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Diverse group of fish with skeletons of bone rather than cartilage

Osteichthyes (,/ɒ//s//t//iː/'/ɪ//k//θ//i//.//iː//z/), also known as osteichthyans or commonly referred to as the bony fish, is a diverse superclass 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, 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 pouchs 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. Early bony fish did not have fin spines like most modern fish, but instead had the fleshy paddle like fins similar 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 expect they gave 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.

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

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 or detritivore 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.

The ocean 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 and tall and long established the biggest ocean 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 , the black marlin, some sturgeon species, and the giant and goliath grouper, which both can exceed in weight. In contrast, Paedocypris progenetica and the stout infantfish can measure less than 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 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