Ray-finned fish, also known as actinopterygians, belong to the class Actinopterygii and represent the majority of the bony fish population, the other being lobe-finned fish or sarcopterygians. Notably, ray-finned fish account for nearly half of all living vertebrate species and constitute the bulk of fish populations worldwide.
They get their name from the Ancient Greek words actino-, meaning ‘having rays,’ and ptérux, which means ‘wings’ or ‘fins’ for being lightweight and having flexible fins, formed from skin webbed over radially extended spines (lepidotrichia) from their skeletons. This structure allows them to change shape and provide better thrust than those with fleshier fins, leading to their extensive diversity and population worldwide.
Presently, 42 orders, 431 families, and approximately 27,000 species are recognized within this class. Teleosts (infraclass Teleostei) are the largest family of ray-finned fish, with nearly 26,000 members (approximately 93%) of the total fish population.
Ray-finned fish vary widely in shape and size. The largest ray-finned fish ever discovered is the now-extinct genus Leedsichthys of pachycormid fish, which measured 54 ft (16.5 m). Among the existing types, the Ocean Sunfish is the most massive, weighing nearly 5,000 lbs (2,300 kg), while the Giant Oarfish is the longest, with an average length of 36 ft (11 m).
The smallest is a member of the carp family, Paedocypris, with females typically measuring 0.3 in (7.9 mm) in length.
These bony fishes are distinguished from other groups by a single dorsal fin, ganoid or leptoid scales that grow lifelong, and a swim bladder. Additionally, they have well-ossified skeletons and jaw bones that can unhinge and expand.
The swim bladder is a highly specialized organ that evolved from the primitive lung systems of early armored fish, like placoderms. It has a smooth, gas-impervious wall that requires air transfer into the chamber via a network of capillaries called the rete mirabile. By adjusting the air level inside the sac to match the water pressure above them, these ray-finned fish can float without moving their limbs.
Though the most developed bladders in modern teleosts are completely internal and separate from the respiratory system, some species, such as gars and arapaima, use them for breathing, similar to the lungs in land-dwellers.
The paired fins of fish consist of a fan-like arrangement of thin spines called fin rays, which are supported at their bases by parallel bony structures known as radials.
The swim bladder enhances buoyancy, allowing the pelvic fins to be positioned just behind the pectoral fins rather than further back, which improves the fish’s agility.
These vertebrates were once covered with rigid ganoid scales (rhomboidal in shape) consisting of layers of spongy bone, dentine, and ganoine (a substance originating from enamel). However, teleosts have only leptoid scales, which are thinner and more flexible, allowing greater maneuverability and faster movement. These circular scales lack the tough enamel and dentine layers of other fishes.
Many teleosts and all the surviving chondrosteans (paddlefishes and sturgeons) have now completely shed their scales.
The branchiostegal rays are long, curved bones emerging from the branchial cavity. Along with the operculum (a bony covering), they form a protective structure over the gills. They allow the mouth cavity to expand, increasing the water capacity and improving circulation. Also, they enable ray-finned fishes to use suction while catching prey.
Although their ancestors had an asymmetric, heterocercal tail with fleshy flaps extending along the axis, most modern species have a homocercal tail (symmetric lobes and a flexible basal joint), which provides buoyancy, uniform thrust, and precise movement.
The neopterygians (subclass Neopterygii) possess a freely hanging maxilla (upper jaw), which generates suction to capture food by expanding the protruding oral cavity. The teleosts show further modifications, such as loose skin flaps along the maxilla and a fully circular mouth to improve suction capabilities.
Like vertebrates, the eyes of ray-finned fish are adapted to detect a wide range of wavelengths, depending on the depth at which they live. Apart from the normal light spectrum, many shallow-water species can perceive ultraviolet light, while others, like salmon and cichlids, can even detect polarized light.
Those living in deep sea or low-light environments have specially evolved long, tube-like eyes to provide better depth perception and improved binocular vision.
The current taxonomy is based on the first Phylogenetic Classification of Bony Fishes by R. Betancur-Rodriguez et al., published in 2013 and updated in 2017 by incorporating the most recent phylogenetic results.
Actinopterygii is currently classified under Cladistia and Actinopteri. Within these two main groups, several orders, like Guildayichthyiformes, Elonichthyiformes, and Saurichthyiformes, are now extinct. Actinopteri is further split into Chondrostei and Neopterygii, with the latter containing the most number of species within the greatly diversified infraclasses, Holostei and Teleostei.
While the group Chondrostei was initially divided into two orders, the Acipenseriformes (sturgeons and paddlefishes) and the Polypteriformes (reedfishes and bichirs), the latter has been newly classified under Cladistia.
Due to their extensive variety, ray-finned fishes inhabit nearly every aquatic zone. While most families (angelfish, carp, and anchovies) appear worldwide in fresh or saltwater, a few are specific to certain regions. Some, like cod, haddocks, sturgeons, and paddlefish, are exclusive to the Northern Hemisphere, while others, like salmon and trout, occur in both hemispheres but typically in temperate waters. Bowfins and gars are native to North and South America, whereas bichirs and reedfish live in the freshwaters of Africa.
