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Actinopterygii

Adapted from Wikipedia · Discoverer experience

A bluefin tuna, a large and prized fish species known for its speed and flavor.

Actinopterygii, often called ray-finned fish, make up almost 99% of all fish species alive today. They are a class of bony fish and include over 30,000 different types. Most of these fish are teleosts, and they are found all over the world in freshwater, brackish, and marine environments — from deep oceans to hidden underground waters and high mountain streams.

These fish are called ray-finned because their fins have thin bony spines called lepidotrichia, which help them move easily through the water. Unlike the fleshy fins of their close relatives, the lobe-finned fish, ray-finned fish fins can change shape and direction quickly. This helps them swim very efficiently.

Ray-finned fish come in many sizes. The smallest living species, Paedocypris, is only 8 mm long, while the largest living ones, like the giant sunfish and the giant oarfish, can weigh over 2,700 kg or grow up to 8 meters long. Even bigger than these, the extinct Leedsichthys from the Jurassic period might have reached 16.5 meters.

Characteristics

Anatomy of a typical ray-finned fish (cichlid)A: dorsal fin, B: fin rays, C: lateral line, D: kidney, E: swim bladder, F: Weberian apparatus, G: inner ear, H: brain, I: nostrils, L: eye, M: gills, N: heart, O: stomach, P: gall bladder, Q: spleen, R: internal sex organs (ovaries or testes), S: pelvic fins, T: spine, U: anal fin, V: tail (caudal fin). Possible other parts not shown: barbels, adipose fin, external genitalia (gonopodium)

Ray-finned fishes come in many different shapes and sizes. One special feature they often have is called the swim bladder, which helps them stay afloat in water. Most ray-finned fishes develop this bladder from a part near their digestive system. In some fish, like bowfins and gars, the swim bladder can also help them breathe air. In others, like the arapaima, it has changed to let them breathe air directly.

These fish also have different kinds of scales. A group called teleosts have special thin, clear scales that grow in layers as the fish gets bigger. These scales are different from the harder scales found in some other fish. Some groups of ray-finned fishes, like sturgeons and paddlefish, have experienced changes in their genes that happened millions of years ago.

Body shapes and fin arrangements

Further information: Fish fin and Diversity of fish

Ray-finned fish come in many different sizes and shapes. They also have special ways of feeding and arrange their fins in unique ways.

Some fish, like tuna, are built for speed with a special tail shape. Others, like the swordfish, are even faster. Salmon use their tail fins to jump over obstacles in rivers. Cod have extra fins that help them move easily through the water. Flatfish have fins that help them stay balanced. The four-eyed fish can see both above and below the water at the same time. Some fish, like the fangtooth, wait to catch their prey by surprise. The anglerfish has a special fin that looks like a fishing rod with a lure. Bichirs have lungs and are closely related to other ray-finned fish. The flying fish has special fins that help it glide through the air.

Reproduction

Most ray-finned fish have separate males and females. In many species, females lay eggs that are fertilized by males outside the body. The young then grow up as free-swimming larvae. However, there are other ways these fish can reproduce. In some, the fish start life as females and later change into males, which is triggered by something inside or outside their body. Changing from male to female is less common. Most of these fish use external fertilization, meaning the eggs are fertilized outside the body. Only a small number of species give birth to live young or care for their eggs, and when they do, it is often the males who help protect the young. One special fish, the mangrove rivulus, can live out of water and can fertilize its own eggs, which helps it survive in its unique habitat.

Classification and fossil record

Ray-finned fish, also called actinopterygians, make up nearly all of the world's fish species. They are divided into groups like Cladistia, Chondrostei, and Neopterygii. The Neopterygii group includes Holostei and Teleostei, which contains most living fish species.

The earliest known ray-finned fish fossils are from about 420 million years ago. These fish have evolved into many different types, living in all kinds of water environments, from deep oceans to tiny streams.

