Teleosts, constituting the largest infraphylum of ray-finned fishes (Teleostei), are distinguished from other bony fish by their movable premaxilla (jaw bone) and specialized jaw musculature. They can protrude their jaws outward, a feature that enhances their feeding capabilities. Additionally, their tail anatomy is unique, with the upper and lower lobes being almost equal in size (homocercal), unlike other fish.
With over 26,000 species classified into more than 50 orders and 448 families, teleosts account for approximately 96% of the total extant fish population. These bony fish inhabit a wide range of environments, from the deepest ocean trenches to high-altitude freshwater lakes, showcasing their remarkable adaptability and ecological diversity.
Being a broad group with high diversity, they vary greatly in size, with the giant oarfish measuring around 25 ft (7.6 m) to the male anglerfish around 6.2 mm (0.24 in) long.
The largest living teleost is the giant oarfish, which can reach lengths of about 25 ft (7.6 m). In contrast, the largest extinct teleost was a member of Leedsichthys, which measured an astonishing 91 feet (27.6 m).
Conversely, stout infantfish (Schindleria brevipinguis) holds the title of the smallest adult teleost and the smallest vertebrate in the world. On average, it measures around 0.33 in (8.4 mm), with males being slightly smaller at about 0.28 in (7 mm) long. Additionally, the ocean sunfish is often considered the heaviest teleost, weighing approximately 2.3 tonnes.
Teleosts are characterized by movable premaxillae in the jaw, homocercal caudal fin, and unpaired basibranchial teeth plates.
The premaxillary bone (located at the tip of the upper jaw) is protrusible and creates a circular opening in the mouth. Such curvature produces low pressure inside the mouth, helping these fish suck prey easily. Once the prey is inside the mouth, the lower jaw and the maxilla are pulled back to seal the mouth.
The premaxilla bears teeth in advanced teleosts, followed by the toothless maxillary bone. The latter pushes the premaxilla and the lower jaw forward. This mechanism, in turn, is facilitated by the contraction of an adductor muscle on the top of the maxilla.
Teleosts also possess pharyngeal jaws, a second set of jaws consisting of five branchial arches (loops of bones supporting the gills). The first three arches are composed of a single basibranchial surrounded by two hypobranchials. The fourth arch comprises pairs of ceratobranchials and epibranchials. While the base of the lower pharyngeal jaws is composed of the fifth ceratobranchials, that of the upper jaw is formed by some additional bones called pharyngobranchials.
The caudal fin or tail is homocercal, with both lobes being almost similar in size. The spine of the tail is modified into a structure called a caudal peduncle.
The upper lobe is supported by elongated neural arches called uroneurals. Additionally, an extra set of bones, the hypurals (form a flattened plate at the rear end of the vertebral column), support the caudal fin.
Like most other fish, teleosts primarily respire through gills. However, various groups have developed additional respiratory structures to adapt to different environmental conditions. Beyond the gills, the swim bladder, which typically aids in buoyancy, can store a small amount of air and assist in respiration for some species.
Teleosts derive their name from the Greek words’ teleios,’ meaning ‘complete,’ and ‘osteon,’ meaning ‘bone.’ This group of fish was first recognized by the German ichthyologist Johannes Peter Müller in 1845. He classified them based on their soft tissue characteristics, which did not take into account the distinguishing features of fossil teleosts.
In 1966, Greenwood et al. proposed a more comprehensive phyletic classification based on the evolutionary history of these fish, which has been further revised over the years.
The taxonomy presented below is based on the Phylogenetic Classification of Bony Fishes by R. Betancur-Rodriguez et al. (2013).
The earliest teleosts are believed to have originated in the Early Triassic Period and diversified into different shapes and sizes during the early Cretaceous. Members of the groups Elopomorpha and Osteoglossomorpha are considered the most basal of all living teleosts.
Teleosts are abundant in all aquatic habitats, including marine and freshwater environments. Some, like the desert pupfish, thrive in hot and saline water bodies of deserts. However, species diversity reduces as we move to higher latitudes.
Members of the groups Elopomorpha, Clupeomorpha, and Percomorpha are exclusively marine, while those of Ostariophysi and Osteoglossomorpha thrive in freshwater. Interestingly, guppies from the order Atherinomorpha and cods from the order Paracanthopterygii can live in both fresh and marine water bodies, showcasing their adaptability to different aquatic environments.
Additionally, pikes (order Esociformes) are restricted to freshwater habitats in the Northern Hemisphere, whereas salmon and trout (order Salmoniformes) are found in temperate waters of both the Northern and Southern Hemispheres.
Teleosts are also found at different elevations. For instance, the brown trout and the scaly osman are found at altitudes as high as 12,530 ft in the lakes of Kashmir, whereas the hadal snailfish is spotted at depths of around 25,300 ft.
