# Osteichthyes

Osteichthyes (/ˌɒstiːˈɪkθiːz/ ost-ee-IK-theez; from Ancient Greek ὀστέον (ostéon) 'bone' and ἰχθύς (ikhthús) 'fish'), also known as osteichthyans or commonly referred to as the bony fish, is a diverse clade of vertebrate animals that have endoskeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes (cartilaginous fish) and the extinct placoderms and acanthodians, which have endoskeletons primarily composed of cartilage. The vast majority of extant fish are members of Osteichthyes, being an extremely diverse and abundant group consisting of 45 orders, over 435 families and 28,000 species. It is the largest class of vertebrates in existence today, encompassing most aquatic vertebrates, as well as all semi-aquatic and terrestrial vertebrates.
The group is divided into two main clades, the ray-finned fish (Actinopterygii, which makes up the vast majority of extant fish) and the lobe-finned fish (Sarcopterygii, which gave rise to all land vertebrates, i.e. tetrapods). The oldest known fossils of bony fish are about 425 million years old from the late Silurian, which are also transitional fossils showing a tooth pattern that is in between the tooth rows of sharks and true bony fishes. Despite the name, these early basal bony fish had not yet evolved ossification and their skeletons were still mostly cartilaginous, and the main distinguishing feature that set them apart from other fish clades were the development of foregut pouches that eventually evolved into the swim bladders and lungs, respectively.
Osteichthyes can be compared to Euteleostomi. In paleontology the terms are synonymous. In ichthyology the difference is that Euteleostomi presents a cladistic view which includes the terrestrial tetrapods that evolved from lobe-finned fish. Until recently, the view of most ichthyologists has been that Osteichthyes were paraphyletic and include only fishes. However, since 2013 widely cited ichthyology papers have been published with phylogenetic trees that treat the Osteichthyes as a clade including tetrapods.

## Characteristics

Bony fish are characterized by a relatively stable pattern of cranial bones, rooted, medial insertion of mandibular muscle in the lower jaw. The head and pectoral girdles are covered with large dermal bones. The eyeball is supported by a sclerotic ring of four small bones, but this characteristic has been lost or modified in many modern species. The labyrinth in the inner ear contains large otoliths. The braincase, or neurocranium, is frequently divided into anterior and posterior sections divided by a fissure.
Early bony fish had simple respiratory diverticula (an outpouching on either side of the esophagus) which helped them breathe air in low-oxygen water as a form of supplementary enteral respiration. In ray-finned fish these have evolved into swim bladders, the changing sizes of which help to alter the body's specific density and buoyancy. In elpistostegalians, a crown group of lobe-finned fish that gave rise to the land-dwelling tetrapods, these respiratory diverticula became further specialized for obligated air breathing and evolved into the modern amphibian, reptilian, avian and mammalian lungs. Early bony fish did not have fin spines like most modern fish, but instead had the fleshy paddle-like fins similar to other non-bony clades of fish, although the lobe-finned fish evolved articulated appendicular skeletons within their paired fins, which gave rise to tetrapods' limbs. They also evolved a pair of opercula (gill covers), which can actively draw water across the gills so they can breathe without having to swim.
Bony fish do not have placoid scales like cartilaginous fish, instead they consist of three types of scales that do not penetrate the epidermis in the process. The three categories of scales for Osteichthyes which are cosmoid scales, ganoid scales, teleost scales. The teleost scales are also then divided into two subgroups which are the cycloid scales, and the ctenoid scales. All these scales have a base of bone that they all originate from, the only difference is that the teleost scales only have one layer of bone. Ganoid scales have lamellar bone, and vascular bone that lies on top of the lamellar bone, then enamel lies on top of both layers of bone. Cosmoid scales have the same two layers of bone that ganoid scales have except they have dentin in-between the enamel and vascular bone and lamellar (vascular and lamellar two subcategories for bone found in scales). All these scales are found underneath the epidermis and do not break the epidermis of the fish. Unlike the placoid scales that poke through the epidermis of the fish.