The classification of ray-finned fishes can be summarized as follows:

  • Cladistia, which include bichirs and reedfish
  • Actinopteri, which include:
    • Chondrostei, which include Acipenseriformes (paddlefishes and sturgeons)
    • Neopterygii, which include:
      • Teleostei (most living fishes)
      • Holostei, which include:
        • Lepisosteiformes (gars)
        • Amiiformes (bowfin)

The cladogram below shows the main groups of living ray-finned fishes and their evolutionary relationships to other fish groups and four-limbed vertebrates (tetrapods). Tetrapods evolved from bony fish during the Devonian period.

The polypterids (bichirs and reedfish) are the closest relatives of all other ray-finned fishes. The Acipenseriformes (sturgeons and paddlefishes) are the closest relatives of Neopterygii, and Holostei (bowfin and gars) are the closest relatives of teleosts. The Elopomorpha (eels and tarpons) are among the most basic teleosts. The earliest known fossil ray-finned fish, Andreolepis hedei, dates back 420 million years and was found in Russia, Sweden, and Estonia. Crown group ray-finned fishes likely originated near the Devonian-Carboniferous boundary. The earliest fossil relatives of modern teleosts are from the Triassic period, although teleosts may have originated during the Paleozoic Era.

Taxonomy

The listing below is a summary of all extinct and living groups of Actinopterygii with their respective taxonomic rank.

ChondrosteiChondrostei (cartilage bone) is a subclass of primarily cartilaginous fish showing some ossification. Earlier definitions of Chondrostei are now known to be paraphyletic, meaning that this subclass does not contain all the descendants of their common ancestor. There used to be 52 species divided among two orders, the Acipenseriformes (sturgeons and paddlefishes) and the Polypteriformes (reedfishes and bichirs). Reedfish and birchirs are now separated from the Chondrostei into their own sister lineage, the Cladistia. It is thought that the chondrosteans evolved from bony fish but lost the bony hardening of their cartilaginous skeletons, resulting in a lightening of the frame. Elderly chondrosteans show beginnings of ossification of the skeleton, suggesting that this process is delayed rather than lost in these fish. This group had once been classified with the sharks: the similarities are obvious, as not only do the chondrosteans mostly lack bone, but the structure of the jaw is more akin to that of sharks than other bony fish, and both lack scales (excluding the Polypteriforms). Additional shared features include spiracles and, in sturgeons, a heterocercal tail (the vertebrae extend into the larger lobe of the caudal fin). However the fossil record suggests that these fish have more in common with the Teleostei than their external appearance might suggest.
NeopterygiiNeopterygii (new fins) is a subclass of ray-finned fish that appeared somewhere in the Late Permian. There were only few changes during its evolution from the earlier actinopterygians. Neopterygians are a very successful group of fishes because they can move more rapidly than their ancestors. Their scales and skeletons began to lighten during their evolution, and their jaws became more powerful and efficient. While electroreception and the ampullae of Lorenzini is present in all other groups of fish, with the exception of hagfish, neopterygians have lost this sense, though it later re-evolved within Gymnotiformes and catfishes, who possess nonhomologous teleost ampullae.

Images

Illustration of a swordfish, showcasing this fascinating marine creature.
A beautiful Atlantic Salmon, a type of fish known for its shiny scales and migratory habits.
Illustration of an Atlantic cod, a type of fish found in the Atlantic Ocean.
A flatfish called the Gulf of Mexico ancylopsetta, a type of fish found in ocean waters.
A picture of Anableps anableps, a type of fish known for its unique upward-gazing eyes.
An illustration of Diaphus metopoclampus, a fascinating deep-sea fish.
A fascinating deep-sea fish called Gonostoma elongatum, glowing softly in the dark ocean depths.
A deep-sea fish called the spiky dogfish, known for its sharp spines.
A deep-sea anglerfish, known for its unique lure used to attract prey in the dark ocean depths.
Illustration of a Beryx decadactylus fish species.
A Polypterus bichir, a unique freshwater fish with bony scales, often kept in aquariums.
Illustration of a regalecus glesne, also known as the king of herrings, a fascinating deep-sea fish.

Related articles

Species
Fish
Giant oarfish
Jurassic
Salmon
Swim bladder

This article is a child-friendly adaptation of the Wikipedia article on Actinopterygii, available under CC BY-SA 4.0.

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