Teleosts advance through the lateral movement of the posterior end of their trunk and the tail. However, this pattern varies based on the niches occupied by the fish.
These fish communicate through sounds produced by stridulation or vibrating their swim bladder.
While some teleosts, like eel, are catadromous and migrate from freshwater rivers to seas during spawning, others, like salmon and striped bass, are anadromous, migrating from seas to rivers. For instance, the European eel migrates from the Atlantic Ocean to breed in the Sargasso Sea. The adults die after spawning, but the newborn young are swayed by the Gulf Stream to the estuaries and rivers of Europe. They then spend their adult lives in streams and ponds.
Teleosts group together into small shoals for different purposes. The shoals that swim synchronously in the same direction are called schools and help fend off predators through collective effort. When attacked by an enemy, these schools scatter and reassemble to confuse the attacker. Moreover, they also aggregate for feeding, as spotting and gathering prey together is easier.
Many teleosts associate in symbiotic relationships with other fishes. For instance, gobies have been found living in association with shrimps in burrows. While shrimp gather food for the fish, the latter cleans the shrimp’s burrows. Similarly, wrasses live symbiotically with larger fish (often inside their mouths), picking parasites from their bodies while staying under their protection.
Most teleosts are gonochoristic, with separate male and female sexes. However, some, like clownfish, are also sequential hermaphrodites, starting life as females and switching to males later in life.
As a diverse group, teleosts have adopted a wide range of reproductive strategies. They are usually iteroparous, having multiple breeding cycles throughout their lives. However, some, like salmon, are semelparous, with a single reproductive episode before death. Although most teleosts are oviparous and lay eggs, some are viviparous (give birth to live young) and provide parental care, especially by males.
Being a diverse group, different species of teleosts use different mating tactics to propagate their races. Some fish, like gobies, are monogamous and form an exclusive bond with a single partner, whereas guppies, cichlids, Nassau groupers, humbug damselfish, and Baltic herring are polygamous, having multiple sexual partners. In polygamy, a single male may mate with multiple females (polygyny), as observed in sunfish, sculpins, darters, and damselfish, or a single female may breed with multiple males (polyandry), like in clownfish.
Polygynous fish, such as Cyrtocara eucinostomus, often engage in lek-breeding, in which the males aggregate to display to the females. They may also guard large groups of females called harems.
Male teleosts usually resort to different courtship techniques to woo their female counterparts. These include color changes, different types of mating calls (clicks, whimpers, booms, grating, and drumming sounds), and visual displays, such as rapid swimming and fin erection. They are also generally larger and more conspicuous than the females (sexual dimorphism), with secondary sexual characteristics increasing their reproductive fitness. For instance, males of minnows develop small bumps called breeding tubercles on their heads, whereas the male green humphead parrotfish grows an ossified ridge on its forehead for headbutting.
Sometimes, males that fail to attract a female use alternative mating strategies. In bluegills, larger and older males (parental males) usually have the best chances of fertilizing females and are busy building nests for the eggs. Meanwhile, smaller satellite males take advantage of this by mimicking females. They quickly enter the nests and fertilize the eggs without the parental males noticing.
Teleosts either release their eggs in the open water or on specific surfaces. Those that spawn in the open water quickly swim to the surface to release their gametes. This type of spawning often occurs in groups.
Most of these fish are oviparous, laying eggs that have been fertilized externally. In contrast, internal fertilization occurs in around 500 to 600 species, including members of the group Otocephala. Here, the males inseminate the females with a penetrating or intromittent organ.
Some teleosts, like splitfins, are viviparous and give birth to live young (nurtured by a placenta-like structure), whereas members of the family Poeciliidae are ovoviviparous, with the young nurtured in the mother’s body by the yolk sac.
A few species, like Nomorhamphus ebrardtii, practice oophagy, where developing larvae feed on unfertilized eggs within the mother’s uterus.
Most marine teleosts lay light, transparent, and buoyant pelagic eggs characterized by thin envelopes. These eggs are dispersed by ocean currents and release planktonic larvae upon hatching, which contain a nutritive yolk sac. In contrast, freshwater teleosts release heavy demersal eggs (that stay near the seabed) with thick, pigmented envelopes. They hatch into relatively well-developed larvae, which can swim and feed upon hatching.
The teleost larvae gradually develop into juveniles, which almost resemble the adults. Depending on the species, this transformation may take a few minutes or hours (damselfish) to weeks (salmon, squirrelfish, gobies, and flatfishes). This stage is characterized by fully developed fins, scales, internal organs, and axial skeleton. With time, the juveniles become sexually mature and reach adulthood.
The sex of teleosts depends on the spawning conditions. In cold water, they produce females, whereas they give rise to males in hot water.
Teleosts can landmark locations through mental mapping. Experiments conducted by scientists using mazes reveal that these fish have spatial memory conducive to such behavior.
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