Osteichthyes

Bony fishes Fossil range: Late Silurian - Recent
Atlantic herring
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Infraphylum: Gnathostomata Superclass: Osteichthyes
Classes
Actinopterygii Sarcopterygii

Osteichthyes are a taxonomic superclass of fish, also called bony fish that includes the ray-finned fish (Actinopterygii) and lobe finned fish (Sarcopterygii).

The Osteichthyes are paraphyletic with land vertebrates, in some classification schemes the tetrapods et al are considered to be members of the Osteichthyes for this reason.

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

Osteichthians are characterized by a relatively stable pattern of cranial bones, rooted teeth, 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.

Replacement bone

One of the best-known innovations of the osteichthians is endochondral bone or "replacement" bone, i.e. bone ossified internally, by replacement of cartilage, as well as perichondrally, as "spongy bone." In the more general vertebrates 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 more basal vertebrates, cartilaginous structures can become superficially calcified. However, in osteichthians, 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 osteichian groups, including the extremely successful teleosts, have evolved away from extensive use of endochondral bone.

The dissection of a bony, or any other fish can prove quite useful to study internal organs.

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, and some sturgeon species.

See also

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

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