Lungfish

Lungfish, or dipnoans, are freshwater vertebrates in the class Dipnoi. They are famous for their unusual respiratory system, which allows them to breathe air, and for features within the Sarcopterygii group, like lobed fins with a strong internal skeleton. These fish are the closest living relatives of tetrapods—the group that includes amphibians, reptiles, birds, and mammals. Only six species of lungfish exist today, found in Africa, South America, and Australia, but their fossils show they once lived worldwide. The fossil record goes back to the Early Devonian, over 410 million years ago. The earliest lungfish were marine, but almost all after the Carboniferous lived in freshwater.

The name "Dipnoi" comes from the Greek *dípnoos*, meaning "with two breathing structures," combining *di-* ("twice") and *pnoḗ* ("breath").

Anatomically, all lungfish have a continuous cartilaginous notochord and a well-developed set of teeth on the palate. Primitive lungfish had marginal teeth and a bony braincase, but modern species have reduced marginal bones and a cartilaginous braincase. In primitive forms, the skull roof was covered with cosmine, but after the Devonian, the roof lay under the skin and lost that covering. Modern lungfish have fused and reduced skull roof bones that don't match those of ray-finned fish or tetrapods. During breeding, the South American lungfish grows feathery appendages from its pelvic fins, thought to help gas exchange around eggs. Through convergent evolution, lungfish developed internal nostrils like tetrapod choanae, and a brain similar to that of amphibians (except the Queensland lungfish, which diverged about 277 million years ago and has a brain like the coelacanth *Latimeria*). Their teeth are unique: rows of odontodes on the palate and lower jaw form a fan-shaped surface that wears into a crushing plate. In some groups, including modern lepidosireniformes, these ridges become blades. Modern lungfish have many larval features, suggesting paedomorphosis, and they have the largest genomes among vertebrates. Their bodies are elongated, with fleshy paired fins and a single caudal fin replacing the dorsal, caudal, and anal fins.

Their lungs are highly specialized. They connect directly to the larynx and pharynx without a trachea. Unlike the simple gas bladders of other air-breathing fish, lungfish lungs are subdivided into many small air sacs, increasing surface area for gas exchange. Most species have two lungs, but the Australian lungfish has only one. These lungs are homologous to tetrapod lungs and, like in tetrapods and bichirs, they extend from the ventral side of the esophagus and gut.

Only the Australian lungfish can rely solely on its gills for oxygen; other species have gills too reduced for that. When using gills, the circulatory system works like a typical fish’s: the conus arteriosus spiral valve is open, certain gill arch vessels are shut or open, and the pulmonary arteries are closed. Water flows unidirectionally through the mouth and gills via a buccal pump, with countercurrent blood flow maintaining a concentration gradient. When breathing air, the spiral valve closes to reduce blood mixing, the third and fourth gill arches open, the second and fifth close to prevent oxygen loss, the sixth arch’s ductus arteriosus closes, and the pulmonary arteries open. The sixth gill still helps remove carbon dioxide (which dissolves easily in water) from deoxygenated blood before it reaches the lung. Airflow through the mouth is tidal, and through the lungs it is bidirectional, with oxygen diffusing in a "uniform pool" pattern.

Lungfish are omnivorous, eating fish, insects, crustaceans, worms, mollusks, amphibians, and plants. They have an intestinal spiral valve instead of a true stomach. African and South American lungfish can survive seasonal droughts by burrowing into mud and estivating through the dry season.