What is a key characteristic of bony fish?

A key characteristic of bony fish is the presence of a swim bladder. This organ is a gas-filled sac that allows bony fish to control their buoyancy and helps them maintain their position in the water column without expending energy. The swim bladder is a defining feature of bony fish (class Osteichthyes) and distinguishes them from cartilaginous fish (class Chondrichthyes), which do not possess this structure.

While fins are indeed present in bony fish, they are also found in cartilaginous fish, limiting the characteristic's uniqueness to bony fish. Additionally, external fertilization occurs in many bony fish species as well as in various other aquatic organisms, not just bony fish, making it less distinctive. Therefore, the presence of a swim bladder is a critical and specific adaptation that highlights their evolutionary adaptations and physiological traits.