## Classification

   ...it is increasingly widely accepted that tetrapods, including ourselves, are simply modified bony fishes, and so we are comfortable with using the taxon Osteichthyes as a clade, which now includes all tetrapods...
Traditionally, Osteichthyes was considered a class, recognised on the presence of a swim bladder, only three pairs of gill arches hidden behind a bony operculum, and a predominantly bony skeleton. Under this classification system, Osteichthyes was considered paraphyletic with regard to land vertebrates, as the common ancestor of all osteichthyans includes tetrapods amongst its descendants. While the largest subclass, Actinopterygii (ray-finned fish), is monophyletic, with the inclusion of the smaller sub-class Sarcopterygii, Osteichthyes was regarded as paraphyletic.
This has led to the current cladistic classification which splits the Osteichthyes into two full classes. Under this scheme Osteichthyes is monophyletic, as it includes the tetrapods making it a synonym of the clade Euteleostomi. Most bony fish belong to the ray-finned fish (Actinopterygii).
| Actinopterygii | ray-finned fish | Actinopterygii, members of which are known as ray-finned fishes, is a class or subclass of the bony fishes. The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines ("rays"), as opposed to the fleshy, lobed fins that characterize the class Sarcopterygii which also possess lepidotrichia. These actinopterygian fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the link or connection between these fins and the internal skeleton (e.g., pelvic and pectoral girdles). In terms of numbers, actinopterygians are the dominant class of vertebrates, comprising nearly 99% of the over 30,000 species of fish (Davis, Brian 2010). They are ubiquitous throughout freshwater and marine environments from the deep sea to the highest mountain streams. Extant species can range in size from Paedocypris, at 8 mm (0.3 in), to the massive ocean sunfish, at 2,300 kg (5,070 lb), and the long-bodied oarfish, to at least 11 m (36 ft). |
| Sarcopterygii | lobe-finned fish | Sarcopterygii (fleshy fin), members of which are known as lobe-finned fish, is a group of the bony fishes. Traditionally, it is a class or subclass that excludes Tetrapoda, a group of typically terrestrial vertebrates that descends from lobe-finned fish. However, under modern cladistic classification schemes, Sarcopterygii is a clade that includes the tetrapods. The living sarcopterygians are the coelacanths, lungfish, and tetrapods. Early lobe-finned fishes had fleshy, lobed, paired fins, joined to the body by a single bone. Their fins differ from those of all other fish in that each is borne on a fleshy, lobelike, scaly stalk extending from the body. Pectoral and pelvic fins have articulations resembling those of tetrapod limbs. These fins evolved into legs of the first tetrapod land vertebrates, amphibians. They also possess two dorsal fins with separate bases, as opposed to the single dorsal fin of actinopterygians (ray-finned fish). The braincase of sarcoptergygians primitively has a hinge line, but this is lost in tetrapods and lungfish. Many early lobe-finned fishes have a symmetrical tail. All lobe-finned fishes possess teeth covered with true enamel. |

## Phylogeny

A phylogeny of living Osteichthyes, including the tetrapods, is shown in the cladogram below. Whole-genome duplication took place in the ancestral Osteichthyes.

## Biology

All bony fish possess gills. For the majority this is their sole or main means of respiration. Lungfish and other osteichthyan species are capable of respiration through lungs or vascularized swim bladders. Other species can respire through their skin, intestines, and/or stomach.
Osteichthyes are primitively ectothermic (cold blooded), meaning that their body temperature is dependent on that of the water. But some of the larger marine osteichthyids, such as the opah, swordfish and tuna have independently evolved various levels of endothermy. Bony fish can be any type of heterotroph: numerous species of omnivore, carnivore, herbivore, filter-feeder, detritivore, or hematophage are documented.
Some bony fish are hermaphrodites, and a number of species exhibit parthenogenesis. Fertilization is usually external, but can be internal. Development is usually oviparous (egg-laying) but can be ovoviviparous, or viviparous. Although there is usually no parental care after birth, before birth parents may scatter, hide, guard or brood eggs, with sea horses being notable in that the males undergo a form of "pregnancy", brooding eggs deposited in a ventral pouch by a female.

