Lungfish are a type of lobe-finned fish, classified within the group Sarcopterygii. This ancient lineage of bony fish includes all living lungfish (Dipnoi) and coelacanths (Actinistia), as well as the ancestors of all four-limbed vertebrates, the tetrapods. The six living species of lungfish are found in freshwater habitats across Africa, South America, and Australia. Their unique combination of fish-like and amphibian-like traits makes them important for understanding the transition of life from water to land.
Defining the Lobe-Finned Fish Group
The classification of lungfish as lobe-finned fish is based on a specific anatomical feature of their paired fins. Unlike the familiar ray-finned fish (Actinopterygii), whose fins are supported by numerous slender bony rays, the Sarcopterygii possess fleshy, muscular fins. These paired fins are constructed around a central, internal skeleton of robust bones and joints.
The structure of the lobe-fin is characterized by a single, stout bone that articulates directly with the shoulder or pelvic girdle. This bone is homologous to the humerus or femur in land vertebrates. This central bone arrangement provides a much greater range of motion and support compared to the fan-like structure of ray-fins.
This skeletal design in the pectoral and pelvic fins is the primary reason lungfish and coelacanths are grouped together in the Sarcopterygii clade. The presence of this lobed fin structure in lungfish is considered a primitive trait within the bony fishes. The fin of the Australian lungfish, for example, is particularly illustrative of this limb-like arrangement.
Unique Adaptations of Lungfish
Within the lobe-finned fish, the six extant species of lungfish (Dipnoi) possess specialized traits that allow them to survive in extreme environments. The most well-known adaptation is the presence of one or two functional lungs, which are modified swim bladders. These organs allow them to breathe atmospheric air, a necessary ability in the stagnant, oxygen-poor water of their tropical habitats.
The African and South American lungfish species are obligate air-breathers, meaning they must surface regularly to gulp air even when water is present, as their gills are often insufficient for their oxygen needs. The Australian lungfish, however, is a facultative air-breather, relying primarily on its gills in well-oxygenated water but using its single lung when oxygen levels drop. This dual-respiratory system has a specialized circulatory arrangement, similar to tetrapods, to direct deoxygenated blood to the lungs.
The African and South American lungfish also exhibit dormancy called estivation, allowing them to survive extended periods of drought. As their freshwater habitats dry up, they burrow into the mud, creating a chamber and secreting a mucus cocoon that hardens around their body. They remain in this state for months, sometimes up to four years, with their metabolic rate slowing drastically. They use their lungs through a small opening in the cocoon until the rains return.
Beyond their respiratory and survival mechanisms, lungfish possess other distinctive anatomical features, such as specialized tooth plates. These plates, which form a fan-shaped crushing surface on the palate and lower jaws, are adapted for grinding and crushing hard-shelled organisms like mollusks. Furthermore, their skeleton is highly cartilaginous, a feature that distinguishes them from most other bony fish.
The Evolutionary Bridge to Tetrapods
The classification of lungfish as lobe-finned fish holds importance in understanding the evolutionary history of vertebrates. Lungfish are considered the closest living relatives to tetrapods, the group that includes all amphibians, reptiles, birds, and mammals. This close relationship positions them at a key point in the transition from aquatic to terrestrial life.
The internal skeletal arrangement of the lobe-fin represents a structural homology to the limb bones of early land vertebrates. While the specific fins of modern lungfish are highly modified, the ancestral pattern of a single bone articulating with the girdle is the fundamental blueprint for the tetrapod limb. This means the genetic and developmental pathways for building a limb were already present in the aquatic ancestors of lungfish and tetrapods.
Fossil discoveries, such as Tiktaalik, are part of the evolutionary lineage stemming from the Sarcopterygii that led to the first land-dwellers. Modern lungfish offer a living system to study the mechanisms that facilitated the water-to-land transition. Recent research on the African lungfish has shown that it can use its pelvic fins to lift and propel itself forward along the substrate.
This capacity for bottom-walking demonstrates that the precursors for locomotion on land were present in aquatic lobe-finned fish millions of years before the appearance of fully terrestrial animals. By studying the biology of living lungfish, scientists can gain insights into the genomic basis for the evolution of limbs, lungs, and other characteristics that enabled vertebrates to conquer the land.