III. Pisces-06. Actinopterygii (I): from Polypteriformes to Gymnotiformes
A brief overview of ray-finned fishes from mullets to herring-like species. Systematics according to Nelson et al., 2016.
We will begin with a general overview of the systematics of ray‑finned fishes. It is presented here according to the authoritative 2016 synthesis. According to this system, 67 orders belong to the ray‑finned fishes. Naturally, not all these orders are equally important for second‑year students studying the general course in vertebrate zoology. The orders that students should have a clear understanding of are shown in the table below in brick‑red colour (their total number – 24).
Class “Osteichthyes” – Bony fishes
Subclass Actinopterygii – Ray‑finned fishes
Infraclass Cladistia – Cladistians
Order Polypteriformes – Polypterids
Infraclass Chondrostei – Chondrostean gnathostomes, Chondro‑osteichthyans
Order Acipenseriformes – Sturgeons
Infraclass Holostei – Holosteans
Division Ginglymodi
Order Lepisosteiformes – Gars
Division Halecomorphi
Order Amiiformes – Amiids
Division Teleosteomorpha
Subdivision Teleostei – Teleosts
Cohort Elopomorpha – Elopoid fishes
Order Elopiformes – Elopids
Order Albuliformes – Albulids
Order Notacanthiformes – Notacanthids
Order Anguilliformes – Eels
Cohort Osteoglossomorpha – Osteoglossomorphs, Bony‑tongued fishes
Order Hiodontiformes – Hiodontids
Order Osteoglossiformes – Osteoglossids
Cohort Otocephala – Otocephalans
Superorder Clupeomorpha – Herrings
Order Clupeiformes – Clupeids
Superorder Alepocephali – Alepocephalans
Order Alepocephaliformes – Slickheads
Superorder Ostariophysi – Ostariophysans
Order Gonorynchiformes – Gonorynchids
Order Cypriniformes – Carps
Order Characiformes – Characins
Order Characiformes – Characins
Order Gymnotiformes – Gymnotiforms
Cohort Euteleostei – True bony fishes
Order Lepidogalaxiiformes – Lepidogalaxids
Superorder Protacanthopterygii – Protacanthopterygii
Order Salmoniformes – Salmons
Order Esociformes – Pikes
Superorder Osmeromorpha – Osmeriforms
Order Argentiniformes – Argentinids
Order Galaxiiformes – Galaxiids
Order Osmeriformes – Osmerids
Order Stomiiformes – Stomiids
Superorder Ateleopodomorpha
Order Ateleopodiformes – Ateleopods
Superorder Cyclosquamata
Order Aulopiformes – Aulopids
Superorder Scopelomorpha
Order Myctophiformes – Myctophids
Superorder Lamprimorpha
Order Lampriformes – Lamprids
Superorder Paracanthopterygii – Paracanthopterygii
Order Polymixiiformes – Polymixiids
Order Percopsiformes – Percopsids
Order Zeiformes – Zeids
Order Stylephoriformes – Stylephorids
Order Gadiformes – Gadids
Superorder Acanthopterygii – Acanthopterygians
Series Berycida
Order Holocentriformes – Holocentrids
Order Trachichthyiformes – Trachichthyids
Order Beryciformes – Berycids
Series Percomorpha
Subseries Ophidiida
Order Ophidiiformes – Cusk‑eels
Subseries Batrachoidida
Order Batrachoidiformes – Toad‑like fishes
Subseries Gobiidae
Order Kurtiformes – Kurtids
Order Gobiiformes – Gobies
Subseries Ovalentaria
8 families incertae sedis (of uncertain placement), including – Pomacentridae – Damselfishes
Order Mugiliformes – Mullets
Order Cichliformes – Cichlids
Order Blenniiformes – Blennies
Order Gobiesociformes – Clingfishes
Order Atheriniformes – Atherinids
Order Beloniformes – Needlefishes
Order Cyprinodontiformes – Pupfishes
Order Synbranchiformes – Swamp eels
Order Carangiformes – Jacks
Order Istiophoriformes – Sailfishes
Order Anabantiformes – Anabantoids
Order Pleuronectiformes – Flatfishes
Order Syngnathiformes – Pipefishes
Order Icosteiformes – Icosteids
Order Callionymiformes – Dragonets
Order Scombrolabraciformes – Scombrolabracids
Order Scombriformes – Mackerels
Order Trachiniformes – Tracins
Order Labriformes – Wrasses
Order Perciformes – Perches
Order Scorpaeniformes – Scorpaenids
Order Moroniformes – Moronids
Order Acanthuriformes – Surgeonfishes
Order Spariformes – Sparids
Order Caproiformes – Caproids
Order Lophiiformes – Anglerfishes
Order Tetraodontiformes – Pufferfishes
Now we can proceed to the overview of individual taxa.
