Osteichthyes (pronounced /ˌɒstiːˈɪkθi.iːz/), also called bony fish, are a taxonomic group of fish that have bony, as opposed to cartilaginous, skeletons. The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of over 29,000 species. It is the largest class of vertebrates in existence today. Osteichthyes is divided into the ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii).
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 ray-finned fish (Actinopterygii); there are only eight living species of lobe-finned fish (Sarcopterygii), including the lungfish and coelacanths.
Traditionally, the bony fish had been treated as a class within the vertebrates, with Actinopterygii and Sarcopterygii as subclasses. However, some recent works have elevated Osteichthyes to superclass, with Actinopterygii and Sarcopterygii as classes.
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 primatively 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.
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