The hingemouth (''Phractolaemus ansorgii'') is a small

freshwater fish Freshwater fish are those that spend some or all of their lives in fresh water, such as rivers and lakes, with a salinity of less than 1.05%. These environments differ from marine conditions in many ways, especially the difference in levels of s ...

that is found only in west central

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

, the sole member of the subfamily Phractolaeminae of the family Kneriidae. The mouth can extend like a small trunk, thus the name, and has just two teeth, both in the lower jaw. The

swim bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled Organ (anatomy), organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their curren ...

has two compartments, and can function as a

lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...

, allowing the hingemouth to survive in

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

-poor environments.

References

* * * Kneriidae Fish described in 1901 Taxa named by George Albert Boulenger Freshwater fish of Africa {{Gonorynchiformes-stub