Infraclass Cladistia – Cladistians
Order Polypteriformes – Polypterids
Probably, the cladistians are the sister group to all other ray‑finned fishes; in fact they represent the earliest surviving lineage of ray‑finned fishes. One family, two genera (Polypterus and the reedfish, or kalamoichthys – Erpetoichthys). The dorsal fin consists of 5–18 finlets. The body is covered with ganoid scales forming a robust armor. They possess a lung used for breathing. They live in Africa.
Infraclass Chondrostei – Chondrostean gnathostomes, Chondro‑osteichthyans
Order Acipenseriformes – Sturgeons
In the past, chondro‑osteichthyan fishes were much more diverse. The largest sturgeon representative is the beluga (Huso huso), which can exceed 4 m in length (reports of individuals up to 9 m exist). It is an anadromous fish of the Black, Azov and Caspian Seas. Belugas may spawn many times in their life, returning to the sea after spawning; old individuals can reach 100 years.
The paddlefish family – Polyodontidae – is represented by two species. Psephurus gladius – the Chinese paddlefish, or psef, lives, as the name suggests, in Asia, in the Yangtze River, and Polyodon spathula – the American paddlefish lives in the Mississippi. They are filter‑feeders with a blade‑like rostrum.
Infraclass Holostei – Holosteans (Bony gnathostomes)
Here we encounter a typical situation in systematics that avoids paraphyletic groups. The next two orders are united in an informal group of bony gnathostomes. But because the ancestors of the subsequent orders also belong to this group, they are included as well. In our course, tetrapods are excluded from the ray‑finned fishes for teaching convenience. One could use another name for this subclass – Neopterygii, the “new‑finned” fishes. However, since we follow the 2016 Nelson et al. system, the older name used by the authors of that system – the one linked to the basal group of neopterygian fishes – is retained.
Order Lepisosteiformes – Gars
This order contains the single family Lepisosteidae – the gars, or armored pikes, represented by two genera and freshwater species distributed in North and Central America. They are ambush predators inhabiting dense aquatic vegetation and can reach lengths of up to 3 m (other sources report up to 4.5 m).
Order Amiiformes – Amiids
The only extant species is the bowfin, Amia calva, found in eastern North America; however, the Mesozoic history of this group includes many diverse forms. The bowfin is a sluggish freshwater predator reaching up to 90 cm in length.
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The ventral fin and its skeleton are absent; sometimes the pectoral fin is also missing. The dorsal and anal fins merge with the caudal fin. Scales are usually absent. They are specialized for entering small holes or burrowing into soft substrate, as marine eels do.
In their life cycle a larval stage – the leptocephalus – is characteristic. To date, the correspondence between leptocephali and adult stages is not fully resolved; larval morphological diversity exceeds that of adult forms.
Some species enter freshwater or are exclusively freshwater. Typical freshwater eels are catadromous (migrate from freshwater to the sea to spawn). The European eel (Anguilla anguilla), which occurs in Ukraine, spawns in the Sargasso Sea. This route was likely established long ago when Europe and America began to separate and the Atlantic Ocean widened. The Sargasso Sea is a region of the Atlantic with very salty water. Eels that leave freshwater spawn there at depths of about 2 km. Leptocephali are carried by the Gulf Stream toward Europe, metamorphose into glass eels, and then enter freshwater.
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Moray eels – mainly marine fishes, although some enter freshwater. They have lost all paired fins and have become benthic ambush predators. In the next photo a moray’s mouth is being cleaned by a shrimp. Morays have high resistance to the toxins of their potential prey. However, as shown in the following video, even they can fail to ingest certain prey. A moray had to release a winged fish (probably already dead) because it could not swallow it.
In typical cases morays hide in burrows and crevices among rocks. Bites from some species can be dangerous to humans. Moray jaws provide efficient prey capture, aided by teeth on the vomer, which protrudes from the mouth as an additional “jaw”.
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"Sea eels" refer to members of a fairly diverse group of families. Many live in burrows they make in soft substrate.
