Ray-finned Fishes

Actinopterygii - ray-finned fishes

Vertebrata; Gnathostomata; Osteichthyes; Actinopterygii

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Show Lateral view of Black Labeo (Carp) in spirit Image
Lateral view of Black Labeo (Carp) in spirit
Show Carp skeleton Image
Carp skeleton
Show Carp skeleton Image
Carp skeleton
Show Fish swim bladder in spirit Image
Fish swim bladder in spirit

The actinopterygians, or ray-finned fish, are one of the two major clades of bony fish (Osteichthyes), the other being the lobe-finned fish, or Sarcopterygians. The Chondrichthyes (cartilaginous fish) are the extant sister clade of the Osteichthyes. Below is a cladogram to show these relationships:

Diversity and Lower Taxonomy:

  • The subclass Actinopterygii comprises some 27,000 species of ray-finned bony fishes, making it the largest radiation of any vertebrate group. As a result, covering the whole group in detail would be extremely complex and, more importantly, hugely baffling! Therefore, the information below aims to highlight the key groups of ray-finned fish in an evolutionary context, explaining how each clade is divided and related.
  • Actinopterygians can be divided into two distinct groups: basal actinopterygians and neopterygians (most extant fishes). The basal actinopterygians comprise two closely related small taxa, the more primitive Polypteriformes (containing a single family of bichirs), and the Chondrostei (containing two families, the sturgeons and the paddlefishes). The following cladogram illustrates these relationships:
  • Living neopterygians can then be split into three groups (see cladogram above), the first two of which are considered primitive neopterygians:
  1. The gars, forming a single family in the order Lepisosteiformes. These are medium- to large-sized (ranging from 1 - 4 metres long) predatory fishes with elongate bodies and jaws, long needle-like teeth, and thick armoured scales.
  2. The bowfin, Amia calva, the single living species forming the Order Amiiformes. The bowfin ranges in length from 0.5 - 1 metre, and is characterised by its long dorsal fin extending across most of the length of the body.
  3. The teleosts, the largest radiation of vertebrate life, exhibiting huge diversity in more than 20,000 species of ray-finned fishes across 40 orders.
  • The clade Teleostei contains four main subgroups, shown in the cladogram below. The highly derived Euteleostei is the largest of the teleosts groups, with around 17,000 species in 375 families. This group shows tremendous diversity, and includes both the largest (the ocean sunfish, reaching up to 3.6 m in length) and the smallest (Paedocypris, a genus of the carp family with females as small as 7.9 mm) extant bony fish species.
  • The Euteleostei comprises three groups:

  1. Order Esociformes - a small group containing pikes and mudminnows.
  2. Division Ostariophysi - a large group including carps, catfishes, minnows, piranhas, and relatives. These 6,500 species form approximately 80 percent of all living freshwater fish species.
  3. Division Neognathi - contains two groups, the Order Salmoniformes (salmon, trout and smelts), and theSubdivision Neoteleostei. The neoteleosts are divided into four groups: three basal groups containing many deep-sea fishes (including the bioluminous lanternfishes), and a single group of highly advanced fishes - the acanthomorphs, or spiny-rayed teleosts (acanth is Ancient Greek for thorn). Spiny teleosts then fall into two groups, the Paracanthopterygii (1200 species of cods, haddocks and anglerfishes) and the Acanthopterygii, a huge clade containing around 12,000 extant species, which dominate the vast majority of the world's open ocean and shallow marine environments. Within this clade is theAtherinomorpha (guppies, killifishes and relatives), and the Percomorpha - by far the most diverse group of fish, containing over one-third of all ray-finned fish species, and exhibiting a fascinating array of body forms - including perches, seahorses, flatfishes, pufferfishes, and tunas


Distribution and Habitat:

As one would expect from their massive diversity, there are certain species of ray-finned fishes present in waters worldwide. However, some clades are much smaller than others, and consequently have much narrower distributions. It would be a huge task to highlight the distribution of all actinopterygian species, or even families, and so the following is a list of the geographical distributions and aquatic environments of the evolutionarily key groups outlined above:

