How does a highly vascularized swim bladder help certain osteoglossomorphs breathe air?

In certain air-breathing osteoglossomorphs—specifically members of the Osteoglossidae, Pantodontidae, and Gymnarchidae families—the swim bladder functions as an accessory breathing organ that allows the fish to survive in deoxygenated water.12

This adaptation is made possible because the swim bladder is highly vascularized, meaning it contains a dense network of blood vessels, and its inner surface is honeycombed with tiny pits. These structural features increase the surface area available for gas exchange, enabling the fish to absorb oxygen directly from air gulped at the water's surface into the bloodstream. This specialized use of the swim bladder as a lung-like structure is a critical survival mechanism for these species in environments where dissolved oxygen levels in the water are insufficient to support them through gills alone.132