The pelican eel (Saccopharynx ampullaceus) is a fairly large (up to almost two metres) fish living at depths of 2–5 km. The pelican eel and its close relatives lack ossified skull bones, ribs, ventral fins, scales and many other structures. They are adapted for swallowing large prey.
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Order Osteoglossiformes – Osteoglossids
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Arapaima – Osteoglossum bicirrhosum – a fish with a characteristic flattened body and a distinctive whisker on the lower jaw. It reaches 120 cm in length and inhabits the Amazon basin and some other South American rivers. Arapaima gigas – one of the largest (or the largest) freshwater fishes, reaching 4.6 m in length, is widespread in the Amazon basin. As shown in the video fragment, the arapaima can crawl quite effectively. As discussed in the section on tetrapod origins, this mode of locomotion is called “concertina”.
Among the osteoglossids is also the African family Mormyridae – the mormyrids. These are usually small fishes (the largest up to 1.5 m) with an elongated rostrum, characterized by electroreception that allows them to survive and feed in the turbid waters of the Nile and other rivers.
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Order Clupeiformes – Clupeids
Planktonic fishes of the water column. The body is laterally compressed, covered with cycloid scales that shed easily. The swim bladder connects to the gut and auditory capsules. They include important commercial species. The family Clupeidae – the herrings – comprises some of the most important fishery species.
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Spawning of herring – a spectacular event.
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Engraulidae – anchovies include small schooling fishes, some of which (e.g., Engraulis encrasicolus – the European anchovy, or “hamza”) are important commercial species. Anchovy schools can resemble giant amoebas.
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Order Cypriniformes – Carps
Predominantly freshwater fishes; the most widespread freshwater fish group. Many resemble clupeids. Characteristic is the presence of the Weberian apparatus – movable ossicles that connect the inner ear with the swim bladder. The body is usually covered with cycloid scales; the mouth is often protrusible, frequently with barbels. A key feature is the shape of pharyngeal teeth. This order includes important aquaculture, commercial and aquarium species.
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A large part of Ukraine’s ichthyofauna belongs to this order. You will become more familiar with these species during field practice. The family Cyprinidae – carps – is probably the most species‑rich vertebrate family, comprising about 367 genera and 3006 species.
Order Characiformes – Characins
A large order of exclusively freshwater fishes, mostly predatory. A characteristic feature (also found in salmonids and catfishes) is the transformation of the second dorsal fin into a fatty fin lacking bony rays. They are common in South and Central America and equatorial Africa. This order includes many aquarium fishes: 18 families, 270 genera and at least 1 674 species.
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Probably the best‑known representatives are the piranhas. The common piranha (Pygocentrus nattereri) is noted for its powerful bite and sharp teeth. Although the notorious reputation of piranhas is greatly exaggerated, they can be dangerous in some cases. Their voracity, attacking a wide range of animal prey, can be exploited in fishing.
For many characin species and genera the name “tetra” is used.
Order Siluriformes – Catfishes
A large order, fairly close to the carps. Mainly freshwater. The body is either naked or covered with bony plates. Most species have barbels on the head (up to four pairs). Most are predators living mainly in warm waters. Many are important commercial species and are popular in aquaria.
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The common catfish (Silurus glanis) – a very large fish of our waters, according to some data reaching 5 m in length and 330 kg in weight.
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Sometimes a catfish may attack a human, although it is more likely defending its territory than trying to eat a person. However, only very large catfish could pose a danger to children. Domestic poultry, small dogs and similar animals are regularly preyed upon by catfish.
The American candiru (Vandellia cirrhosa) is much smaller (10–15 cm) yet far more dangerous. It is one of the few vertebrate species with a parasitic feeding habit.
Order Gymnotiformes – Gymnotiforms
Eel‑like body shape. Characterized by electric organs derived from modified muscle or nerve cells. 5 families, 30 genera, 134 species.
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Electric eel (Electrophorus electricus) – a large (up to 3 m and 40 kg) South American fish capable of generating series of electric discharges with voltage and current sufficient to endanger human life. This is both a hunting method (the discharge paralyzes the prey) and a defense mechanism. The video shows a caiman that received a series of electric eel discharges.
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A large part of Ukraine’s ichthyofauna belongs to this order. You will become more familiar with these species during field practice. The family Cyprinidae – carps – is probably the most species‑rich vertebrate family, comprising about 367 genera and 3006 species.
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The common catfish (Silurus glanis) – a very large fish of our waters, according to some data reaching 5 m in length and 330 kg in weight.