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All aquatic amniotes (reptiles, birds and mammals) have thick and impermeable cutes that preclude cutaneous respiration, and thus rely solely on the lungs to breathe air. When underwater, the animal is essentially holding its breath and has to routinely return to the surface to breathe in new air.
Aerial respiration is the 'gulping' of air at the surface of water to directly extract oxygen from the atmosphere. Aerial respiration evolved in fish that were exposed to more frequent hypoxia; also, species that engage in aerial respiration tend to be more hypoxia tolerant than those which do not air-breath during the hypoxia.
Many microscopic aquatic animals, and some larger but inactive ones, can absorb sufficient oxygen through the entire surface of their bodies, and so can respire adequately without gills. However, more complex or more active aquatic organisms usually require a gill or gills.
An aquatic system lacking dissolved oxygen (0% saturation) is termed anaerobic, reducing, or anoxic. In water, oxygen levels are approximately 7 ppm or 0.0007% in good quality water, but fluctuate. [5] Many organisms require hypoxic conditions. Oxygen is poisonous to anaerobic bacteria for example. [3]
In both aquatic and terrestrial respiration, the exact mechanisms by which neurons can generate this involuntary rhythm are still not completely understood (see Involuntary control of respiration). Another important feature of the respiratory rhythm is that it is modulated to adapt to the oxygen consumption of the body.
Gills usually consist of thin filaments of tissue, branches, or slender tufted processes that have a highly folded surface to increase surface area.The high surface area is crucial to the gas exchange of aquatic organisms as water contains only a small fraction of the dissolved oxygen that air does.
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Aquatic animals generally conduct gas exchange in water by extracting dissolved oxygen via specialised respiratory organs called gills, through the skin or across enteral mucosae, although some are evolved from terrestrial ancestors that re-adapted to aquatic environments (e.g. marine reptiles and marine mammals), in which case they actually ...