A significant number of species (41%) live in freshwater, with Southeast Asia being the most densely populated. Families like carp, pikes, catfishes, and piranhas prefer mostly rivers and lakes, while eels, herrings, and seahorses are marine inhabitants living in coral reefs. Several groups, such as guppies, cods, and anglerfish, can survive both in salty and freshwater, while gars exclusively occupy brackish waters.
Ray-finned fish can be herbivores, carnivores, omnivores, zooplanktivores, or detritivores. They are generally opportunistic feeders, feeding on anything they encounter.
Typically, ray-finned fish have two distinct sexes, male and female, with the latter spawning eggs and the former fertilizing them. Mating usually happens in four ways: monogamy, polygyny, polyandry, and polygynandry.
Monogamy, the practice of maintaining a single partner for multiple seasons, is typical among hawkfishes and blennies. Some, like sea basses and darters, show polygyny, with the males having several partners, while anemonefishes display polyandry, the females mating with various males. Herrings and wrasses prefer polygynandry (or promiscuity), where both sexes mate with different partners throughout the breeding season.
However, some forms of alternative mating, like synchronous or sequential hermaphroditism, exist in various circumstances.
Synchronous (or simultaneous) hermaphrodites like black hamlets contain ovarian and testicular tissue inside their bodies and can exhibit whichever trait is most advantageous.
Sequential hermaphroditism is the sex change of an individual at some point in their life. Protogyny, commonly seen in wrasses, parrotfish, and groupers, is the most popular type, in which fish begin life as females and later convert to males. Some, like moray eels and clownfish, prefer the reverse, starting as males and then transforming into females, a process called protandry. Either external or internal factors can trigger these sex changes.
Most ray-finned fish undergo external fertilization to produce offspring. Some, like the mangrove rivulus, can self-fertilize, producing both sperm and eggs internally. However, this often causes inbreeding. To avoid this unwanted situation, some individuals develop into males at temperatures below 66°F (19°C) and fertilize the eggs. In mouthbrooding fish like cichlids, the eggs undergo insemination within the female’s mouth.
Around 21% of the species are either viviparous (give birth to live young) or ovoviviparous (eggs hatch inside the mother’s body), where males show greater parental care than females. Conversely, 79% of egg-laying or oviparous teleosts do not participate in child-rearing.
While most temperate fish eggs hatch between 3 to 20 days after laying, those of most coral reef species hatch within just a day.
Unlike other vertebrates, these fish undergo a larval stage and have indeterminate growth (they keep growing throughout their lives). These transparent, free-swimming larvae are quick and responsive, and by the time they attain maturity, they can easily navigate predator-infested waters.
Smaller ray-finned fishes generally live shorter lives, with many surviving in the wild for less than a year. However, some species have much longer lifespans than predicted, living for decades in captivity. For example, largemouth bass and European perch can survive for 24 to 25 years, while some sturgeons and rockfish can live between 80 to 150 years.
Many ray-finned fish, like parrotfish and wrasses, are active during the day for foraging (diurnal). However, some, like tarpon and groupers, feed at dusk (crepuscular), while groups like ponyfishes and cardinalfishes prefer the nighttime (nocturnal).
Most species exhibit migratory traits. They go on short, daily trips to feed and avoid predators, while long ones are generally reserved for reproductive activities. Others may spend their entire lives in either marine (oceanodromous) or freshwater regions (potamodromous).
Those that migrate between fresh and salty water are called diadromous. Some, like salmon, grow in saltwater and travel to freshwater to spawn (anadromous), while others, including anguillid eels, do the reverse and spawn in salty water (catadromous). Conversely, others, like gobies, move between different salinities for feeding (amphidromous).
These fish use visual and olfactory clues and factors like ocean currents, temperature, salinity, and the earth’s magnetic field to find their way back after completing extensive journeys. They can easily find locations with accuracy using their remarkable memory.
Sometimes, these fish can form aggregates or shoals for various purposes, such as feeding, breeding, or seeking protection. Extremely large groups become schools and can contain millions of individuals. They move as one entity, often contorting into random shapes in search of food or avoiding danger.
Ray-finned fish primarily communicate through vision, mechanoreception (spatial awareness, touch, balance, and hearing), electroreception, and chemoreception (smell and taste).
Apart from the basic modes of nutrition, many fish are opportunistic feeders and can switch between the modes based on food availability.
Many groups, such as catfish, cod, wrasses, and minnows, maintain dominance hierarchies determined by age, size, sex, previous experience, and residency. Typically, males are dominant over females, having access to better feeding grounds, habitats, and ocean currents.
Territoriality is another form of aggressive behavior in various groups, such as barracuda, freshwater eels, surgeonfishes, and cichlids. Their interactions involve vocalizations, displays, chasing, and biting that occur at territorial boundaries.
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