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Oxygen diffuses into water from air and therefore the top layer of water in contact with air contains more oxygen. This is true only in stagnant water; in running water all layers are mixed together and oxygen levels are the same throughout the water column. One environment where ASR often takes place is tidepools, particularly at night. [34]
The weatherfish is long and thin which allows it to burrow through the substrate and navigate through places that deeper bodied fish would have trouble with. It grows up to 30 cm (12 in) in total length , [ 3 ] though there are fishermen who say they have caught longer, up to 45 cm (17.7 in). [ 4 ]
In shallow water fish, the ratios closely approximate that of the atmosphere, while deep sea fish tend to have higher percentages of oxygen. For instance, the eel Synaphobranchus has been observed to have 75.1% oxygen, 20.5% nitrogen , 3.1% carbon dioxide , and 0.4% argon in its swim bladder.
The fish is an air breather, using its labyrinth organ, which is rich in blood vessels and opens into the fish's mouth, [19] an advantage in oxygen-deprived water that is often found in the Amazon River. This fish is able to survive in oxbow lakes with dissolved oxygen as low as 0.5 ppm.
The fish becomes inactive in waters below 10 °C (50 °F); [10] at this temperature they breathe almost no air; however, with increasing temperature their air breathing increases. [7] Their preferred temperature range is between 12–26 °C (54–79 °F), with 18 °C (64 °F) the temperature of maximum activity. [ 41 ]
Having no lung-like organs, modern amphibious fish and many fish in oxygen-poor water use other methods, such as their gills or their skin to breathe air. Amphibious fish may also have eyes adapted to allow them to see clearly in air, despite the refractive index differences between air and water.
Certain amphibious fish also evolved to breathe air to survive oxygen-deprived waters, such as lungfishes, mudskippers, labyrinth fishes, bichirs, arapaima and walking catfish. Their abilities to breathe atmospheric oxygen are achieved via skin-breathing, enteral respiration , or specialized gill organs such as the labyrinth organ and even ...
The west African lungfish also has the ability to breathe atmospheric air by rising to the water's surface and taking a gulp of air, which it must do every half an hour or so to survive. [10] While they do possess lungs, there is minimal oxygen exchange through them; the lungs are primarily used for atmospheric respiration. [9]