Lungfishes are an ancient group of freshwater vertebrates known for their ability to breathe air. Belonging to the class Dipnoi, these animals possess a dual respiratory system, utilizing both gills for aquatic respiration and specialized lung-like organs for breathing atmospheric air. This adaptation allows them to survive in environments where water oxygen levels can fluctuate dramatically. Their lineage extends back over 410 million years, providing insights into evolutionary transitions.
Unique Anatomical Features
The most notable feature of lungfishes is their lungs, which are modified swim bladders connected to the alimentary tract, similar to amphibian lungs. These lungs are subdivided into numerous smaller air sacs, significantly increasing the surface area for gas exchange. While most lungfish species have two lungs, the Australian lungfish has only one.
Lungfishes also possess fleshy, limb-like fins, supported by a well-developed internal skeleton. These fins allow some species, like the West African lungfish, to propel themselves along the substrate, sometimes even lifting their bodies. Their bodies are elongated and eel-like, covered with cycloid scales. Their mouths feature tooth plates, effective for crushing hard-shelled organisms, a common part of their diet.
Survival Adaptations and Behavior
Lungfishes have developed strategies to survive in environments prone to drying out. Their most well-known adaptation is estivation, a state of dormancy that allows them to endure periods of drought. During estivation, African lungfishes burrow into the mud of drying riverbeds or lakebeds and secrete a mucus cocoon. This cocoon hardens, providing a protective barrier against dehydration and allowing them to survive for months, sometimes up to four years, without water or food.
While encased in their cocoons, their metabolism slows significantly, conserving energy. They switch from excreting toxic ammonia to less-toxic urea, which accumulates in their bodies. When water returns, the lungfish rehydrates and emerges from its cocoon, resuming normal activity and feeding. Lungfishes are opportunistic omnivores, consuming a variety of aquatic animals, including crustaceans, mollusks, and smaller fish. In aquatic habitats, they surface periodically to gulp air, a behavior that becomes more frequent as water oxygen levels decrease.
Global Distribution and Species Diversity
Six living species of lungfish are recognized, distributed across three continents: Africa, South America, and Australia. These species are grouped into three families.
The Australian lungfish, Neoceratodus forsteri, is the sole member of the family Neoceratodontidae, found exclusively in the Mary and Burnett River systems of southeastern Queensland, Australia. This species is considered the most primitive of the living lungfishes.
The South American lungfish, Lepidosiren paradoxa, represents the family Lepidosirenidae. It inhabits swamps and slow-moving waters within the Amazon, Paraguay, and lower Paraná River basins.
The largest group, the African lungfishes, belong to the family Protopteridae and include four species within the genus Protopterus: the Marbled lungfish (Protopterus aethiopicus), the Gilled lungfish (Protopterus amphibius), the West African lungfish (Protopterus annectens), and the Spotted lungfish (Protopterus dolloi). These African species are concentrated primarily in the equatorial belt, extending from Senegal to Mozambique.
Evolutionary Significance
Lungfishes are called “living fossils” and hold an important position in the study of evolution. They are considered the closest living relatives of tetrapods, the group that includes all four-limbed land vertebrates like amphibians, reptiles, birds, and mammals.
The presence of well-developed lungs and limb-like fins in lungfishes provides insights into the evolutionary transition from aquatic to terrestrial life. Their ability to breathe air and use their fins for locomotion on a substrate suggests that these adaptations may have predated the full emergence of land animals. Studying lungfishes helps researchers understand the morphological, physiological, and genetic changes that occurred as vertebrates moved from water to land.