Lung fish form a strange group of very primitive fish that can breathe from the air. All the living species of this group live in the southern hemisphere of the planet and, as aquatic animals, their biology is very determined by this means.
Dipnoi or pulmonary fish are a group of fish belonging to the sarcopterygii class, where fish with lobed or muscular (fleshy) fins are classified.
To date, six species of lungfish are known, which are grouped into two families. The Lepidosirenidae and Ceratontidae. The lepidosirenids are divided into two genera: Protopterus in Africa with four live species and the genus Lepidosiren in South America, with a single species. The family Cerantodontidae only has one species, in Australia, Neoceratodus fosteri, which is the most primitive lungfish.
Characteristics of lungfish
They have two working lungs when they're adults. These come from the ventral wall of the pharynx termination. In addition to the lungs, they have gills, but they take care only of the 2% of the breathing of the adult animal. During their larval stage, these fish only breathe through the gills.
They have nasal orifices, but do not utilize them for getting air, but for carrying out olfactory functions. Their body is covered by very small scales which graft under the skin.
They live in shallow continental waters, and during the dry season, they bury themselves in the mud and hibernate. They seal the mouth with a cap of mud which has a small orifice from where enters the air necessary for breathing.
How pulmonary fish breathe
Lung fish have two lungs and have a circulatory system with two circuits. These lungs have ridges and walls to increase gas exchange, as well as being very vascularized.
To breathe these fish rise to the surface and keep their mouths open, thus expanding the mouth cavity forcing the air to enter. Subsequently, they close the mouth, compress the oral cavity and the air passes to the anterior pulmonary cavities.
While the mouth and the anterior cavity of the lung remain closed, the posterior cavity contracts and exhales the inhaled air in the anterior respiration, making it come out from the opercules (where the gills are usually found in the fish with aquatic respiration). Once the air is exhaled, the anterior chamber contracts and opens, allowing the ria to pass to the posterior chamber where the exchange of gas will take place.
Lepidosiren paradoxa
Lepidosiren paradoxa Fitzinger (Lepidosiren paradoxa) is found in the waterways of the Amazon and other parts of South America. Its appearance is reminiscent of that of an eel and can exceed one metre in length.
It lives in shallow and preferably still waters. When summer and drought come, a den is built in the mud to keep the humidity, leaving an orifice from which to perform lung breathing.
Coanoitto
El Protopterus annectens or coanoitto is a lungfish that lives in Africa. It also looks like an eel, although it has very elongated and filamentous fins. It inhabits the countries of West and Central Africa, but also some Eastern regions.
It is a nocturnal animal and during the hours of light remains hidden in the aquatic vegetation. During the dry season, it digs a hole where it slips vertically so that the mouth remains in contact with the surface. If the water level falls lower than their hole, they begin to secrete the mucus that they need to keep hydrated.
Neoceratodus forsteri
The Neoceratodus forsteri, is an Australian fish (Neoceratodus forsteri), living south-east of Queensland, in Australia, in the rivers Burnett and Mary.
Unlike other lungfish, the Neoceratodus forsteri has only one lung, so it cannot live with air breathing alone. It lives in deep river areas, hidden during the day and moving slowly in the muddy bottom during the night.
They are large animals, more than one metre long in adulthood and can weigh up to 40 kg.
When the water level decreases in the dry season, these fishes remain on the bottom, as having only one lung they also need the aquatic respiration through the gills.