Bony fish

Bony fish are fish that have a skeleton made up of bone and cartilage, in contrast to the cartilaginous fish, which have no bones. There are over 29,000 species of bony fish that grouped together into a taxonomic Superclass called Osteichthyes, making them the largest taxa of vertebrates in existence today. Some other characteristics of bony fish include the ability to swim forward and backwards. They also have a gas-filled swim bladder used for buoyancy as well as a sound receptor, and gills with covered gill slits. In reproduction the eggs are usually fertilized externally and the sexes are separate. In addition, they have smooth and overlapping scales, a jawed mouth terminal that has teeth in most cases, and a two-chambered heart. 

Osteichthyes was recently elevated from a taxonomic class into a superclass that is separated into two different classes, Actinopterygii and Sarcopterygii. Actinopterygii consists of the the ray-finned fish, which makes up most of the species of Osteichthyes(about 27,000 species). They inhabit both freshwater and marine environments. An example of an Actinopterygii is the ocean sunfish, which is the most massive bony fish in the world. Sarcopterygii, or lobe finned fish, consists of coelacanths and lungfish, and are paired with rounded fins. Sarcopterygii makes up only eight living species. 

Anatomy
Osteichthyes have a patter of rooted teeth, cranial bones, and medial mandibular muscle in the lower portion of their jaw. The labyrinth in the ear of Osteichthyes contain large otoliths. Their head and pectoral girdles are covered with dermal bones. The neurocranium is divided into anterior and posterior divisions by fissure. They also have a swim bladder or a lung. Osteichthyes has no spine support for fins, so the fins are supported by lepidotrichia (bone fin rays). Also, they have operculum to help them breathe even when they are not moving through the water. Their digestive tract starts with the pharynx with gill rakers or special pharyngeal. Then food enters the esophagus where it moves on to the stomach. The stomach is a muscular sac-like pouch that secretes digestive acids. Food then moves to the pylorus that sometimes secretes enzymes. After the pylorus comes the small intestine that brings in bile and pancreatic secretions. This then moves to the large intestine with many folds that leads to the anus in the anterior end of the fish where wastes are excreted. 

Scales
Some Osteichthyes, like Lepisosteiformes (gars), have Ganoid scales which are thick diamond-shaped scales covered in a substance called ganoin. Other Osteichthyes have cycloid scales that grow with the fish, are thin, more flexible, and overlap each other. These scales show growth rings in some species that can be used to determine the growth rate or the age of the fish. Another kind of scale is the ctenoid which are thin flexible scales where the posterior end has comb-like teeth, and are believed to reduce drag on the fish when swimming in the water. Other fish are scaleless (catfish, sculpin, eel), sacrificing protection for more flexibility and maneuverability. 

Reproduction
The reproduction among organisms in Osteichthyes varies greatly, but is all sexual reproduction with separate sexes. The Coelacanth for example, gives birth to about 5 to 25 live young after 13 months gestation, that can survive immediately after birth on their own. However, the female salmon lays her eggs, and then males will fertilize the eggs as the female deposits them. These eggs hatch and eventually become salmon.

Examples

 * The ocean sunfish is the biggest member of Osteichthyes, weighing up to 5,000 lb. and as long as 11 feet.
 * The longest of the bony fish are the oarfish which can be as long as 50 ft.
 * Another large fish is the Atlantic blue marlin, with a maximum published weight of about 1,800 lb. and a maximum total length of around 16 ft 3 in.
 * The swordfish uses its sharp, sword-like bill to kill prey and protect itself. The swordfish reach a maximum size of around 14 ft and 1,190 lb.

Evolution and Fish
Certain breeds of guppies have exhibited population traits contrary to evolutionary assumptions of senescence, the study of population aging and endurance on a scale of generations.