ateleopodiformes: gelatinous snout, 12 species in 4 genera; long caudal and anal fins.
• Aulopiformes: 13 marine families; structures grouped by their pharyngobranchial elements.
• Myctophiformes: 2 families, including lanternfish.
• Lampriformes: about 50 species with a deep, elongated body, scale-like features, found at depths of 100 to 1000 m.
• Polymixiformes: named for their long hyoid barbels; found in tropical and subtropical waters.

Freshwater and Coastal Orders

• Percopsiformes: perch and trout relatives, such as Aphredoderus sayanus.
• Ophidiiformes: eels, pearlfishes and brotulas, such as Carapus sp.
• Gadiformes: elongated body, cycloid scales, spiny fin rays, homocercal caudal fin, gas-filled swim bladder; includes hake and cod.
• Batrachoidiformes: about 70 species in 19 genera, including toadfishes.
• Lophiiformes: anglerfishes (frogfish, deep-sea species) with broad, flat heads.
• Mugiliformes: mullets; the swim bladder is closed, two dorsal fins, forked caudal fin, dorsal and anal fins opposite, smooth body.
• Atheriniformes: two dorsal fins with flexible spines, one spine in front of the anal fin, weak or absent lateral line.
• Beloniformes: needlefishes and their relatives.
• Cyprinodontiformes: aquarium fish, including live-bearers and killifish such as Fundulus heteroclitus; they live where water is rich in oxygen at the surface and have adaptations to environmental disadvantages.