## Examples

The giant sunfish is the heaviest bony fish in the world. In late 2021, Portuguese fishermen found a dead sunfish near the coast of Faial Island, Azores, with a weight of 2,744 kilograms (6,049 lb) and 3.6 metres (12 ft) tall and 3.5 metres (11 ft) long established the biggest giant sunfish ever captured.
The longest is the king of herrings, a type of oarfish. Other very large bony fish include the Atlantic blue marlin, some specimens of which have been recorded as in excess of 820 kilograms (1,810 lb), the black marlin, some sturgeon species, and the giant and goliath grouper, which both can exceed 300 kilograms (660 lb) in weight. In contrast, Paedocypris progenetica and the stout infantfish can measure less than 8 millimetres (0.31 in). The beluga sturgeon is the largest species of freshwater bony fish extant today, and Arapaima gigas is among the largest of the freshwater fish. The largest bony fish ever was Leedsichthys, which dwarfed the beluga sturgeon as well as the ocean sunfish, giant grouper and all the other giant bony fishes alive today.

## Comparison with cartilaginous fishes

Comparison of cartilaginous and bony fishes :
| Characteristic | Sharks (cartilaginous) | Bony fishes |
| Habitat | Mainly marine | Marine and freshwater |
| Shape | Usually dorso-ventrally flattened | Usually bilaterally flattened |
| Exoskeleton | Separate dermal placoid scales | Overlapping dermal cosmoid, ganoid, cycloid or ctenoid scales |
| Endoskeleton | Cartilaginous | Mostly bony |
| Caudal fin | Heterocercal | Heterocercal or diphycercal |
| Pelvic fins | Usually posterior. | Mostly anterior, occasionally posterior. |
| Intromittent organ | Males use pelvic fins as claspers for transferring sperm to a female | Do not use claspers, though some species use their anal fins as gonopodium for the same purpose |
| Mouth | Large, crescent shaped on the ventral side of the head | Variable shape and size at the tip or terminal part of the head |
| Jaw suspension | Hyostylic | Hyostylic and autostylic |
| Gill openings | Usually five pairs of gill slits which are not protected by an operculum. | Five pairs of gill slits protected by an operculum (a lateral flap of skin). |
| Type of gills | Larnellibranch with long interbranchial septum | Filiform with reduced interbranchial septum |
| Spiracles | The first gill slit usually becomes spiracles opening behind the eyes. | No spiracles |
| Afferent branchial vessels | Five pairs from ventral aorta to gills | Only four pairs |
| Efferent branchial vessels | Nine pairs | Four pairs |
| Conus arteriosus | Present in heart | Absent |
| Cloaca | A true cloaca is present only in cartilaginous fishes and lobe-finned fishes. | In most bony fishes, the cloaca is absent, and the anus, urinary and genital apertures open separately |
| Stomach | Typically J-shaped | Shape variable. Absent in some. |
| Intestine | Short with spiral valve in lumen | Long with no spiral valve |
| Rectal gland | Present | Absent |
| Liver | Usually has two lobes | Usually has three lobes |
| Swim bladder | Absent | Usually present |
| Brain | Has large olfactory lobes and cerebrum with small optic lobes and cerebellum | Has small olfactory lobes and cerebrum and large optic lobes and cerebellum |
| Restiform bodies | Present in brain | Absent |
| Ductus endolymphaticus | Opens on top of head | Does not open to exterior |
| Retina | Lacks cones | Most fish have double cones, a pair of cone cells joined to each other. |
| Accommodation of eye | Accommodate for near vision by moving the lens closer to the retina | Accommodate for distance vision by moving the lens further from the retina |
| Ampullae of Lorenzini | Present | Absent |
| Male genital duct | Connects to the anterior part of the genital kidney | No connection to kidney |
| Oviducts | Not connected to ovaries | Connected to ovaries |
| Urinary and genital apertures | United and urinogenital apertures lead into common cloaca | Separate and open independently to exterior |
| Eggs | A small number of large eggs with plenty of yolk | A large number of small eggs with little yolk |
| Fertilisation | Internal | Usually external |
| Development | Ovoviviparous types develop internally. Oviparous types develop externally using egg cases | Normally develop externally without an egg case |