Osteichthyes

Osteichthyes Fossil range: 420–0 Ma PreЄ Є O S D C P T J K Pg N
Blue runner
Scientific classification
Kingdom: Animalia Phylum: Chordata Superclass: Osteichthyes Huxley, 1880
Included groups
Actinopterygii Sarcopterygii
Excluded groups
Tetrapoda

Osteichthyes /ˌɒstiːˈɪkθi.iːz/, popularly referred to as the bony fish, is a diverse taxonomic group of fish that have skeletons primarily composed of bone tissue, as opposed to cartilage. The vast majority of fish are members of Osteichthyes, which is an extremely diverse and abundant group consisting of 45 orders, and over 435 families and 28,000 species. It is the largest class of vertebrates in existence today. The group Osteichthyes is divided into the ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii). The oldest known fossils of bony fish are about 420 million years ago, which are also transitional fossils, showing a tooth pattern that is in between the tooth rows of sharks and bony fishes.

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, whereas on a traditional view, Osteichthyes includes only fishes and is therefore paraphyletic. However, recently-published phylogenetic trees treat the Osteichthyes as a clade.

Characteristics

Guiyu oneiros, the earliest known bony fish, lived during the Late Silurian, 419 million years ago). It has the combination of both ray-finned and lobe-finned features, although analysis of the totality of its features place it closer to lobe-finned fish.

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 lungs (a pouch on either side of the esophagus) which helped them breathe in low-oxygen water. In many bony fish these have evolved into swim bladders, which help the body create a neutral balance between sinking and floating. (The lungs of amphibians, reptiles, birds, and mammals were inherited from their bony fish ancestors.) They do not have fin spines, but instead support the fin with lepidotrichia (bone fin rays). They also have an operculum, which helps them breathe without having to swim.

Bony fish have no placoid scales. Mucus glands coat the body. Most have smooth and overlapping ganoid, cycloid or ctenoid scales.

Classification

Traditionally, Osteichthyes is considered a class, recognised on having a swim bladder, only three pairs of gill arches, hidden behind a bony operculum and a predominately bony skeleton. Under this classification systems, the Osteichthyes are paraphyletic with regard to land vertebrates as the common ancestor of all Osteichthyes includes tetrapods amongst its descendants. The largest subclass, the Actinopterygii (ray-finned fish) are monophyletic, but with the inclusion of the smaller sub-class Sarcopterygii, Osteichthyes is paraphyletic.

This has led to an alternative classification, splitting the Osteichthyes into two full classes. Paradoxically, Sarcopterygii is under this scheme considered monophyletic, as it includes the four classes of tetrapods. Most bony fish belong to the ray-finned fish (Actinopterygii); there are only eight living species of lobe-finned fish (Sarcopterygii), including the lungfish and coelacanths.

Actinopterygii	ray-finned fish	Actinopterygii, or ray-finned fishes, constitute 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, however, 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) or lobe-finned fish constitute a clade (traditionally a class or subclass) of the bony fish, though a strict cladistic view includes the terrestrial vertebrates. The living sarcopterygians are the coelacanths, lungfish, and the tetrapods. Early lobe-finned fishes have fleshy, lobed, paired fins, which are 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

The phylogeny of living bony fishes

	Osteichthyes classification
Sarcopterygii Coelacanthimorpha Coelacanthiformes Rhipidistia Dipnomorpha Ceratodontiformes Tetrapodomorpha Actinopterygii Cladistia Polypteriformes Actinopteri Chondrostei Acipenseriformes Neopterygii Holostei Amiiformes Lepisosteiformes Teleostei Elopomorpha Elopiformes Albuliformes Notacanthiformes Anguilliformes Osteoglossocephalai Osteoglossomorpha Osteoglossiformes (incl. Hiodontiformes) Clupeocephala Otomorpha Clupei Clupeiformes Alepocephaliformes Ostariophysi Anotophysa Gonorynchiformes Otophysa Cypriniformes Characiphysae Gymnotiformes Characiformes Siluriformes Euteleosteomorpha Lepidogalaxiiformes Protacanthopterygii Argentiniformes Galaxiiformes Salmoniformes (incl. Esociformes) Stomiatii Osmeriformes Stomiatiformes Neoteleostei Ateleopodiformes Eurypterygia Cyclosquamata Aulopiformes Ctenosquamata Myctophiformes Acanthomorpha

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 members of the family scombridae such as the swordfish and tuna have achieved various levels of endothermy. They can be any type of heterotroph: omnivore, carnivore, herbivore, or detritivore.

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 ocean sunfish is the largest bony fish in the world, while the longest is the king of herrings, a type of oarfish. Specimens of ocean sunfish have been observed up to 3.3 metres (11 ft) in length and weighing up to 2,303 kilograms (5,077 lb). 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, the dwarf pygmy goby measures a minute 15 millimetres (0.59 in).

Arapaima gigas is the largest species of freshwater bony fish. The largest bony fish ever was Leedsichthys, which dwarfed the beluga sturgeon, ocean sunfish, giant grouper, and all the other giant bony fishes alive today.

Comparison with cartilaginous fishes

Cartilaginous fishes can be further divided into sharks, rays and chimaeras. In the table below, the comparison is made between sharks and bony fishes. For the further differences with rays, see sharks versus rays.

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

See also

• Acanthodians - relatives of the bony fish
• Ostracoderm - armoured jawless fish.
• Prehistoric fish