Osteichthyes

Osteichthyes, or bony fish, are the most diverse and abundant class of vertebrates. This group includes a vast array of species from tiny minnows to massive tuna. Their defining characteristic is a skeleton made of bone. Unlike cartilaginous fish, most bony fish have a swim bladder, an internal gas-filled sac that helps them maintain neutral buoyancy in the water, so they don't have to constantly swim to avoid sinking. Their bodies are typically covered in overlapping scales and they have paired fins for steering and propulsion. They breathe through gills covered by a protective bony flap called an operculum. Osteichthyes can be found in almost every aquatic environment, from freshwater lakes and rivers to the deepest parts of the ocean. This class is further divided into two subclasses: ray-finned fishes, which make up the majority, and lobe-finned fishes, which are thought to be the ancestors of tetrapods.