Lungfish

African and South American lungfish can easily be distinguished by their appearance, but they also share many similar characters. In fact, some zoogeographers have used the close link between these two lungfishes to provide supporting evidence of an early land connection between South America and Africa. Lungfish are eel-like, with a long, narrow, tubular body and small scales well embedded in the body of the fish. Both the pectoral and pelvic fins of lungfish are elongated and somewhat threadlike. In Lepidosiren the pelvic fins are modified to function as accessory respiratory organs, with feathery margins receiving an increased blood supply—they function like a pair of gills. The primary respiratory organs of Lepidosiren are a pair of lungs with a single opening on the floor of the mouth. These lungs have furrowed walls and receive a full supply of blood. The young of both species have long, feathery, external gills located behind the head which are lost in the South American lungfish at 1.6 in (4 cm) in length, and at 5.9 in (15 cm) in the African lungfish. Both South American and African species reach a length of 3.3-6.6 ft (1-2 m).

When lungfish are in water, they breathe air by rising to the surface and sticking the tips of their nasal opening and mouth out of the water, so as to empty their lungs and take in fresh air. In most fish, the nostrils are pouchlike. However, the jaw construction of the lungfish is modified so that there is an opening from the nasal sac to the inside of the mouth. This internal nostril allows the fish to breathe air at the surface without opening its mouth and swallowing water. Because lungfish breathe by lungs instead of gills, air is essential for their survival; if lungfish are forced to remain underwater, they will drown.

Lungfish live in areas with temporary water bodies, such as shallow swamps, the stagnant backwaters of river courses, and small creeks. These areas are prone to dry out during the dry season. As the water recedes, lungfish burrow into the mud, forming a hollow at the end of the tunnel. The lungfish curl up, tail over head, keeping the nostrils clear of dust and dirt, and secrete a mucous cocoon. Air enters the burrow through a small hole at the top of the dried mud. Lungfish are able to remain in a period of aestivation (dormancy) for the duration of the dry season and have been known to survive as long as four years, although usually a year is all that is necessary. During the aestivation period, lungfish experience a drop in their metabolism and obtain adequate nutrition from the stored fat in their tail. Breeding takes place in the wet season following the reestablishment of the water bodies they inhabit. The male of the South American lungfish guards the eggs during the period of incubation and hatching.