  • Polypteriformes (bichirs and reedfishes) - Africa; freshwater.
  • Chondrostei (sturgeons and paddlefish) - Northern Hemisphere; coastal and freshwater.
  • Lepisosteiformes (gars) - North and Central America and Cuba; fresh and brackish water.
  • Amiiformes (bowfin) - North America; freshwater.
  • Osteoglossomorpha (elephant fishes and relatives) - Worldwide; mostly tropical freshwater.
  • Elopomorpha (true eels and relatives) - Worldwide; mostly marine.
  • Clupeomorpha (herrings, anchovies, and relatives) - Worldwide; mostly marine.
  • Esociformes (pikes and mudminnows) - North America, Western Europe and Northern Eurasia; freshwater.
  • Ostariophysi (carps, catfishes, piranhas, and relatives) - Worldwide; mostly freshwater.
  • Salmoniformes (slamons, trouts, and relatives) - Temperate Northern and Southern hemisphere; freshwater.
  • Paracanthopterygii (cods, haddocks, and anglerfishes) - Northern hemisphere; marine and freshwater.
  • Atherinomorpha (guppies, killifishes, and relatives) - Worldwide; surface-dwelling; freshwater and marine.
  • Percomorpha (perches, seahorses, tunas, and relatives) - Worldwide; mostly marine.


  • The skeleton of the paired fins is formed from many small bones, called fin rays, in a fan-like arrangement, which are supported at the bases of the fins by parallel rows of bones called radials. All living actinopterygians except bichirs and reedfishes (Order Polypteryformes) also have branching rays in unpaired fins.

  • Modified the ancestral fish character of a breathing lung into a swim bladder to aid and adjust buoyancy. Although it is easy to assume that lungs are an adaptation unique to terrestrial vertebrates (tetrapods), it is likely that lungs evolved in early armoured fishes called placoderms in the seas of the Silurian and Devonian around 415 million years ago, and are therefore a shared ancestral character of all bony fishes, including their tetrapod descendents. The swim bladder is a smooth-walled (i.e., non-alveolar) modified lung that is virtually impermeable to gas. Therefore, gas can be gulped - or indeed transferred from the bloodstream in many more derived teleosts - into the swimbladder of ray-finned fishes to reduce the overall density of the body, and afford neutral buoyancy. This allows fish to remain stationary in the water column, and thus waste less energy. As the pressure of the water column on the body changes with depth, actinopterygians must regulate the volume of air in their swim bladders to remain neutrally buoyant. Some ray-finned fishes, such as the gars of North and Central America and Cuba, are secondary air-breathers, who have evolved an alveolar lung to survive in their low-oxygen environment.

  • Single dorsal fin. Some fishes, such as salmons and catfishes, have an additional fin positioned just posterior of the dorsal fin, called the adipose fin, which is small, soft and fleshy.

  • Ancestrally, ray-finned fishes were covered in tough rhomboidal ganoid scales, formed of a thick layer of spongy bone, covered with dentine, followed by ganoine (a substance derived from enamel - the material that covers human teeth). However, the more derived fish groups have modified the structure of their scales to increase flexibility and reduce weight, in order to improve locomotory efficiency - allowing individuals to become more advanced predators, as well as more evasive prey. Living species within the teleost clade have reduced scales that are circular, flexible, thin, and overlapping craniocaudally - cycloid scales. One group, the spiny-rayed teleosts (Order Acanthomorpha), have further modified their scales to a form termed ctenoid (cten is Ancient Greek for comb), which bear comb-like spines on the posterior edge.

  • While many basal actinopterygians possess a primitive heterocercal tail - one in which the fin lobes are different in length (asymmetrical) - there is increasing symmetry towards the teleost group, with teleosts themselves possessing a homocercal tail, which is completely symmetrical in appearance (the images clearly show this in a species of carp).

  • The jaws of ray-finned fish have undergone many modifications through their evolution. Early forms had simple snapping jaws with weak jaw-closing muscles, which were used to grab prey. The neopterygians then lost the connection between the cheek bones and the posterior of the upper jaw (maxilla). This caused the maxilla to be rotated forwards and to the side when the jaws were opened, increasing the volume of the oral cavity to create a suction effect, drawing prey items into the mouth. Further skeletal jaw modifications, alongside flaps of skin around the maxilla then allowed teleosts to have a highly protruding tube-like mouth, whose fully circular opening produced a stronger and highly directional suction force into the oral cavity, which also retained prey on jaw closure. These adaptations to increase suction were crucial in the evolution of ray-finned fishes as active predators, as lunging towards prey items in water actually acts to push them away by forcing a flow of water towards them.

Labelled image of a typical ray-finned fish skeleton:

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