Osteichthyes

Bony fish Temporal range: Late Silurian - Recent
Atlantic herring
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Infraphylum:	Gnathostomata
Superclass:	Osteichthyes Huxley, 1880
classes
Actinopterygii Sarcopterygii

Osteichthyes (/ˌɒstiːˈɪkθiːz/), also called bony fish, are a taxonomic Class of fish that includes the ray-finned fish (Actinopterygii) and lobe finned fish (Sarcopterygii). The split between these two classes occurred around 440 mya.

In most classification systems the Osteichthyes are paraphyletic with land vertebrates. That means that the nearest common ancestor of all Osteichthyes includes tetrapods amongst its descendants. Actinopterygii (ray-finned fish) are monophyletic, but the inclusion of Sarcopterygii in Osteichthyes causes Osteichthyes to be paraphyletic.

Most bony-fish belong to the Actinopterygii; there are only eight living species of lobe finned fish (Sarcopterygii) including the lungfish and coelacanths.(Some species of lobe-finned fish have jointed bones.)

They are traditionally treated as a class of vertebrates, with subclasses Actinopterygii and Sarcopterygii, but some newer schemes divide them into several separate classes.

The vast majority of fish are osteichthyes. Osteichthyes are the most various group of vertebrates, consisting of over 29,000 species, making them the largest class of vertebrates in existence today.

Characteristics

Osteichthyans are characterized by a relatively stable pattern of cranial bones, rooted, medial insertion of mandibular muscle in 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 fissure. Osteichthyans have a lung or swim bladder. 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. They also have a swim bladder which helps the body create a neutral balance between sinking and floating. They also are able to see in color, unlike most other fish.

Replacement bone

One of the best-known innovations of the osteichthyans is endochondral bone or "replacement" bone, which is bone ossified internally, by replacement of cartilage, as well as perichondrally, as "spongy bone." In vertebrates, in general, there are various types of calcified tissue: dentine, enamel (or "enameloids") and bone, plus variants characterized by their ontogeny, chemistry, form and location. But endochondral bone is unique because it begins life as cartilage.

In lower vertebrates, cartilaginous structures can become superficially calcified. However, in osteichthyans, the circulatory system invades the cartilaginous matrix. This permits the local osteoblasts (bone-forming cells) to continue bone formation within the cartilage and also recruits additional, circulating osteoblasts. Other cells gradually eat away at the surrounding cartilage. The net result is that the cartilage is replaced from within by a somewhat irregular vascularized network of bone. Structurally, the effect is to create a relatively lightweight, flexible, "spongy" bone interior, surrounded by an outline of dense, lamellar periostial bone. Since this bone now surrounds other bone, rather than cartilage, it is referred to as periostial rather than perichondral. This is the unique endochondral bone from which the osteichthians derived their name, as well as many structural advantages. However useful endochondral bone may be, it is also much heavier and less flexible than cartilage. Thus, many modern osteichthyan groups, including the extremely successful teleosts, have evolved away from extensive use of endochondral bone.

Respiratory system

Bony fish breathe through their gills with the help of their operculum, a hard plate that allows osteichthyes to breathe without moving. This means energy does not have to be wasted swimming.

Metabolism

Osteichthyes are ectothermic (cold blooded), meaning they don't have to eat as much and that their temperature is dependent on the water's. They can be any type of heterotroph: omnivore, carnivore, herbivore, or detrivore. Most species have a stomach.

Body covering

No placoid scales. Mucous glands coat the body. Most have scales of sort: ganoid, cycloid, or cytenoid. These scales are smooth and overlapping.

Appendages

All osteichthyes have paired fins along with two eyes, nostrils, a mouth and an operculum.

Skeleton

The skeleton is made of bone and cartilage. The skeleton is almost completely calcified. The vertebral column, cranim, jaw, ribs, and intramuscular bones make up the skeleton.

Reproduction

Osteichthyes can by hermaphrodites. Fertilization is usually external, but can be internal. Development is usually but can by oviparous, ovoviviparous, or viviparous. Although their is usually no parental care after birth, before birth parents may scatter, hide, guard or brood eggs.

Examples

The ocean sunfish is the most massive bony fish in the world (but not the longest one; that honor goes to the oarfish). Specimens of ocean sunfish have been observed up to 3.33 m (11 ft) in length and weighing up to 2,300 kg (5,070 lb). Other very large bony fish include the Atlantic blue marlin, some specimens of which have been recorded as in excess of 820 kilograms (1,807.4 lb.), the black marlin, some sturgeon species, the giant grouper and the goliath grouper, both which can exceed 400 kg (880 lb) in weight. In contrast, the dwarf pygmy goby measures a minute 1.5 cm.

The Arapaima gigas is the largest species of freshwater bony fish.

The largest fish ever was leedsichthys, which is a gigantic bony fish.

See also

• Ostracoderm - armoured jawless fishes.
• Acanthodians - relatives of the bony fishes