Bony fish, classified as Osteichthyes, are the most successful and numerous group of vertebrates on Earth. This vast assemblage contains over 28,000 known species, accounting for nearly 99% of all fish and more than half of all living vertebrate species combined. Their success has allowed them to colonize virtually every aquatic environment, from the deepest oceans to the highest mountain streams. The bony fish group is divided into two major extant classes: the Ray-finned fish (Actinopterygii) and the Lobe-finned fish (Sarcopterygii).
The Shared Characteristics of Bony Fish
The defining feature of Osteichthyes is the presence of an endoskeleton made primarily of bone tissue, which distinguishes them from cartilaginous fish (Chondrichthyes) like sharks and rays. This skeletal reinforcement provides a rigid internal structure that supports their muscular system. Bony fish also possess the operculum, a hard, bony flap covering and protecting the delicate gills on each side of the head.
The operculum actively pumps water across the gills, creating a continuous flow of oxygenated water without the fish needing to swim constantly. This mechanism contrasts sharply with the reliance of most sharks on ram ventilation. Another significant shared trait is the swim bladder, a gas-filled organ located beneath the backbone. This organ allows the fish to precisely control its buoyancy in the water column, expending minimal energy to remain suspended at a specific depth, unlike cartilaginous fish, which rely on large, oil-filled livers for lift.
Ray-Finned Fish (Actinopterygii)
The Ray-finned fish, Actinopterygii, constitute the overwhelming majority of all bony fish species. Their class name describes their defining anatomical feature: fins supported by flexible, bony spines called lepidotrichia. These rays connect directly to the internal skeletal elements, giving the fins a highly maneuverable, fan-like structure.
The vast evolutionary radiation within this group is largely attributed to the Teleostei, or modern bony fish, which comprise about 96% of all living fish. Teleosts evolved a highly mobile jaw structure, allowing them to protrude their mouths rapidly to capture prey through suction, a major factor in their ecological dominance. Their diversity covers everything from the sleek tuna and salmon to the minute anglerfish and seahorses.
Their success is also linked to flexible, lightweight scales, which provide protection without sacrificing speed or maneuverability. Adaptations like the efficient homocercal tail, which has symmetrical upper and lower lobes, contribute to their superior swimming performance in diverse habitats.
Lobe-Finned Fish (Sarcopterygii)
The Lobe-finned fish, Sarcopterygii, are characterized by a fleshy, robust pair of pectoral and pelvic fins joined to the body by a single, stout bone. These fins contain internal skeletal elements and musculature, creating a structure that resembles the limbs of land vertebrates. Sarcopterygii hold a significant place in evolutionary history because they represent the lineage that gave rise to all tetrapods, including amphibians, reptiles, birds, and mammals.
Today, this ancient group is represented by only a few surviving species across two main groups: the Coelacanths and the Lungfish. Coelacanths were once thought to be extinct until their rediscovery in 1938, earning them the moniker “living fossils.” The two extant coelacanth species live in deep marine waters and retain primitive features, such as an oil-filled notochord and a hinged skull.
The six living species of Lungfish are found in freshwater habitats across Africa, South America, and Australia. These fish are notable for their ability to breathe air using a modified swim bladder that functions as a lung, allowing them to survive in oxygen-poor or seasonal water bodies. In some species, such as the African lungfish, this adaptation allows them to burrow into the mud and estivate, or enter a state of dormancy, for months during dry periods.