| Air-breathing organ (ABO) |
A structure or body surface of a  fish having the specialized structural capacity for aerial gas exchange; utilized by air-breathing fishes to supplement the normal respiratory mode which is aquatic respiration using gills. |
| Amphibious air breathing |
The mode of air breathing when a fish is out of water. |
| Aquatic surface respiration (ASR) |
A hypoxia-driven behavior in which a fish surfaces and places its the mouth as close to the water surface as possible in order to ventilate the gills with the upper few millimeters of water that remains well oxygenated because of atmospheric diffusion. |
| Autapomorphy |
A specialization unique to one group and thus not useful for establishing relationships. |
| Bimodal respiration |
The capacity to exchange respiratory gases in both air and water and to do so either simultaneously or sequentially. |
| Continuous air breathing |
The frequent and regular occurrence of air gulping, even when aquatic oxygen levels are sufficient to sustain aquatic respiration. This behavior often indicates the use of ABO gas for functions other than respiration such as buoyancy. |
| Facultative air breathing |
Emergency aquatic air breathing initiated in response to a diminished capacity for aquatic respiration usually associated with aquatic hypoxia or some other stress factor. |
| Hypoxia-inducible factor (HIF-1α) |
Integrated hypoxia response that activates genes whose protein products either increase O2 transfer (i.e., erythropoiesis, Hb affinity increases, angiogenesis, etc.) or upregulate metabolic adaptation by regulating anaerobiosis and O2 consumption rate. HIF-1α induction is an ancient adaptation that first appeared in eukaryotic cells and thus long preceded the origin of metazoans. |
| Lung |
The principal aerial respiratory organ of all vertebrates and some primitive air-breathing fishes. An outpocketing along the vertebrate digestive tube having a vascular epitheilial surface that functions for aerial gas exchange through its connection to the body surface by a duct through which air can be inhaled and exhaled. |
| Obligatory air breathing |
A high degree of physiological dependence on air breathing for the maintenance of basal oxidative processes or to prevent suffocation. This is usually associated with a reduction in gill surface area that occurs with progressive specialization for air breathing. |
| Physoclistous gas bladder |
A gas-filled structure within the body of the fish that has lost its connection to the body surface and thus cannot function for gas exchange but retains functions for buoyancy and either or both sound production and sound reception. |
| Physostomous gas bladder |
A gas-filled structure within the body of a fish that is connected to the body surface by the pneumatic duct that allowing inflation or deflation. In different fishes, functions of this organ may include aerial respiration as well as buoyancy and both sound reception and production. |
| Placoderm |
Armored Paleozoic fish that were the first vertebrates to have jaws. |
| Plesimorphy |
A primitive trait. |
| Pneumatic duct |
Tube linking the pharynx to either the lung or physostomous gas bladder which functions for the passage of air for either air breathing, volume control, or both. |
| Synapomorphy |
A specialized trait shared by two or more groups, which implies that an ancestral group also possessed this trait. |
| Tetrapodomorpha |
The group of vertebrates consisting of the tetrapods (four-limbed vertebrates) and their closest sarcopterygian (lobefin fish) relatives which invaded the land during the Devonian period. |
