Teleost

Teleostei, commonly known as teleosts, represents the largest group of ray-finned fishes within the class Actinopterygii. These fish are incredibly diverse, making up approximately 96% of all living fish species on Earth today. The group contains over 26,000 species organized into about 40 orders and 448 families. Because they are so widespread, they occupy almost every aquatic environment. They range from the massive ocean sunfish, which can weigh over 2,000 pounds, to the tiny male anglerfish, *Photocorynus spiniceps*, which measures only one inch long.

The most significant physical difference between teleosts and other bony fish is their specialized jaw structure. Teleosts possess a movable premaxilla, which is the bone at the very front of the upper jaw. This bone is not attached to the neurocranium, or the braincase. When the fish feeds, the maxilla acts as a lever to push the premaxilla outward. This creates a circular opening that lowers the pressure inside the mouth. This mechanism works like a vacuum to suck prey into the mouth.

By using this suction, teleosts can grab prey without the risk of pushing it away by simply closing their jaws. In more advanced species, the premaxilla is the main bone that holds teeth.

Beyond the outer mouth, teleosts have a second set of jaws located in the throat called pharyngeal jaws. These are composed of five branchial arches, which are bony loops that support the gills. In more basal, or primitive, teleosts, these jaws mainly serve to transport food. However, in more derived teleosts, these structures have become much more powerful. The left and right ceratobranchials fuse to form a single lower jaw. These pharyngeal jaws also include specialized muscles that allow the fish to grind their food before swallowing.

Teleosts also feature a specific tail structure known as a homocercal caudal fin. In a homocercal tail, the upper and lower lobes are roughly equal in size. This differs from many other fish where the spine extends into the upper lobe of the tail. In teleosts, the spine ends at the caudal peduncle, which is the narrow base of the tail. To support the upper lobe, they have elongated neural arches called uroneurals. This skeletal arrangement contributes to the flexibility and speed seen in many species.

The history of teleost classification began with the German ichthyologist Johannes Peter Müller in 1845. He named the group after the Greek words *teleios*, meaning "complete," and *osteon*, meaning "bone." While Müller focused on soft tissue, modern scientists use DNA to understand their true relationships. In 1966, researchers including Greenwood provided a more stable classification system. We now know that while the oldest stem groups appeared in the Triassic period, teleosts likely began evolving during the Paleozoic era. They saw a massive increase in size and abundance following the Cretaceous-Paleogene mass extinction about 66 million years ago.

Within the teleost family, there are several distinct lineages with unique adaptations. The Elopomorpha, which includes eels, have thin, leaf-shaped larvae called leptocephali. The Osteoglossomorpha, or elephantfishes, are defined by a unique bony element in their tongue. Another major group, the Ostariophysi, includes many freshwater fish like carps and catfishes. These fish possess a Weberian apparatus, which is a series of bones connecting the swim bladder to the inner ear. This allows them to hear much better by transporting vibrations from the bladder to the ear.

Teleosts are also highly significant to human society and global economies. The fishing industry harvests many species for food, and many others are raised on commercial farms. Some people catch teleosts for sport, while others keep them in aquariums for beauty. Beyond food and hobbies, they are vital to the scientific community. Researchers use teleosts in the fields of genetics and developmental biology to understand how life works. Their ability to adapt to so many different roles makes them a central part of the Earth's biological systems.